Proph3t

Hi guys, this is a carry on from my other drama filled thread at https://www.audiworld.com/forums/sho...php?p=24279691

Its a '96 A8Q with a ABZ 4.2 V8, 4speed CML tiptronic.

Long storey short:
-Had a no-crank situation, replaced ignition switch.
-Would crank now but not fire. I had upset the dodgy repaired crank angle sensor connector while trouble shooting the previous issue. So I replaced the sensor.
-Car started after adding 20L of high octane (98) fuel, was parked on a hill during previous fixes.
-Drove exactly 5.0km, then misfired and stopped. Towed it to parents home.
-Checked fuel pressure at filter, crap. Replaced filter. Checked pressure at pump, crap also.
-Removed pump and it runs strong in a bucket. Pump wiring gets 12v. Reinstalled, and pumped fine.
-Did a test drive for 10km, no troubles. Drove up steep hill back to parents home, misfired all the way up, but was pretty strong still.
-Drove all the way back to my house with a slight mis taking off from a level intersection, drove like a beauty apart from that.
-Parked on a slight hill idling, raised revs to 2000rpm, starting missing.
-Started it this morning, fired up perfectly first go, no smoke, no fuss, sat idling with a miss or 2, pretty rough.
-VAGCOM pulled no codes, except the usual brake light one because my foot wasn't on the brake pedal, and the airbag code, because its not plugged in due to the ignition switch change. None relating to misfires or anything.

So what causes a miss with no codes? My guesses are:

-Vacuum leak on warm up and under load. The mechanic who did some work on the gearbox a few years back said if I ever get idle issue, that he'd be looking at the vacuum hoses at the rear of the engine as some looked a little ordinary. I took a look this morning after I remembered and one looks perished and its carrying the weight of all the other hoses its connected to, and kind of drooping off its connector on the throttle body assembly, right next to where I was working on the crank angle sensor wiring, I may have upset it then, and its finally broken through and getting worse.

-Pump not picking up correctly. Blockage? Water in tank? Bad regulator?

-Fouled plugs. It was a bit smokey when I got it to run. had been 6 weeks since the ignition switch failure. I run the genuine 4 point BOSCH plugs, probably due for a change shortly anyway.


Any other ideas? Seems like one thing just leads to another.

Thanks guys.

...oh, another little issue...only one speaker works now from the Bose headunit. The rear right door. The others are all dead. I'm assuming I've blown the amplifier fuse or something. No idea where thats located.

Prospeeder

well. Misfire recognition might not be part of the engine management system. It is pretty old. your on the right track for a vacuum leak or bad plugs or coils low fuel pressure ect...

Proph3t

Great, so do we know which ECUs have Misfire recognition? I'm pretty sure I've cleared misfire codes from it when I first got VAGCOM and clearly codes hadn't been cleared for years by the previous owners. I still don't understand how the ECU can detect a misfire.

Anyway, I've bought a snake light to check this pump assembly fitment, and will remove the suspect vacuum house I can see on the throttle body. Thanks

Proph3t

So without touching anything after 2 days, it started first go and idled fine. As it warmed up it began to miss-fire off and on, 1 cylinder only by the sound and feel.

I pulled out the suspect vacuum hose (077133784L) and it actually pulled off without needing to undo the worm clamp, and discovered it was nearly completely broken right through.

So I'll need to source and install one of those, then go from there.

Adjuster

So, any un-metered air leak, will cause a misfire issue it appears.

Vac leaks, or air leaks anyhwere can be the culprit, only it depends on how much of a leak, as to when it shows up. (IE: Engine load.)

A small leak might never pull a code, but will cause the CEL to flash at high RPM/Load....

Or the other side is clogged fuel system parts, or failing pump. Dirty injectors, clogged filter or failing pump will also cause the random misfire code, and flashing CEL, rough running due to dropped out injectors etc.

I'm assuming the ECU cuts the injectors on the misfire cyc to save the engine from damage. Then will re-set once it's started again, or until there is a misfire again.

My 2001 S8 will not pull codes now around town, but put some load on the engine, and it will flash, and run it at a constant speed over 65mph, and it will drop injectors, and run like crap until it's shut off, and started again.

I'm going to check this hose, don't think it's the issue, and as Sam has indicated, might pull my injectors and have them cleaned.

Proph3t

Ok, so fitted the new vacuum hose, cranked her over and she started and idled fine. Gave her a rev, all trumpets were loud and proud. No sign of any issues.

But alas!...

...went for a drive and got about 3km down the road (probably just warmed up) and noticed a very slight miss under light acceleration. Couldn't feel it, but could hear it. It progressively got worse, and of course it waited til I was across the other side of a major intersection no where near a side street before flashing the CAT light and running very rough, basically undrivable. Was also idling rough when I pulled over and the CAT light continued blinking. A wide body sedan on the side of a narrow major road was not popular, so I had no choice but to fire her up again and sneak her up the road into a side street.

I haven't ever seen the CAT light through any of this trouble, so at least I might have some codes to go by now, but the car is across town and it's pouring rain, so it'll have to sit there til it cools down and I'll sneak her home, and pull some codes.

Why are these things never straightforward? Hopefully I've been a noob and forgotten to plug a vacuum hose in or something.

EDIT: from what others are saying, its pretty commonly coilpack or ignition amp. Least they are easy to test...but have to get codes out first.

Proph3t

So snuck it to my fathers house...well towed half the way as once it warmed up, the CAT light went crazy and it was undrivable.

VagCom shows exactly what I thought, but not really helpful...but it shows the issue is on both banks.

I only have the free version, but I was able to verify the 5 digit codes from elsewhere...

VCDS-Lite Version: Release 1.1
Sunday, 10 June 2012, 17:25:19.
Control Module Part Number: 4D0 907 557 E
Component and/or Version: 4,2L V8/4V MOTR AT D02
Software Coding: 04263
Work Shop Code: WSC 00000
VCID: 2E2F8DBBF919
6 Faults Found:

16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -

16685 - Cylinder 1: Misfire Detected
P0301 - 35-00 -

16689 - Cylinder 5:
P0305 - 35-10 - - Intermittent

16686 - Cylinder 2: Misfire Detected
P0302 - 35-00 - -

16688 - Cylinder 4:
P0304 - 35-10 - - Intermittent

16687 - Cylinder 3:
P0303 - 35-10 - - Intermittent


EDITED CODES AS THE MEANINGS and ORDER WERE WRONG.

I believe cylinder 1 & 2 are the main issue with the "35-00" codes and actual "Misfire detected", while the others showing as "35-10" and "Intermittent", most likely because the engine is leaning 3,4,5 out or something.

Proph3t

No ideas anyone?

Proph3t

So after a weekend of testing, I'm still no closer to a solution...

Here's my VAGCOM codes and steps taken: (codes cleared between each step)

**Warmed up and driven. Missing bad, crawled home barely. Cat light flashing.
6 Faults Found:
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -
16685 - Cylinder 1: Misfire Detected
P0301 - 35-00 -
16689 - Cylinder 5: Misfire Detected
P0305 - 35-10 - - Intermittent
16686 - Cylinder 2: Misfire Detected
P0302 - 35-00 - -
16688 - Cylinder 4: Misfire Detected
P0305 - 35-10 - - Intermittent
16687 - Cylinder 3: Misfire Detected
P0303 - 35-10 - - Intermittent


**Cleared codes, just let idle, not warmed up, no CAT light (doesnt show until warmed up)
2 Faults Found:
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -
16685 - Cylinder 1: Misfire Detected
P0301 - 35-00 -


**Swapped coils between banks, cleaned plugs. Warmed up, CAT light flashing.
5 Faults Found:
16514 - Oxygen (Lambda) Sensor B1 S1: Malfunction in Circuit
P0130 - 35-00 - -
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -
16685 - Cylinder 1: Misfire Detected
P0301 - 35-00 -
16686 - Cylinder 2: Misfire Detected
P0302 - 35-00 - -


**Unplugged B1 S1 Sensor
1 Fault Found:
16518 - Oxygen (Lambda) Sensor B1 S1: No Activity
P0134 - 35-00 - -

**NO MISS!! Warmed up, no CAT light, drove perfectly, smelt a little rich. Thought I was onto something. I cleaned up and went to put the car away and order an oxygen sensor, then it wouldn't even drive up the driveway, bad miss and CAT light flashing.

5 Faults Found:
16518 - Oxygen (Lambda) Sensor B1 S1: No Activity
P0134 - 35-00 - -
17541 - O2 Sensor Heating Circuit, Bank1 +2 - Sensor 1, Electrical Malfunction
P1133 - 35-00 -
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -
16685 - Cylinder 1: Misfire Detected
P0301 - 35-00 -
16686 - Cylinder 2: Misfire Detected
P0302 - 35-00 - -


..so I'm confused. I think my model only has 1 oxygen sensor per bank, so removing the sensor fixed the issue initially pointing to a bad sensor. But the fact I was getting a CAT light again and misses on the same bank point to a second sensor (right?).

What else should I check for cylinder 1 and 2? Coils and plugs have been moved. I know everything is physically ok as it, most of the time, runs fine, revs free, starts fine etc until it warms up, oxygen sensor kick in, then it goes crazy.

I compared the oxygen sensors from each bank. Engine off, the suspect sensor was 2.5ohms, and the other was 3.0ohms. They had the same wire continuity etc. The picture is of VAGCOM straight after stopping the engine with just cylinder 1 missing, no cat light flashing. As you can see one of the oxygen (lambda) voltages is very low, and was sporadic when running the other didn't seem to change much, so maybe it was the stuffed one. Both these voltage were still there when one sensor was unplugged (???)

Anyone make sense of this? Any ideas.
Thanks guys.

Adjuster

My experience with O2 narrowband sensors is that you want to see voltage jumping very fast if they are working correctly.
The "hertz" rate, or point where the voltage jumps will slow down as they age, and fail. (So no voltage change anymore, or very slow.)

Wideband 02's are not the same at all, and I'm not sure what type of sensor the A8 has.

It would make sense that as your sensors warm up, the engine stops working, since it goes from a set map that the engine runs on cold, to one that is a closed loop, constantly changing and "trimming" fuel to get best economy and emmisions results.

Hopefully, new O2 sensors will cure your ride?