Rich9928p

Andrew,

The 928 S4 and newer engines have two separate ignition circuits. Ignition circuit 1 (C1) is comprised of cylinders 1,4,6,7, the right side coil and distributor. Ignition circuit 2 (C2) is comprised of cylinders 2,3,5,8, the left side coil and distributor.

You are correct about the cylinder numbers that are shut off and the fact that Adam's problem is a mix of circuits 1 and 2 [which rules out the relay as the source of the problem if ONLY those cylinders are affected]. However, I disagree with the statement that it is basically right side or left side (of the engine that is shut down?) if that is what you intended to say.

For the sake of clarification of this system, the diagram below using your cylinder number and position diagram, illustrates the position of spark and fuel injection circuits 1 and 2.

The 1989 and newer 928s have fuel injection circuits that mirrors the ignition circuits, and the injection cut-off relay works such that if the combustion temperature is low from one "ignition circuit" then the four fuel injectors are shut off for that circuit.

.... Front ....
5=C2 .... 1=C1
6=C1 .... 2=C2
7=C1 .... 3=C2
8=C2 .... 4=C1

stealth

Rich, we all appreciate your having that post ready to cut/paste easily .

As you may recall (thanks for posting the info in my thread last week), I have an issue currently-- which now occurs even on the first start of a cold engine, I can't drive it even once it smooths out because of the relay-- where the green LED comes on. I'm mildly suspicious of the relay now, even though there is a genuine issue with letting the car sit for days, because I'm skeptical that 20 seconds of running a cold engine would be enough for the relay to make a judgement...but I want to correct the root cause, anyway, so I'm going to replace ignition components on 1,4,6,7 and see what happens before testing relay.

What I don't understand is how having two exhaust temp sensors would work if the monitoring circuit were watching two cylinders on each side of the engine for each half of the monitoring circuit. If ignition circuit 2,3,5,8 fails, wouldn't the mean temperature on each side of the engine as monitored by the exhaust be similar, thus preventing detection by temperature differential (two cylinders working on each side of the engine)? Or is the difference between having the "inner" cylinders firing and "outer" cylinders firing going to cause a temperature difference due to more dissipation from the "outer" cylinders or something like that?

Vilhuer

Temp probes and relay monitor cylinders 3 and 7 IIRR. If either one of those have problems corresponding led lights up etc. So temp probes do not even try to find out if all four have problems or not. It's just presumed that's the case when 3 or 7 is showing symptoms.

AO

Rich-
My bad... I was thinking of how the distribtors are set up. Pass side 1,4,6,7 and driver side 2,3,5,8. You are right of course. Sorry for any confusion.

Bill Ball

Yes, as Erkka as saying, it is using the temp as a marker for a failed ignition (hence, "ignition monitoring system"). So, if one of the monitored cylinders is colder, it presumes the ignition to that half of the cylinders has failed and shuts off the gas to that half to prevent raw gas from going into the exhaust and starting a cat fire. The catless Euro cars have a factory "bypass" relay that disables the monitoring system.

So, Porsche believes that with a dual ignition system in this powerful V8 that drivers may not notice the loss of ignition to 4 cylinders and just drive on into a cat fire. Seems kind of odd to me that you would not notice the loss of power, but truth is, it has happened to some Rennlist members.

For people who are experiencing a failure in this system with NO ignition problems, this probably represents a bad sensor. They can be tested. They can be removed and reseated. They can be replaced. Or you can take the route I did and install the bypass relay and trust that you will sense a loss of 4 cylinders yourself.

928SS

interesting thought, but if the plugs were wet that would mean the injectors were still pumping fuel... unless they stopped after the sensors decided to cut them off. but then the car would run ok at 1st when cold, then die as the fuel was cut.

heinrich

i mean water

Bill Ball

Based on the cylinders affected, it's been established the ignition monitoring system is not the issue here. So far, Adam has a freaky problem that doesn't fit any common system failures.

heinrich

Bill I've had sleepless nights over this one. And I have concluded the most likely solution would be to just ignore the issue and move on .... Adam ... fuel pressure? Spark plugs?

atb

Last night, two of the inboard injectors were swapped with two of the outboard injectors. No change in running condition.

I swapped the brains out of my 88 and put them in this car.
It ran better, but still way off.

Timing light showed all 8 plugs were firing.
Pulled injector clips while motor was running, no change when 2,3, and 7 were pulled. When I would pull 6, the motor would die every time, when I pulled the others, the rpms would just drop.
Put a noid light module on each clip per Bill's suggestion, the harness is pulsing at all eight plug ends.

Ran the car for a bit, had the owner take it for a few laps around the complex, and did a plug check. #6 six appears to be firing now, but 2,3, and 7 were slightly wet when pulled, but didn't show any burned carb deposits that would indicate that they were firing correctly.

I've got fire, I can't see how the primary ignition system could be the problem.

I swapped around the injectors, and the same holes are still not firing correctly, which I think rules out the injectors themselves.

I'm getting pulses at each clip, so the harness seems to be functioning correctly.

The only other option cam timing. That's the only condition where you can have fire, fuel, and compression, not not have a car run correctly. Somewhere, somehow maybe the cams got misaligned. I spent a lot of time getting them degreed in perfectly. My index marks still line up, neither of the cams are fully advanced or retarded in their key slots, or otherwise indicate that the cams turned in relation to the cam wheels. I'm definitely at a loss, but its really the last thing to rule out before I instill the help of a tribal shaman to ward off any evil spirits.

AO

Since you verified that you have spark, fuel, and compression, I'll re-direct you to my post (#18) earlier in this thread...

Hey I just had a thought about a towel being stuck in there. If that were the case then the compression would be significantly lower in those cylinders. So I can't imagine that that's the case here... but you might want to pull and injector and get a scope in there just to make sure...

928SS

how's the O2 and NOX readings? might give a clue if it's overly lean/rich... sure the flappy is ok? what if it's just laying in there improperly?? any rattling noises in the intake area?? if the 2nd set of chain driven cams are off, that could make a mess too - and you wouldn't see it from the cam belt timing marks, since they only indicate 1 set of cams...

atb

Hey Rob,

Indexed both cam wheels at the same TDC point.

If you mean that the intake cams are off from the exhausts, not possible. The alignment marks were aligned properly when I installed them. (Straight up at 45 degrees BTDC, point outward 9 o'clock and 3 o'clock at TDC. This motor even had the copper colored alignment links in the chain. Pretty cool.

No rattling, no rags, the intake wasn't taken apart.


Pulling cam covers tonight just to make sure.

928SS

it's a bugger for sure!!! hope you find it soon!!!

Bill Ball

OK, noid light shows that the injectors for the bad cylinders are getting an electrical pulse, but swapping the injector does not change the behavior - same cylinder affected. Now only 3 cylinders are affected. (Sorry, no point here - just summarizing).

I had one cylinder drop out at idle which had everything it should to fire. It was due to a nearby vacuum leak leaning it out. It would run fine once off-idle. BUT you have wet plugs in the affected cylinders. This really points to lack of spark or air then.