loprado

Ahhh haaa..thanks John!

Success.....relay is behind a foam 'protector' behind the carpet.....

And (drumrooooollll) red light as soon as the revs drop!

so the 6 million $ question...what now? Simply replace it?

loprado

....sorry forgot to add....
why is it tripping in the first place? What is the cause of it.....

Thanks everyone for your help in finding this gremlin....

928Quest

From the second description I think this problem is simipler than we first thought.

Does it behave this way just after you have wash the car? Have you checked the leads going to both coils? Your description sounds like something mine was doing when water would get into the drivers side coil lead. It would intermittently run on four cylinders. The wire started to corrode, green gunk. replaced the coil wire, no problems since.

"....Idle speed is steady when this occurs. When it happens the car has almost no acceleration at all, almost like it's running on 4 cylinders, and vey rough. I haven't observed an black smoke but then again its something I haven't looked for when this has happened. I switch the car off straight away and then fire it up again after a few minutes which seems to fix the issue....."

loprado

Hi Chuck,
Never observed it after being washed or rain. From memory it has happened on a very cool night from a cold start...and also on quite a warm day after stopping for fuel and then restarting.
On cold or hot day, warm or cold car..nothing common!..but something to check, thanks

Bill Ball

Hey, I should trust my intuition more. I mentioned this relay, but didn't THINK it could be the problem. Glad it was easy to find.

This system detects differences in temperature between cylinders that are fed by the two coils. If it sees a significant difference it concludes half the ignition system has failed and it cuts the fuel injectors to cylinders served by that side to prevent a cat fire from unburnt fuel. So, others mention checking the coil connections. RED indicates the right (passenger US) side system is at fault.

Might as well service the ignition system (plugs, rotors, caps, wires), if it hasn't been done recently if you don't find simply a loose or corroded coil wire connection. If all is up to snuff, but the relay still trips, then the temperature sensors are suspect. They can be tested and replaced at something like $250 each.

Porsche also sells a bypass relay (928.615.175.00). If the car runs perfectly with the relay and there is no other evidence of any ignition system issues, you can run on the relay until you figure out the fault. I had 3 false trips and decided to install the bypass relay and forego the system. I'm counting on my ability to feel a signficant ignition failure on my own.

You can temporarily bypass this system by pulling the relay and connecting pins 2 to 4 to 5 on the connector and leaving the relay out.

My intuition again is that since your car runs fine until the system trips, this is a false alarm. Bypass it as a test.

Mongo

HOLY CRAP!!! My 1987 928 S4 does the same exact thing!!! My car would cold-start fine. However, shut it off when warm, come back 15-30 minutes later, the car won't hold an idle. It fluctuates like crazy between 600 RPM to 200 RPM, then stalls!

I have replaced my MAF and this helped it only slightly. I have managed to trace it to a faulty Idle Stabilizer Valve. Have you changed yours yet? I know this repair is next in the book on my car along with every damned vacuum line under that intake manifold (DOH)

Good luck on finding the solution...let me know if you beat me to it!

loprado

Hi Andy,
Mine doesn't fluctuate or stall though...so maybe different issue? Yours sounds like the ISV from what I've read.
With my ignition relay red diode tripping I will check caps, wires etc.
If still tripping I think I will bypass. The car runs perfect all other times....

Bill Ball

Yep, loprado, that's a good plan.

John Speake

Hi Loprado,
There's not much to add to Bill's excellent answer, except to say that the red LED circuit suggests a problem with cyl #4 which is rear passenger side for your LH drive car.

loprado

Thanks everyone!
John....car is RHD!

loprado

Ok here's the update and a few q's!
Spent the morning yesterday checking all relays, fuses, wiring, etc. All looked good. Checked both coils, caps, etc. One looked a little grotty but not too bad. Cleaned it up and looked like new. Swapped both coils to see if I could get the relay light to change to green from red if this was the problem. 1/2 day later relay tripped again (red light still) coil not the problem. Let it sit until cold, checked all plugs and MAF. Some plugs little oily but not too bad, MAF and filters all look good. Started again, now on a cold start relay trips within 10 seconds! Bypassed relay with 2-4-5 connections.....all good. Engine running smooth, starting first crank (never had a starting issue anyway) so a few q's!
1. If this realy is an ignition overheat relay (ie. detecting significant temp differences between cylinders), how could this trip on a cold start?
2. Where are these sensors?
3. What's the purpose of the ignition relay (IV)? How is this different to the overheat relay?
4. How is the bypass relay sold by Porsche different to me connecting 2-4-5?

The more I'm learning, the more I want to know!!

Thanks all for you help, much appreciated....

Bill Ball

Relay has an 18 second delay. The fact that it trips immediately argues it is faulty, although you do report you got a half day after some clean up. Same side trips despite coil swap and clean-up - points to a bad sensor. You can see the sensors from underneath the car, screwed into the block undeneath cylinder 4 and 8. One bulletin from Porsche describes loosening the sensor, turning it a quarter turn and re-tightening. Also, you can move them to cylinders 3 and 7, as they were placed on later models. You would need new seals to do that.

The ignition relay (IV) supplies power to the ignition when you turn the key. The montioring relay is an add on starting in 89.

I don't believe the bypass relay does anything more than jump the contact as you have done.

You report some plugs are oily.....they should not be. They should be dry and light tan colored. Drive the car some more and watch this. If it is oil and not just soot, then you may have worn valve stem seals. Jeeze. Shouldn't happen. Clean the plugs. Swap them. Make sure you are getting good spark. If it is soot, you are rich for some reason. If some of the plugs are oil-fouled, the car would not run so great and that could be tripping the monitoring system. You did do a chip swap, didn't you? Was that an LH or EZK chip?

loprado

Thanks Bill...I don't think the half a day is relevant...sometimes previously I could go for months without it tripping, sometimes one day (as happened yesterday).
Also I did say immediately/10 seconds, but I guess this could have been 18 seconds? Will it still trip on a cold start after 18 seconds?....another confusing thing is it has never happened whilst driving, only in N. Maybe I'm missing the point...does the sensor check the temp of the cylinder? Could this trip after 18 seconds from cold?? Maybe a fouled plug would cause this although I think I would notice this.
Some plugs were a little oily, I did clean them and will keep an eye on them. Although car is running fine (until it was tripping)...
I didn't do a chip swap......do I need to?

Bill Ball

There have been others who have had this thing go off at idle but not when driving, as you are seeing. Yes, it measures the temperature near cylinders 4 and 8 and compares them based on the voltage output from one sensor relative to the other. They can be tested for voltage output. The specs are in the WSM (get CD from Jim Morehouse). So, if there is a difference at rest, then one of the sensors is bad, and it will trip when cold. Perhaps once hot, the sensor is OK.

Leave the chips alone. Sorry, I mixed you up with somone else. Go out and do some spirited driving and pull the plugs.

loprado

That I can do!