VOLVO V90 FREQUENTLY ASKED QUESTIONS

Most of the time when you have a tire that is making a thumping noise its going to have a bump on the tire which usually comes from a broken "belt" on some types of tires such as radials. Other causes for this sound would be that a shock absorber is not functioning correctly or has loosened from its mount.

The term ABS means ANTI-LOCK BRAKING SYSTEM. The ABS System automatically controls braking at the front wheels individually and the rear wheels as a unit (in most cases) preventing the wheels from locking during a hard braking situation. This enables the driver to maintain STEERING control and also shortens the distance to a complete stop. In cars without ABS, the brake master cylinder actually applied steady pressure to each wheel when the brakes were applied causing the wheels to look under heavy breaking. This led to wheel lockup, loss of steering control & lengthy stopping distances. ABS systems use a hydraulic pump that applies modulating pump pressure during heavy braking. The ABS system in effect pumps the brakes for the driver a consistent frequency. This pulsing is considerably faster than even the most experienced driver could accomplish if he were to attempt to this manually!

Converting from R12 to R134 freon seems to be a subject that is in debate throughout the country as to what is necessary to complete the conversion. Many mechanics say that in-order to convert from R12 to R134 you should replace the compressor, receiver drier and expansion valve. It has been our experience with car AC systems that the original systems will convert to R134 without major complications. We do however suggest replacing the receiver dryer, all O-rings (included in the conversion kit along with the 134 fittings), the refrigerant oil and rid the system of all of the old oil by flushing the system.

The A/C system has lubricating oil that will leak out when the freon escapes. A very likely place to look is the ends of the A/C hoses where the hose is joined to the fitting.

There are Vacuum hoses that operate the ductwork in the dash. Check each outlet vent in every setting, including the windshield defroster. If one or more has no air flow, the vacuum hoses (or 1 hose) could be disconnected. Find it and re-attach it to the vent.

A roaring noise that can be heard when turning on the A/C can generally be attributed to a faulty A/C Compressor but can also come from a faulty A/C idler pulley. Compressor failure should be done by a professional technician only. Idler pulley failures can be identified by removing the belt and turning the idler by hand to see if the bearing has failed!

In most cases the problem is with the alternator. Check the fluid level in the battery. If it is low, add water to bring it to the correct level. Even if it is a no-maintenance battery you can usually get the vent covers off without damaging them. Charge up the battery to a full charge. With the car running, use a voltmeter to check the voltage across the positive and negative post of the battery. It should read approximately 14 volts. If not, the alternator may be faulty. Turn off the ignition and check the voltage of the battery. It should be above 12 volts. Do not start the car for several hours, preferably overnight, then check the voltage again. If it is within 1/2 volt of the first reading, the battery is probably good.

NOTE: One of the most common issues with batteries not being charged correctly is faulty voltage regulators. The brushes typically wear out which causes the alternator not to charge correctly.

The first check the brake fluid. Fill it to the proper level. If the fluid level is ok, there could be a problem with a sticking relay contact and in hot weather, the higher humidity would make it worse. This may also be caused by a problem in the ABS Control Unit or one of the ABS sensors.

Brake Booster failures can often be identified by noting that you hear a slight air leakage inside the vehicle that seems to change tone when applying the brakes. When the brake booster fails you will often note that the brake pedal feels hard and more force is required to stop the vehicle. Brake booster replacement is the repair!

This can be caused because the pads are a little loose on their mounts, which itself is not a problem. When you change direction, they will slide up or down until they hit the caliper bracket. They could be dragging slightly on the rotors if there is any buildup on the caliper pistons or if the sliders are a little rusty.

Brake Master cylinder failures can often be diagnosed by noting that when sitting at a stop sign or stop light with your foot on the brake pedal that the pedal seems to continue to move toward the floor very slowly. The rubber seals in the master cylinder tend to wear around the edges causing leaking past the seals. Replacement of the master cylinder is the repair!

In Most cases the problem with a hard brake pedal can be attributed to a faulty ABS pump. When the pump fails the pedal gets hard because the pump is no longer assisting during the braking process. In most cases the pumps are included in the ABS hydraulic units and cannot be replaced seperately. The cost of the New complete units range between 1500.00 to 2500.00 and used units are usually around 475.00 depending on the model.

Most car manufacturers do not recommend turning rotors because the majority of time after turning them they are below the recommended minimum thickness. The recommended minimum thickness is generally stamped on the rotors. This is not to say that car rotors cannot be turned. They can be turned one time but will generally warp within one year because of how thin they become.

In most cases when you apply the brakes and the car pulls to the left or right this can be attributed to a sticking brake caliper. Depending on the application some calipers can be rebuild and in other cases the faulty caliper must be replaced. If your vehicle is equipped with ABS brakes you may find that the valve body in the ABS system is malfunctioning.

NOTE: ABS brakes are under VERY HIGH PRESSURE and should only be worked on by those who are properly trained!

A Low pitched clicking noise that comes from the cabin fan is an indication of bearing failure within the cabin fan. Cabin fan access will depend on the car. In most cases the cabin fan is located under the hood and under the false bulkhead. In some cases the cabin fan is located inside the car under the dash.

Cleaning a car: a piece of cake, right? Certainly, if you follow the procedures suggested by the Car Care Council. While it's the most basic procedure in car care, it does deserve some thought. The first step in cleaning the car is to wash it. Give it a good rinsing from top to bottom, including the wheels and inside the fenders. Always clean the tires and wheels before washing the body, and don't use the same mitt for both. This way you'll avoid contaminating the vehicle's paint with debris from the wheels and tires.

Use a good tire cleaner with a stiff brush, to improve your tires' appearance even if you don't have white sidewalls or white letter tires.

Next, clean the wheels with a wheel cleaner that removes the brake dust, which often blackens the front wheels. Application of these cleaners vary, so be sure and follow the directions on the container.

Now it's time to wash the body. Use a product sold specifically for automobiles. (Household cleaners can strip the wax from the paint and damage the finish.) Starting at the top, wash one section at a time, thoroughly rinsing away the soap. Work your way down toward the front, sides, and rear of the vehicle. Clean the rear last since it usually has the largest accumulation of dirt and grime, which can contaminate the wash mitt. Wash the inside door jams about once a month.

To rinse, remove the spray nozzle from the hose. Starting at the top, let the water cascade down the surfaces of the vehicle.

Then, to avoid water spotting, dry with a chamois or other product made for this purpose.

Now is an excellent time for waxing, which not only protects the finish but also makes subsequent washing easier. Before proceeding, look for foreign particles on the paint. Use a car cleaner, available at auto supply stores, to remove contaminants imbedded in the paint.

Once the surface is clean, apply the wax, following the manufacturer's instructions for application of the product. Often they recommend not using the product in direct sunlight

Proper Washing And Waxing Add Value To Vehicle
More than one-third of car owners use damaging non-automotive products when washing their cars-products that could contain harmful detergents, abrasives and additives. And almost half of motorists don't ever wax their vehicles.

"Waxing at least twice a year is recommended for maximum protection, yet surveys show that 48 percent of motorists don't wax their vehicles at all," said Jeffrey Webb, director of retail marketing at Turtle Wax, Inc. "That's leaving money on the table at trade-in time, as a clean, well-maintained car can be worth up to 50 percent more than one in 'fair' condition, according to the Kelley Blue Book."

Motorists should avoid dish detergent, which contains harsh chemicals that, intended to cut through grease, will strip away the wax finish on your car. Some are hard to rinse off and leave streaks. For best results, a formulated automotive wash is recommended, one that gently lifts the dirt and grime while protecting the finish.

STEP 1 Don't wash cars in direct sunlight. Do wash cars in shade or in cooler temperatures in the early morning or late afternoon.
STEP 2 Don't use dish detergent. Do use a formulated car wash
STEP 3 Do fill your bucket with warm water
STEP 4 Do use a soft terrycloth towel or washing mitt.
STEP 5 Do spray the car often with water.

STEP 6 Don't scrub the car all at once. Do complete one section at a time, rinsing repeatedly to prevent the soap from drying on the paint.

STEP 7 Do use soft terrycloth towels or scratch-free fabric to dry the vehicle.
STEP 8 Don't neglect waxing the vehicle. Do prep the car for waxing using cleaner/polish to remove contaminants

The problem usually comes from a tire being out of balance that can be balanced out by anyone who has a tire balancing machine wether it be the "bubble" type or a computer ran type of balancer. A loose, worn or out of adjustment wheel bearing, worn tie rod ends, worn out ball joints, excessive wheel run out, bump on the tire, or a bent axelshaft can also cause these vibrations.

Your Volvo may have a faulty Converter if your car will not accelerate or has severely diminished power. If your Volvo has these symptons, the converter may be clogged. If it is clogged, it may glow red while the engine is running. That happens when excessive unburnt fuel causes the converter to overheat as it burns off the unburnt fuel. One quick test can help to confirm if the converteris clogged. Disconnect the exhaust from the header to keep the exhaust gases from flowing through the converter. If the converter is the problem your Volvo's power will be restored to normal with the header pipe disconnected.
The only legal repair is to replace the converter with a new unit.

The easiest way to bleed the clutch is to use a pressure bleeder. You apply pressure to the reservoir with air and loosen the nut on the line to the master first. Second, make sure solid fluid with no air comes out of the line. Third, tighten the line back down at the master cylinder while the fluid is still coming out. Follow the same procedure to bleed the slave as well.

Clutch Failures can often be identified by noting that the engine will rev up but the vehicle will not be propelled forward. This is an indication of a worn out clutch disc or pressure plate. It is our recommendation that the pressure plate, Disc, throw-out bearing, Slave & pilot bearing all be replaced when servicing the clutch. These components go through considerable movements daily and all will deteriorate and be in need of replacement at some point as well.
Other possible things could be having oil on the disc from a leaking engine seal

These problems usually expose themselves when the pressure plate or flywheel is warped, the clutch disc is worn or coated with oil from a leaking seal, worn out splines on the clutch disc, or loose or worn engine/transmission mounts.

Make sure the hydraulic fluid is at the normal level. If you have had to add fluid recently, you most likely have a leak. Check that the bleeding screw is not loose. If there are no leaks the problem is either the clutch slave cylinder or the master cylinder is failing. In most cases fluid loss can be found where metal hoses change into rubber hoses. The leaks can often be found at the junctions where the two meet.

If you have fluid on the master cylinder dust cover and pedal the rear seal in the master cylinder has probably failed. If you have it on the slave cylinder then in most cases the plunger seal has went bad or you have loose lines or a bleeder valve.

When working on hydraulic clutch systems its nothing for the pedal to be going to the floor, usually just a few minutes of bleeding and good to go, but when the pedal just goes out...well thats a different story. Lots of times when the pedal just goes to the floor without much warning its going to be the master cylinder or lines have burst. Just check around the master cylinder for any leaks, sometimes all it needs is a few seals and its good to go. Have someone to press the clutch while looking for a leak in the lines, when they apply pressure (thats if there is any fluid left in the system) then fluid should come out where the line is damaged.
It could be that the reservoir may be out of fluid (which you have a leak that needs fixing). Some more uncommon things that can happen is having a broken clutch release bearing or fork or release cable.

The reason for the spongy or soft pedal is because of air in the system. You need to bleed it out. But first you need to find the source of the leak. Check the engine compartment and the area where the car is parked for signs of leakage. If there is a lot of leakage, you probably have a bad clutch slave cylinder (providing that you have a hydraulic clutch)

If your clutch pedal does not come up off the floor then most likely the fork is in a bind in the bellhousing or you have a broken release bearing or fork. On other things that can happen is there may be a bind in the master cylinder or bad master cylinder.

Internal coolant leaks can be very harmful to your engine and can lead to engine failure if not taken care of very quickly.
The way to tell if you have an Internal coolant leak would be by your coolant level dropping with no signs of it on the ground. You can check by inspecting your oil. If the oil has a milky white tent to it then you have an internal coolant leak.
Coolant leaks as such would be a cylinder head gasket being blown, you can usally tell this by the oil having the milky white tent to it and also having an excess amount of condinsation in the tail pipe(s). This can cause lots of problems if not taken care of as soon as possible, it could possibly bust pistons, bend rods, and cause the cylinder sleeve to bust due to the little bit of capacity that the cylinder already has and having the extra coolant filling that up and when the piston come up it has no where to go on an intake stroke and thats when stuff bends and bust.
Another place it could be leaking from is the cylinder head if it is cracked. Most of the time a cracked cylinder head comes from overheating an engine.

Coolant leaks are a common problem with just about any vehicle old and new. The older cars most of the time will have deteriorated or damaged hoses going to the radiator or for the heating system. Some problems come from just clamps being loose and others can come from water pump seals and gaskets, from the radiator core or heat core, engine drain, or water jacket freeze plugs leaking or have come completely out.

Other things that can cause problems is if something has punctured the radiator by it coming up off the road or if a coolant hose comes in contact with a moving part.

Coolant loss can be potentionally a dangerous thing if it goes unattended. It can come from over filling the coolant system, overheating the engine and pressure forces it out, internal and external leaking (check our other FAQ on these two subjects), or a faulty radiator cap. If you have problems with the radiator cap most of the times is because the recommended pressure for your car is less then what the cap is made for. Always be sure to get the right cap for your car to avoid any problems with coolant loss or overheating. Always fill your coolant resivoir to what your car is made to handle, don't over fill this thinking it will last longer because you will get the exact opposite reaction.

The most common cruise control failures on the car are: pedals switch failures, The cruise control vacuum line cracks (on cars with vacuum controlled systems) & Control module failures. The pedal switch failures are the most common problem with the cruise control not functioning correctly. What generally occurs is that the pedal switches become mis-adjusted or the ears get broken off the switches causing a no contact condition.

Worn or cupped tires usually comes from a poor alignment most of the time, but worn shock asorbers, bad wheel bearings, excessive wheel or tire runout, or worn out ball joints can all be the likely cause for tire wear.

Reason for the light not to come on as the key is turned on can be:

Warning light bulb is defective or blown
Fault in the printed circuit, dash wiring or bulb holder

The light will stay on if any of the following are happening:

Faulty aternator or charging circuit
Alternator drivebelt defective or out of adjustment
Alternator voltage regulator inoperative

Don't rely on a jack when working under the vehical. Always use a jack stand and place it under the recommend jacking points specified by the car manufacture.

Don't attempt to loosen tight fasteners such as wheel lugs while the vehical is on the jack because it could fall.

Don't start the engine without making sure that the vehical is either in neutral for manual transmission or park for automatics and that the parking brake is set.

Don't remove the radiator cap from a hot cooling system, let it cool or if necessary using a cloth to cover up the cap and release pressure gradually.

Don't attempt to drain the engine oil until you are sure it has cooled to the point that it will not burn you.

Don't touch any part of the engine or exhaust system until it has cooled enough to avoid burns.

Don't siphon toxic liquids such as gas, antifreeze, and brake fluid by mouth or allow them to remain on your skin.

Don't inhale brake dust due to it possibly containing asbestos for older models

Don't allow spilled oil or grease to remain on the floor due to being able to slip on it and getting injured.

Don't use loose fitting wrenches or other tools which may slip and cause injury. Also you'll be looking for a new bolt or nut after that ones rounded off.

Don't push on wrenches when loosening or tightening nuts or bolts, its better to pull but if you have to push use an open hand to avoid scraping your knuckels if the wrench slips.

Don't attempt to lift heavy parts alone, always have someone to help you.

Don't rush or take unsafe short cuts to finish a job.

Don't allow children or animals in or around the vehical while you are working on it.

Do wear eye protection when using power tools and when working under the vehical.

Do keep loose clothing and long hair well out of the way or moving parts

Do make sure that any hoist used has a safe working load for the task at hand.

Do get someone to check on you periodically when working alone on a vehical.

Do carry out work in a logical sequence and make sure that everything is assembled correctly.

Do keep chemicals and fluids tightly capped and out of reach of children and pets.

Do remember that your vehicle's safty affects that of yourself and others.

If your brakes are dragging the most common cause is the caliper(s) are sticking and usually just need replacing. Other things that could cause this problem is having restricted brake lines and its not allowing the pressure to come off the caliper. It also could be an incorrectly adjusted parking brake cable.

The following may be the cause of your engine backfire:

Emission system not functioning properly
Ignition timing Incorrect
Faulty Secondary ignition system (cracked spark plug insulator, Faulty plug wires, distributor cap and/or rotor)
Carburetor or fuel injection system malfunctioning
Vacuum leak at fuel injectors, intake manifold or vacuum hoses

Generally, A roaring noise during belt rotation (without the A/C on) will often be caused by an idler pulley bearing going bad. There are occasions where Alternator bearings will also create a roaring noise during belt rotation. In-order to diagnose either of these problems simply remove the belt and rotate each of the pulleys by hand. The pulley failure will make itself apparent by noting that the noise begins when rotating the faulty pulley.

BELT REMOVAL SHOULD BE BY AUTHORIZED PERSONNEL ONLY! PHYSICAL INJURY CAN OCCUR!

Some reasons are:

Idle speed too high
Excessive engine operating temperature
Incorrect fuel octane grade

A loud clanking noise from the engine belt area can often be attributed to a faulty crank shaft pulley (harmonic Balancer). Crankshaft pulley's are often designed in 2 parts & pressed together from the factory. The problems often come from the material that holds the two part pulley together. These materials will deteriorate over time causing the pulley to separate leading to one of the following symptoms: A loud clanking noise as previously described or a loud squeak upon initial start up. You can also identify this problem when you find that your engine continually throws the drive belts off the pulley for no apparent reason.

Reasons for the engine not to start while the starter is being opertated would be:

Dead battery (recharge, use jumper cables, or push start)
Battery terminals are loose or corroded
Battery ground strap loose or broken
Engine ground strap loose or broked
Starter motor (or solenoid) wiring loose or broken
Automatic transmission selector in wrong position, or inhibitor switch defective
Ignition/Starter switch defective
Major mechanical failure (seizure)
Starter or solenoid internal fault

Fluid leakage is quite common on almost any vehicle type. The first thing one must do is to attempt to identify the fluid color in-order to locate the leakage area. Red fluid is transmission fluid, Dark Brown or Black fluid is most likely engine oil, Light brown fluid is Power steering and pink or green fluid is antifreeze. Listed below is the most common issues with fluid leakage:
Dark Brown or Black fluid: Most likely engine drain plug leak or engine oil pan gasket.
Red Fluid: Transmission Pan Gasket failure
Pink or Green Fluid: Water Pump failure or radiator hose failure
Light brown Fluid: Power steering hose failure

This can be caused by vapor lock or a sticking fuel pump check valve. A warmed up motor produces a great deal of external heat. Fuel must be kept cool to flow properly through the fuel lines and into the combustion chamber. If the fuel system is exposed to the engine heat, the fuel will vaporize and interrupt the flow of fuel to the motor. This is called vapor lock. A fuel pump has a check valve to prevent the fuel from flowing back toward the fuel tank and to keep some pressure to allow the motor to start easily without having to re-pump the fuel all the way to the motor. This check valve can wear over time causing it to stick and making it harder to start the vehicle.

Battery discharged or low
Fuel system malfunctioning
Injectors leaking
Distributor rotor carbon tracked

Some reasons that would cause an engine not to start when hot could be:

Air Filter Clogged
Fuel not reaching the carburetor or fuel injection system
Injectors leaking
Corroded battery connections, especially ground

Things that can cause the enigne to idle ruff or miss at idle would be:

Vacuum leakage
Air filter clogged
Fuel pump not delivering sufficient fuel to the fuel injection system
Leaking head gasket
Timing belt or sprockets worn
Camshaft lobes worn
Spark plugs worn or not gapped properly
Faulty spark plug wires
Incorrect ignition timing
Uneven or low compression

Reasons for this could be:

Incorrect ignition timing
Excessive play in distributor shaft
Worn rotor, distributor cap or wires
Faulty or incorrectly gapped spark plugs
Carburetor or fuel injection system malfunctioning
Faulty coil
Brakes binding
Automatic transmission fluid level incorrect
Clutch Slipping
Fuel filter clogged and/or impurities in the fuel system
Emission control system not functioning properly
Low or uneven cylinder compression pressures

Things that can cause a vehical to miss while driving:

Fuel filter clogged and/or impurities in the fuel system
Low fuel output at the fuel injection system
Faulty or incorrectly gapped spark plugs
Incorrect ignition timing
Cracked distributor cap, disconnected distributor wires or damaged distributor components
Leaking spark plug wires
Faulty emission system components
Low or uneven cylinder compression pressures
Weak or faulty ignition system
Vacuum leak in carburetor or fuel injection system, intake manifold or vacuum hoses

Tools needed to change the oil in your Volvo:

Jack and pair of jack stands (unless you have availabilty to a lift)
Oil Drain Pan
17MM Wrench (size varies depending on model but most 1990-98 are 17MM)
Oil Filter Wrench
Protective gloves should be worn

PROLONGED CONTACT WITH ENGINE OIL CAN CAUSE A NUMBER OF SKIN DISORDERS, INCLUDING CANCER! MAKE SURE TO ALWAYS WEAR PROTECTIVE HANDS AND EYE WEAR TO AVOID CONTACT WITH SKIN OR EYES. WASH YOUR HANDS OR ANY OTHER PARTS THAT WERE EXPOSED TO THE OIL AS SOON AS POSSIBLE!

Engine oil leaks can be very hard to pin point if it has been leaking for a while, everything is covered in oil but here are a few locations where it could be coming from:

Oil pan gasket and/or oil pan drain bolt seal leaking
Oil pressure sending unit leaking
Valve cover gasket leaking
Engine oil seals leaking

Easiest way to find the leak is to clean the engine bay of as much residue as possible. Once its clean you can then start looking for the cause of the leak.

Oil leakage from the Valve Cover Gasket is a common problem on most cars. What generally happens is that the gasket heats up and cools down so many times that the gasket eventually hardens up to a point where it no longer seals. Replacement of the valve cover gasket(s) is the only repair

Most times overcooling is not the issue on most cars, but it does happen.
The most common problem is a bad thermostat. What happens to it is that the thermostat is not opening properly and letting the coolant stay in the radiator too long and its cooling it too much. Most cars do not operate properly untile they get up to a certain temperature and lack on power.
Another problem could lie within the cooling fans. Most fans work off what a contact switch tells them, so if your car is equipped with a contact switch and have electric fan(s) then the switch can be telling the fans that the coolant is hot causing the fans to stay on cosistantly causing the coolant to be more cool then what it needs to be.

The most likely cause is using gas with a lower octane rating than recommended. Check your owner's manual. Another cause is carbon buildup around the valves keeping them from closing properly. A couple of treatments with a fuel additive may help. Follow the directions and make sure you use it on a full tank of gas. This is especially important on Turbo vehicles because of the added air that is forced into the combustion chamber. High test fuel = better performance.

Other reasons for this would be ignition timing incorrect, carburetor in need of adjustment or fuel injection system malfunctioning, improper or damaged spark plugs or wires, worn or damaged distributor components, faulty emission system, or a vacuum leak.

Some reasons why the engine rotates but will not start can be:

Fuel tank is empty
Battery is discharged (if engine is turning slowly)
Battery terminal connections loose or corroded
Leaking fuel injectors, faulty fuel pump, pressure regulator
Fuel not reaching carburetor or injectors
Ignition components damp or damaged
Worn, Faulty or incorrectly gapped spark plugs
Broken, loose or disconnected wiring in the starting circuit
Loose distributor is changing ignition timing
Broken, loose or disconnected wires at the ignition coil or faulty coil

This can be caused by:

Low oil level
Idle rpm too low
Short in wiring circuit
Faulty oil pressure sending unit
Worn engine bearings and/or oil pump

If you crank your car up after it has been sitting for several days or overnight and it smokes out the tailpipe for a few seconds upon initial start up then the problem is more than likely leaking valve guides/Valve stem seals. What generally occurs is that the guides/seals wear due to the stress of the valve moving up and down so many times. This basically causes excess clearance between the valve guide and the valve stem. Minor seepage occurs during the rest period allowing oil to leak into the combustion chambers leading to morning smoke!

One common cause of stalling is vacuum leaks. Check all vacuum hoses and fittings. If your vehicle is equipped with emissions equipment such as a charcoal canister, the end of the hoses sometimes crack over time creating a vacuum leak. Stalling can also be caused by an erratic Fuel Control Module. Note: On some vehicles this is called the Electronic Control Module. This might be indicated if the motor almost stalls when you accelerate from a stop and then suddenly slams up to full power.

Other reasons could be that the idle speed is incorrect, fuel filter clogged and/or water and impurities in the fuel system, distributor components damp or damaged, faulty emissions system components, faulty or incorrectly gapped spark plugs, or faulty spark plug wires.

Reasons for an engine to start then stop immediately could be:

Loose or faulty electrical connections at distrbutor, coil or altenator
Insufficient fuel reaching the carburetor or fuel injection system
Faulty ballast resistor
Malfunctioning fuel injection system
Faulty fuel injection relays

Here are some reasons it may be stumbling:

Spark plugs fouled (dirty with gas or oil)
Carburetor or fuel injection system malfunctioning
Fuel filter clogged
Incorrect Ignition Timing
Intake Manifold air leak

The most common cause for engine surging is a vacuum leak in one of the hoses, other causes could be intake air leak, fuel pump or carburetor faulty, loose fuel injector harness connections, defective ECU. Check all vacuum hoses including the ones for the emission components.

Engine vibration in drive or reverse may be due to faullty or failing engine mounts. What usually happens is the rubber portion of the engine mounts deteriorate causing the vibration from engine rotation to travel through the vehicle. You may be able to find the faulty mount by shifting from drive to reverse with the engine running and your foot on the brake. This cause the engine mounts to come under load and you may see the telltale signs of movement that indicate a bad mount.

Some reasons for the mileage being lost can come from dirty or clogged air filter(s) which is the most common problem. Other more technical reasons would be due to timing not being correctly set, there is a problem with the emission system, the carburetor or fuel injection system is not working properly, or low tire pressure or wrong sized tire.

Another issue that can cause less mileage on a tank of gas would be wheel alignment. If the tires are toed in or out (toe is referring to if the front of the tires are turned in at each other or pointing out away from each other). Toe can be effected by pot holes and hitting curbs. You can tell if the tires are toed in by the car wanting to dart back and forth on the road and toed out by it having slower acceleration and works the engine harder to do the same amount of speed as before.
Most times it can be adjusted back to specs but on rare occasion such as a wreck or hitting a curb hard you may have to replace parts such as tie rods, tie rod ends, and any steering part that was damaged.