cparke

1998 Audi A4 Quattro V6 30V (AHA engine)

I am told after shutting off the engine, smoke was seen coming out from under the hood, but I wasn't there and my wife of course refused to open the hood so I could get a clue as to where it was coming from. The car did not overheat, oil and coolant levels are fine, all instrument cluster dials were reading normal. Car had been on highway but was being driving locally immediately prior to the incident.

When I got to the car, no smoke (though a faint smell of unburned fuel in exhaust smoke), here is the VAG-COM fault codes I got:

Address 01: Engine
Control Module Part Number: 4D0 907 551 AH
Component and/or Version: 2.8L V6/5V MOTR AT D01
Software Coding: 06251
Work Shop Code: WSC 06335
9 Faults Found:

17549 - Load Calculation Cross Check: Implausible Value
P1141 - 35-10 - - - Intermittent
17746 - Camshaft Position Sensor (G40): Open or Short to Plus
P1338 - 35-10 - - - Intermittent
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -
16685 - Cylinder 1: Misfire Detected
P0301 - 35-00 - -
16690 - Cylinder 6: Misfire Detected
P0306 - 35-00 - -
18014 - Rough Road/Engine Torque Signal from ABS: Electrical Malfunction
P1606 - 35-00 - -
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -
16685 - Cylinder 1: Misfire Detected
P0301 - 35-00 - -
16690 - Cylinder 6: Misfire Detected
P0306 - 35-00 - -

Readiness: 0000 0000


Clear the codes, restart the engine, car runs fine, drove it home (including with highway and local driving) without incident, no codes present when I pulled into the garage.

Background: This car has been running with a rich (but not too rich) multiplicative fuel trim problem on Bank 1. I have been suspecting that the fuel injector for cylinder 1 is bad and the cause that problem, but haven't had a pressing need to possibly change it without a CEL until this incident just suddenly happened. Car recently had new coils, wires, and spark plugs installed, so that shouldn't be the cause here. The cause of my engine problems has been very elusive.

The first 2 intermittent fault codes have been sitting in the ECU without causing the check engine light to go on at all for 3 weeks prior to this incident. Those codes were in there when I took the car back from a repair shop which didn't know what it was doing and wanted to change my MAF after I already had changed it myself.

Fuel trims in measuring block 32 were last logged 6 days ago as follows (unfortunately, I didnt re-log them after this incident before resetting the ECU):

Group 032: O2S control learning values (before Cat. Conv.) [B=Bank S=Sensor]
-0.8 % @idle B1S1 +/-10%
-10.2 % @part B1S1 +/-10%
-3.1 % @idle B2S1 +/-10%
-0.8 % @part B2S1 +/-10%


Now, the part that most puzzles me most about this incident is how/why the misfire also happened on Cylinder 6. Cylinder 1 didn't surprise me, I've seen misfires on that one and elsewhere on bank 1, but this is the first time I saw both cylinder 1 and cylinder 6 misfire together, and this happened completely out of the blue without any warning or clue (car was running great just prior to the incident, as well as immediately after, there was no CEL on prior to the incident). Any thought on how a bad cylinder 1 fuel injector could somehow cause cylinder 1 and cylinder 6 to simultaneously begin misfiring together? Is there something else that could explain this bizarre incident.

Finally, is my car wired correctly? On the right (passenger side), cylinders 1,2,3 are plugged in from front to back (cylinder 1 towards front of the car, cylinder 3 towards the firewall). On the left (driver's side), cylinders 4,5,6 are also plugged in from front to back (cylinder 4 towards the front of the car, cylinder 6 towards the firewall). On the coils, cylinders 1 and 6 are connected on opposite ends of the same coil paired section (I can check the other two sections if you think it is important).

Much appreciation for your thoughts. I sure would like to nail this engine problem once and for all.


P.S. We drove the car some more now, and the misfires happened again on the local travel section! (seems to happen when accelerating from a stop or near stop) This time, the car started shaking and the CEL started blinking for just 10 seconds, then it stopped shaking and CEL went steady. VAG-COM faults are mostly similar:

Address 01: Engine
Control Module Part Number: 4D0 907 551 AH
Component and/or Version: 2.8L V6/5V MOTR AT D01
Software Coding: 06251
Work Shop Code: WSC 06335
6 Faults Found:

16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-10 - - - Intermittent
16685 - Cylinder 1: Misfire Detected
P0301 - 35-10 - - - Intermittent
16690 - Cylinder 6: Misfire Detected
P0306 - 35-10 - - - Intermittent
18014 - Rough Road/Engine Torque Signal from ABS: Electrical Malfunction
P1606 - 35-00 - -
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-10 - - - Intermittent
16685 - Cylinder 1: Misfire Detected
P0301 - 35-10 - - - Intermittent

Readiness: 0000 0100

diagnosticator1

And cylinders 1 and 6 are one of the three pairs, so if 1 AND 6 are experiencing misfires, it's most likely the ignition coil. If the coil was OK, and for example, cyl 1 had a bad spark plug, there would by a misfire DTC for cylinder 1 only. The ignition coil is actually three separate independent coils within the same package.
Another clue pointing to a bad ignition coil, is the symptom where the misfires are provokes on acceleration, but don't manifest at light loads. The spark ignition voltage required to spark jump the plug gap, increases rapidly along with higher cylinder peak compression pressure at the spark point, that increases along with throttle valve opening angle. A weak ignition coil, can't developer the higher secondary coil output voltage needed with high cylinder pressures.

cparke

Misfires didn't start happening until just now.

diagnosticator1

The evidence points to that, The problem could be a bad plug wire also.

cparke

No more misfiring on two paired cylinders at the same time. Good call.

I still see CYL-1 misfiring too much (8-16 times) when I rev at idle, though none really detected while actually driving. The plug and plug wire were newly installed 4 months ago with the coil that got replaced.

At this point, I'm thinking the plug gap on CYL-1 was set wrong when they installed it, or there is a compression issue on CYL-1. Over 4-months time, the intermittent misfiring of CYL-1 weakened the coil to the point where a misfire on CYL-1 caused CYL-6 to misfire too. So, I'm concerned this will happen again if I can't get CYL-1 fixed right.

Could a possibly bad camshaft position sensor be contributing to this problem? Firing order for this car: 1-4-3-6-2-5

diagnosticator1

Considering that spark plugs for cylinders 1 and 6 fire together, then the plug wires at the plugs can theoretically be swapped 1<>6, Then if the recently observed misfires with cyl 1 moves to cyl 6, then the only reason would be if the plug wire for cylinder 1 was defective somehow. If nothing changes with the swap, then it's probably the plug in cyl 1, or it must be a fuel issue on cylinder 1, and not ignition related.

cparke

BTW - the wire to cyl. 1 isn't long enough to reach to cyl. 6. Firing order shouldn't be disturbed either, I would only try it by swapping the wires entirely so the 1/6 cylinders are still plugged in correctly to the coil.

I installed a new fuel injector on CYL-1, and swapped injectors on 2&3 with 5&6. Misfiring on CYL-1 at idle revving still happens, everybody else running normal. No abnormal misfires while driving on any cylinder.

Also, did I mention than bank 1 has been suffering from a "rich, but not too rich" multiplicative fuel trim problem (-10.2 - -12.7) for all 4 months? I worry the intermitten misfiring on Cyl. 1 over 4 months weakened the coil, and will do it again if I don't fix the problem. Also, I have to believe the misfiring on cyl. 1 and this too rich condition on bank 1 are related, say the exhaust valve or a valve seal?

My theory goes, as I weakly understand these things, at idle speeds the valve causes misfiring on the cylinder 1 because some of the exhaust gas is staying in the cylinder or something like that? At multiplicative, however, some of the air-fuel mixture is going out the exhaust and the O2 sensor thinks the system is running rich. Computer is pulling back fuel, causing the whole bank 1 to actually run lean. Sound plausible?

Now, if this is true, what can I or should I do about this?

diagnosticator1

low compression is most evident at idle speed, when the longest time for the leak down in pressure to occur. As rev/min increases, and throttle valve opening angles increase, two factors involved that reduce the operant effect of the compression leak. Those factors are; 1st, the greater throttle opening increases the air charge mass in the cylinder when the compression stroke begins, raising dynamic compression pressure, and second, the time period when the compression leakage occurs vs the relative flow rate of the leak, decreases the pressure lost during the compression stroke, so the compression pressure at the firing point is higher at higher speeds. Both factors together, minimize the loss of cylinder charge and help normalize the compression pressure as rev/min increases.
The multiplicative trim to rich, is also the worst at idle speed, and the relative excess fuel decreases as rev/min increases. Check all of the fuel injectors on the effected bank for leaky fuel injectors. Also perform a compression/leak down test.

cparke

Additive trim is fine on both banks. Just multiplicative on bank 1 is high. There are no misfires on cyl. 1 when actually in idle. Only as I rev the engine while in park and RPM is increased does CYL. 1 start misfiring. Doesn't happen while car is in gear and driving either.

Fuel injectors on bank 1 have been swapped with the other bank which didn't have the problem, and CYL. 1 has a new one installed now.

If spark gap or wire were bad on CYL-1 because somebody screwed up or plug/wire is defective, it would consistently misfire all the time regardless of RPM and driving conditions, wouldn't it?

As always, results with this car on this problem are mixed. I guess compression/leak down test is the only thing left to do, correct?

diagnosticator1

Then the misfires may only happen during acceleration, due to the increased cylinder pressures causing spark blow out as the ignition voltage required increases with cylinder pressure. At some point, the required voltage exceeds the capability of the defective plug wire. It may also be a spark plug defect.

Regardless, if the compression is weak, the variation in crankshaft angular velocity (speed dip,) that occurs when cylinder 1 is on the power stroke, will be detected by the ECU as a missfire, even if there is no actual misfire occurring. The ECU uses only the variable of crankshaft angular velocity/time to determine if a missfire event has occurred or not. The missfire monitor does not evaluate the ignition system secondary activity in any way. Misfires are only "inferred" by secondary effects on other measured variables, specifically, crankshaft angular velocity, or (variation of) instantaneous crankshaft speed.

Considering the possible phantom misfires caused by weak cylinder compression, a compression/leak-down test both cold and hot, is indicated next.