Engine Problems and Diagnostics


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Problems and Diagnostics

Engine Problems & Diagnostics

Diagnostic Fault Codes
Wandering Idle
Throttle Pots & Idle switches
Cold Start Problems
Idle Adjustment
Crankcase Breather Hose
"Limp-Home" Mode
Exhaust Smoke
Erratic Coolant Temp Gauge
Radiator Fan
Alternator Rebuild
Oxygen Sensor
Starter Replacement


Fault Codes
The first thing to do if you're having engine problems is to dump the fault codes from the computer. It's easy to do with a homemade VAG 1115 LED test light. For more info, go to the Diagnostic Fault Codes page.

Wandering Idle
Symptoms: Idle will constantly bounce between 1600 and 400 RPM, sometimes stalling. This is a very common problem with these cars.

Idle Stabilizer Valve (ISV)

These typically cause problems because of either a bad electrical connection or some kind of internal gunky build-up.
  1. Locate the ISV. On the early 1990 cars, it's on the side of the airbox. It has two rubber vacuum hoses attached and one electrical connector. I have colored the diagram of the passenger side of the engine compartment to the right (ain't I sweet?). In the picture, the ISV is colored yellow, the rubber hoses are blue and the electrical connection is red. The later '90 and all '91 cars have a slightly different arrangement with the ISV mounted on the boot between the throttle body and the airbox.
  2. A quick test to see of the ISV is causing the problem is to unplug the electrical connector while the problem is occurring. If the idle is now steady (higher or lower speed is normal), then the ISV is the likely culprit.
  3. Check the contacts on the electrical connector. If they are wet, corroded, or loose, clean them up with contact cleaner (available at car stereo shops or Radio Shack) and reconnect securely. If you're lucky, this will cure the problem.
  4. If problem persists, remove ISV by loosening the two hose clamps and detaching the hoses, then unscrew the ISV from the plastic airbox lid.
  5. THOROUGHLY spray carb cleaner into the two hose connections on the ISV and allow to soak inside the unit, then pour it out. Repeat this process several more times until the stuff you pour out looks clean. Better yet, fill a small container with carb cleaner and let the ISV soak in it for an hour or two, then spray it a few more times. When you think you've cleaned it enough, clean it some more!
  6. If you have an early '90 car with the Hitachi ISV (pictured at left below), then you can try a more thorough cleaning procedure:
    1. To disassemble, remove (cut off and discard) the crimped-on sheet metal anti-tamper end cover.
    2. Before unscrewing the brass end plug, mark its position relative to the housing so it can be replaced to the same position, thus maintaining the same spring tension on the valves.
    3. Disassemble the unit completely and clean all parts with solvent.
    4. Lube the two shuttle bearings with a drop of silicone oil on each.
    5. Reassemble the unit making sure the brass end plug is returned to it's original position.
    6. Reinstall valve on air box and insure no air leaks at the hose connections.
    7. Adjust idle speed as described below.
  7. Reattach the ISV and hopefully the problem is gone, because a new ISV costs about $240!
If cleaning the ISV and it's electrical connections don't fix your idle problems, look for vacuum leaks (like the idle screw hole, there is an O ring that might have decomposed) and check out your idle switch before you replace the ISV. A new ISV is expensive. If you are faced with replacing your ISV, please note that there are two varieties:

Cold Start/Idle Problems
(Ray Calvo)
Symptoms: After a cold start, the car stalls or threatens to. Also, somewhat erratic idle.
Check the rubber connector from the filter housing/air sensor to the throttle manifold for cracks that would allow unmeasured air to enter. (approx. $50) Also check for any other sources of vacuum leaks.

Idle Adjustment
Unofficial Idle Adjustment (may only work on pre-March '90 cars)
(Bernie Benz) This procedure sets the idle speed to minimize the effects of sudden engine braking when the throttle is slammed shut.

  1. With engine at operating temperature, disconnect the ISV electrical connector.
  2. Adjust the idle speed with the throttle valve air bypass adjustment screw to 700 to 750 RPM. (This is just below the speed at which the ECU computer will control idle.) This sets a minimum idle speed well above zero.
  3. Reconnect the ISV elec. connector, and check the idle speed. The computer should control idle at 800 to 900 RPM. If it's faster, the ISV is not accurately following computer control, the sign of a sticky valve.
Official Idle Adjustment
Warning: Bentley says the idle and CO content are interrelated, and must be adjusted together. The following procedures may be good for fine adjustment; however, they may cause more problems than they fix.

Pre March 1990 production date

  1. Clamp crankcase ventilation hose behind fuel pressure regulator, then remove oil dipstick and cover tube with a lint-free rag to prevent oil spray. (This step is probably more important for the CO adjustment rather than idle adjustment, so can probably be skipped.)
  2. Under driver's side glove/map compartment (near steering column) you should find two diagnostic connectors, with two jacks on each one. Bridge/short the lower jack between each connector (A1 and B1).
  3. Start car, turn off all accessories, make sure car is warm but radiator fan is not running, check idle speed. Should be 800 RPM +/ 50
  4. Adjust idle speed to spec at adjusting screw on throttle body (accel.cable is sticking out of it, adj. screw is pointing approx.to the air cleaner).
Post March 1990 production date
  1. Remove rear electrical connector from carbon canister shutoff valve.
  2. With engine idling and all accessories off, install spare fuse in the top of the fuel pump relay.
  3. Adjust idle speed to spec at adjusting screw on throttle body (accel.cable is sticking out of it, adj. screw is pointing approx.to the air cleaner).
Crankcase Breather Hose
(Wylie Bean)
These cars (the early ones, at least) came with a rubber crankcase breather hose. The hose comes out of the lower engine block on the driver's side, then runs toward the firewall a few inches, then turns to the passenger side to enter a metal tube. In time, the hose becomes very soft and mushy, and may collapse or rupture. The replacement is a metal tube with two short lengths of rubber hose clamped to each end. The P/N for the metal tube is 054 103 227 A, but make sure you ask for the kit that has all the necessary pieces with it. Replacement is straightforward, but frustrating because of limited access and the rigidity of the metal replacement compared to the original rubber hose.
  1. For easier access to the lower clamp, unbolt the metal support wire that holds the two hydraulic hoses that run from the pump toward the firewall.
  2. Look just below/behind distributor, to the back of the engine (a light helps here), and you'll see where a long metal breather tube (runs transverse across engine bay and behind engine) connects to the rubber breather hose. To disconnect the two, first loosen the mounting bolt that holds the metal tube behind the engine block. This will allow you to maneuver that tube (helpful during reassembly, too). A medium sized screwdriver will get in there, from the top. If you still have difficulty getting to the clamp, consider removing the battery for easier access
  3. The bottom of the hose mounts on the driver's side of the block, just below the bracket that holds the plugs for the 02 sensor. Loosen the hose clamp here.
  4. Take note of the placement of the hose--you'll need to re-route the new one in the same place.
  5. The hose will probably be in such horrible shape that when you pull it off the lower mount it will break apart, causing a bunch of debris to fall into the crankcase. To avoid this, be very careful as you remove it.
  6. The new hose is metal, with two rubber ends. The hoses are tight enough so that they fit over the metal piece without the need for clamps, and Audi doesn't provide any for this purpose. Use the old clamps just to tighten the hose ends down over the mounts, not onto the metal replacement. For extra security you can buy a medium and a small sized hose clamp to clamp the rubber ends onto the metal center section.
  7. Replacement is opposite of removal. Getting everything lined up can be difficult, so be patient. You can connect either end first, depending on which way you find to be easier.
"Limp-Home" Mode
The first thing to do is check the fault codes. They should point you in the right direction. If they don't, or you need a quick fix on the road, there are some things to check. A fairly common cause of this is actually just a poor connection on the fuel pressure regulator, or the ground wire that comes out of that connector and attaches (usually) to the air accumulator on the intake manifold. The 20V is completely timed by the computer, so distributor placement doesn't have to be too exact. There is a big black connector that is a few inches in front of the distributor cap towards the front of the engine, this is the multipoint connector that has the O2 sensor, distributor controls, engine management etc on it. If that is half off or corroded/wet the car will act funny. Try taking that connector apart, cleaning it, and putting it back together. You might want to put in some anti-corrosion spray in there too.

Smoking Exhaust
(Phil Payne) Here are some general rules of thumb that apply to any car:
Symptoms Possible Cause Fix
White smoke, acrid smell Vaporised coolant Probably from head gasket leak (usually fails at back of an I5 engine) -- replace head gasket (duh!)
Blue smoke on shifting, burnt oil smell Valve stem oil seals Find the right mechanic, and it's a surprisingly cheap job
Blue smoke on overrun w/o shifting, vaporised oil smell More serious wear, perhaps rings Very rare in Audis, common in Beemers
Grey or black smoke, just plain stinks Seriously rich mixture Adjust mixture (duh!)

Erratic Coolant Temperature Gauge
(Pablo Carselle)
Symptoms: Coolant temperature gauge is erratic or non-functional; possibly no heat from the climate control.
There are two possibilites. The first thing to check is the thermostat, because it's easier and cheaper. If that's not the problem, you may need to replace the multi-function temperature switch per instructions below.

  1. Follow upper radiator hose to housing attached to block. On lower part of this housing there is a multi-function sensor with a rubber boot with 4 or 5 wires sticking out , and a bronze or copper colored hexagonal shape. It is threaded to housing (normal thread direction).
  2. With engine cold or lukewarm: Run a small screwdriver carefully around the perimeter of the boot, between the boot and the sender body. The intent is to separate any stickiness. Push down the boot from the upper edge and work it down. (Do not pull down the wires).
  3. There is a plug in connector inside with 3 or 4 contacts. The boot going down will disconnect the plug.
  4. Prepare the replacement part next to you so that you don't have to walk away. Place absorbent towel or rag underneath the sender.
  5. Using an open end wrench loosen sender and finish extraction by hand but maintaining slight upward pressure to avoid excessive coolant loss.
  6. When sender completely loose, pull it out and plug opening with left hand index finger. Make sure you have the flat washer on the sensor you pulled. If it lodged into the hole, pry it out. The new one will leak with two on there.
  7. With right hand (quickly without panic) insert new sender and initiate thread. Do not over-tighten with wrench (brass threads).
  8. Reconnect plug redress boot and clean the coolant from the block etc. If you broke the tabs, don't worry, just about every replaced sensor has broken tabs.
  9. Warm up engine check for leaks and enjoy your new temp reading and ECU performance.
Radiator Fan
(Paul Royal)
If you need to replace the fan, consider getting a 6-blade VW part (P/N 811 119 113 B) for about $60. It's a bit noisier than the stock unit, but pulls more air. It does not come with new screws and bolts, so don't lose the originals. When removing the old fan, note that on the lower driver's side of the fan at the very bottom of the fan shroud there is some black wiring that feeds down to the sensor at the bottom right of the radiator. Do your best not to pinch this wire and make sure on install of the new fan that you have not pulled it up so that there is not enough slack to reconnect it to the sensor. You will probably need to cut a tie wrap that is holding that wire in place in order to get at the bottom nut. Consider the following two wiring updates while you're in there.
Wiring Updates
(Peter Schulz)
The wiring harness to the fans on these cars appears to be a bit short. Mechanical and thermal stresses may cause a failure at the wiring connectors. To fix or prevent the problem, splice in some extra wire to provide strain relief. At the same time, you should add in a fuse to the ground wire as detailed below.
(Scott Justusson)
In later cars, Audi put an 80 amp fusable link into the ground wiring for the radiator fan. This setup is used to avoid sending 80+ amps of 12V to the fan ground if the fan bearings freeze up . This could possibly cause an electrical fire, or at a minimum it will fry your electrical harness.

Here are the Audi P/Ns for retrofitting a ground fuse to the fan wiring:

Or, you can install a generic fuse (60 amp should be fine, and it's easier to find than 80 amp). If you ever blow the fuse, you need to get a new fan motor ASAP.

(Bob D'Amato)
Symptoms: Engine makes a strange rattle noise when you blip the throttle, heard most predominantly when the rpms fall from +- 2000 rpm back down to idle.
Distributor #: 034 905 205J from 90-91 Coupe Quattro and 90-91 90 Quattro 20V.
  1. Mark the general direction the rotor is pointing. Timing is done by the computer, so you don't have to be too exact.
  2. Remove distributor. You first need to remove the hold-down bracket. You can't really see it, but it's there. The bracket is secured by a 13mm bolt, which may be covered by an anti-tamper cover. You will need a box wrench, preferably a ratchet box wrench, because there is very little room to swing the wrench.
  3. Hold shaft by cam gear and move parallel to shaft (end play). This is your noise.
  4. Drive out Clevis pin holding on cam gear CAREFULLY. Cam gear is plastic and breaks easily. (Experience talking here...)
  5. Add high quality washer/spacers between the cam gear and the body of the distributor, until there is no more play. The spacers should be stainless steel with an inside diameter of 12.7mm (.5"). The thickness may vary by the amount of runout.
  6. Put cam gear back on, drive in Clevis pin.
  7. Put distributor back in car.
NOTES: Use very thin stainless washers to get the right spacing. You don't want it too tight, the distributor must turn freely.

New Gear Installation and Timing Considerations

(Bernie Benz)
I.  Preliminaries: Bentley ref pgs:  3B: 28.47, early 7A: 28.250
                  (The Audi distributor alignment tool is not necessary.)
1.  Rotate the engine to #1 TDC.  Rotor pointing to #1 plug terminal and0-degree flywheel mark at pointer edge. Get this right!
2.  With dist cap and Hall sensor connector removed from the dist and other junk out of the way as much as possible, note the angular position relativeto horizontal (3B), or vertical (7A) of the Hall sensor connector and the rotor (these will be opposite each other) for later return to original oriention purposes.
3.  Remove the anti-tamper cover, if any, on the dist hold down clamp nut, located on the stud in the head under the dist location hole, remove nut and clamp.  I removed the anti-tamper device by hacksawing a slot across the top of it and then unscrewing it with a flat blade screwdriver.  They usually aren't on very tight.
4.  Pull dist from its locating hole, noting that the rotor turns clockwise about 1/4 inch as the dist gear is withdrawn from its mating cam gear.  Upon reassembly it must go back in with this same final removal orientation to end up in the same original position.

II. Plastic Gear Removal:

1.  Don't try to drive out the retaining pin yet!  Remove the gear plastic from its steel hub by hacksawing 2 or more diagonal cuts thru the plastic to the hub, pry or chew off the pieces.
2.  Note that both ends of the groove pin are swaged larger than the pin center dia.  Saw off the smaller pin end and file it flush with the gear hub.
3.  Support the gear hub and shaft in a suitable V block or vice jaws and drive out the pin using a 5/32" dia pin punch or an old nail on the filed end.
4.  Remove the gear hub and retain the shim washer pack for reassembly.
5.  Remove the shaft, not loosing the fiber thrust washer at the top end, and clean the dist body and Hall rotor.  If there is any oil in the distributor, service the crankcase breather system.  (This distributor doesnot use a shaft seal component, just a reverse spiral groove on the shaft in the top portion of the upper bushing.)

III.  New Bronze Gear Installation:

1.  Install the original shim pack on the shaft, followed by the new gear. Align the smaller gear pin hole with the shaft pin hole.  (Do not use the larger pin hole, it is a vendor goof.)  Support the gear hub on the far side (not on the gear teeth) and tap the roll pin part way into the shaft.  Check the dist shaft axial clearance.  It must not be tight, but should have 2 to 5 thou end play, a "just perceptable free movement" feel without binding. If not so, remove the gear and adjust the shim pack, + or -, as required. An additional .003" thick shim is provided with the gear and roll pin to
faciliate this adjustment.  Add or subtract different thickness shims until the end play is the closest to the "feel" spec.  Unlikely, but if more or different thickness shims are necessary, they can be cut from paper and added into the center of the shim pack.
2.  Drive the roll pin thru the assembly and center it in the gear.  Recheck the axial clearance "feel".

IV.  Distributor Installation:  without VAG alignment tools.

1.  Remove the black plastic shield which may have been covering and seperating the Hall area of the dist. body from the rotor.  Not all 20V dist have this shield, which may be replaced just prior to reinstalling the dist cap.
2.  Notice that there is a line embossed on the top surface of the Hall rotor immediately under the hot end of the dist rotor, and a mating line notched into the top edge of the distributor housing opposite the Hall connector and just above the Hall sensor unit.  When finally in place and locked down these two lines must line up with each other with the engine at TDC.
3.  With the dist and rotor lined up about as it was as you removed it (you forgot where that was, didn't you?) insert the dist, moving the rotor as necessary to get the gears to mesh.  Once all the way seated in place, while applying a slight counterclockwise finger bias to the rotor (thus eliminating gear mesh clearance), adjust/rotate the dist body to exactly line up the two marks, and lock it down (lightly, you may have to move it again) with the clamp and nut.  If you are not happy with the resultant orientation angle of the Hall connector, pull the dist just enough to free
the gears and rotate the rotor one gear tooth in the same direction that you would like to move the Hall connector when the two marks are again lined up.
The distributor will work in any oriention, as long as the two lines are lined up at TDC, the actual oriention just facilitates the electrical connections to the Hall and dist cap.
4.  All 200 owners who elected to short cut and not remove the intake manifold per Bentley, must now curse loudly at the Audi gods for the inaccessability of the whole dist system and the dist cap hold down clamps in particular.  7A owners need not comply.
5.  "Reinstall all components in reverse order of removal".  More of Bentley's sage advice.  If the engine won't start, recheck your TDC position for accuracy and then the distributor marks alignment for its accuracy. They must be right on.  Do not attempt to move the dist with the engine running as you did with those old point type dist, as this dist does not determine engine ignition timing.  Thank you for flying united.

Alternator Rebuild
(Tom Jelecanin)
Bosch P/N 30091 is the brush/voltage regulator block, and should be available for ~$20. You can remove the brush/Vreg block w/o removing the alternator -- just disconnect the battery and remove the two slot head screws from the back of the alternator w/ a right angle blade screwdriver (wouldn't want to accidently short across the B+ termanal with the battery connected now, would we??). New spec. for the brushes is 10mm protruding from the plastic, (the longest edge) and min wear spec. is 5mm.

Drill out the top of the soldered connection of the old brushes because they are not only soldered, but crimped in place, then they shoot out of their sleeves. Make sure you don't lose the springs that are behind them with the small insulator sleeves on the brush wire. Then place the new brushes (along with the springs and teflon insulator) in the reg. assembly. Fold the wire over so that the brushes don't try to pop out while you're soldering, then try to crimp the hole the wire came through. Solder the wires, and trim the excess wire off (~ 1.5" - 2").

Oxygen Sensor
Oxygen sensors have a limited life-span. Audi recommends replacing them every 60K miles. If your gas mileage has plummeted or you've failed an emissions test, you might want to consider replacing your old O2 sensor. The official replacement has the correct wires to mate up with the harness, but it's also over $100. You can get a generic three-wire sensor for much less, and just splice the wires into the existing ones. One common version is Bosch P/N 13913 and costs about $40. It's for some model of Ford and can be found at most auto parts stores, just ask for it by the P/N. If you prefer to have the original connector, you can get either Bosch P/N 13957, or NGK P/N 25040 (double-check these numbers to be sure).

Here are some instructions for replacing the sensor: (rngmeister)

  1. Disconnect negative battery terminal.
  2. Disconnect the two plugs from their carriers on the driver's side of the block, just to the right side of the carrier for the plug wires.
  3. Remove battery and pop out silver insert from underside of tray (optional, but helps with the view--may not be necessary if your sensor is in the cat rather than the downpipe).
  4. Tie a string around the plug, so when you pull it through, you can attach the new wiring and direct it back through the same way with relative ease.
  5. Jack front of car up, turn wheels left to lock.
  6. Remove passenger side front wheel. This should give you access to the sensor, which is in the exhaust downpipe or in the front of the cat.
  7. There are a couple of stays that keep the wires in place; open them up, then pull the wires through and untie the string.
  8. Loosen and remove the old sensor. You can use a 22mm offset open-end or a box-end wrench, or you can buy a special O2 sensor socket that has a cutout in the side for the wires.
  9. Wipe off the downpipe so you can install the new sensor without getting any crud on the threads. The new sensor should come with anti-seize lube on the threads. If yours doesn't make sure you put some on!
  10. Install the new sensor and tighten to 37 ft-lbs.
  11. Tie the plug/wires to the end of the string and re-route them up to the holder. Tighten stays to keep wire in place, making sure if there is any extra (there might be a longer wire than the original) it is out of the way of any heat/moving parts.
  12. Plug it back up and secure in the plug carriers.
  13. Replace wheel, lower car down, replace battery and tray insert, connect cable.
  14. Fire it up and enjoy your smoother running, better gas mileage, and cleaner emissions.
Starter Replacement
(Mike Sherrill)
Needed: metric sockets ranging from 19mm down to 12mm (one 17mm deep-well) and an extension arm may help provide leverage on the top starter bolt.
  1. Remove lower engine compartment shield.
  2. Remove battery cover, disconnect and *remove* battery, remove battery seat on the battery shelf - you should be able to see down toward the transmission.
  3. Remove top and bottom of air intake box, rubber intake hose, throttle body (may be optional).
  4. Disconnect starter positive lead and black solenoid wire.
  5. Use 19mm socket, possibly with breaker bar or handle extension to loosen the top, rear-facing bolt for the starter (it's on the edge of the tranny bell-housing).
  6. Underneath the car, passenger side, use a 17mm deep-well socket to loosen lower, front side nut which secures the starter. Make sure you hold on to the starter--it's heavy! I think you'll need a 16mm wrench to hold the bolt on the other end to keep it from spinning.
  7. Remove starter, save nuts, bolts, etc. Note connection points.
  8. Install new starter in position, slip the lower bolt thru it's mounting hole, then scoot over to the top side and hold the starter in position while you tighten the top bolt.
  9. tighten the lower starter mounting nut.
  10. Re-attach wires to the solenoid.
  11. Re-install battery, attach wires.
  12. Re-assemble throttle body, rubber intake hose, airbox, etc.
  13. Re-install lower engine compartment shield.
  14. Wash hands, then start 'er up!

See also: Engine Upgrades

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