the flyin' scotsman

another update: the passenger side coil was replaced (Ignition Cct I) with a known good unit as well as a brand new coil wire. A short run where I took the car through its paces and loads with the cover off the IMS ...........all worked very well proving the IMS system worked as it should.

Thanks all for your inputs.....cheers

the flyin' scotsman

spoke to soon.....the issue is back

didnt have the cover off so unsure if its the same LED thats lit. Shall drive again this aft.

SeanR

Put the bypass relay so you can eliminate that. I've had several of those IMS relays fail.

the flyin' scotsman

thanks Sean....as before the symptoms didnt reoccur although this time I didnt quite hit warp speed/revs........kept below 4k in lower gears

soontobered84

Malcolm,
The issue with my IMS was the driver side plastic coil protector was actually pushing the coil wire out of the coil after it had clicked into position. I would take the coil wire out to check it and resecure it and the engine would be great. After a few minutes, the driveline vibrations coupled with the pressure of the plastic coil protector would "disconnect" the coil wire from the coil.

You might check your coil wire connection at the coil. It may just not be secure in the coil.

the flyin' scotsman

thx John........the issue does seem somewhat revs related as I described it happens when the engine is under load beyond 4k rpm (hard acceleration in lower gears) although if the coil wire was coming loose the relay may reset but the wire would remain not firmly connected.

I'll recheck the coil wires and with the jumper I'll get it figured.

dr bob

Malcolm--

The thermocouples and relay respond to weak or no combustion that's typically but not always caused by ignition failure. If you have a weak injector over one of the thermocouples, for instance, there may be a bit of lean misfire under load that might cause the relay to shut down the offending cylinder. Look at the spark plugs directly over the thermocouples for evidence of lean operation. A little sleuthing with the IR thermo gun may help some too.

The protection is kicking in for a reason.

Bill Ball

So, your IMS is tripping and the red LED is indicating it believes cylinder 7 is colder than cylinder 3 (in 89, this would be cylinder 4 colder than 8). Mine used to do this when I would hit a particular pothole on the way to work. The car would go into 4 cylinder mode, an IMS LED would light up (visible if I had the footwell carpet out), I would pull over, turn off the ignition, restart. It would reset and the car would run fine from then on until I encountered that pothole again.

The system is designed to detect what it believes is a partial ignition system failure, shutting off fuel to the cylinders served by the ignition circuit it believe has failed in some way in order to prevent a fire from unburned fuel going into the exhaust. I know of such fire incidents in earlier dual ignition 928s not equipped with the IMS (pre-89), principally cars equipped with catalytic converters.

This system can trip if one of the injectors to cylinder 3 or 7 is not delivering proper fuel, as Dr. Bob mentioned, or if one of the thermocouples is dirty or faulty or it's wiring is faulty.

If after thorough investigation you determine the ignition system is operating properly and the IMS alarm is false, you can bypass the system with a jumper or a bypass relay that is available, intended for cars not equipped with catalytic converters.

I don't recommend this, but I have driven with the bypass relay installed for probably 10 years. Someday I will diagnose why my IMS falsely tripped and repair it. I may be fooling myself, but I believe I would quickly notice if I had a real ignition circuit 1 or 2 failure and pull over immediately.

Also, it is possible to have a bad plug or wire on a cylinder other than 3 or 7, which could drop raw fuel into the exhaust and not be detected by the IMS. So, the IMS is imperfect, but it was Porsche's solution to a real potential problem with dual/split ignition systems and everyday drivers who may not detect an ignition circuit failure. I know of one very experienced '86 928 owner who drove on with a coil failure unknowingly until the cats caught on fire and things got a bit hot inside the cabin.

Mrmerlin

Malcom you also stated that you replaced the ignition caps,
we had a car that ran fine till the engine warmed up,
then two of the plugs would lose their fire this came from a brand new cap ,
swapping in an old cap the engine ran fine.

What is your status with your MAF have you tried it another car?

Ed Scherer

Malcolm, what ever happened with this?

I'm trying to chase down an intermittent (has reared its ugly head after about 30 minutes of driving about three times now) IMS issue and have been reading various related threads.

On mine, I just got done checking all the ignition wires (both ends) and everything looked good (although I found and removed some oil in most of the spark plug wells, which didn't make me too happy, although maybe it's not that bad since I did my top-end refresh about 6 years ago). All ignition wire terminals looked pristine.

Actually, I'm not even 100% sure that it's IMS, as I hadn't yet removed the cover that lets me see the IMS LEDs. I need to go for a long drive at night and verify that what's going on is really from the IMS being triggered. It sure feels like it.

dr bob

Ed--

Remember that the IMS protects you from -anything- that causes uneven combustion and exhaust temps. Ignition is one, a partially plugged injector will do it too. At least the relay will tell you which ignition system is a problem. Then you get to decide it it's the specific cylinders you are measuring with the thermocouples, or the whole group supported by that ignition system. Misses on other cylinders won't trip the relay.

WallyP

Ummm, Bob - Perhaps "set" would be a better term than "bank". It's two cylinders per side in each set...

Ed Scherer

Mine might be a self-inflicted wound. Upon reading other posts and TSBs, I noted that TSB 29203, 07/07/1992, "Ignition Monitoring System - Diagnosis" (which, other than diagrams) is reproduced in a post from Rich Andrade, contained this diagnostic procedure; I have emphasized part of it:

Why is that significant? Well, that "rotate the temperature sensor" stuff indicates that the sensors being moved will affect their output. And, a few months ago, I did just that (possibly, if bending is functionally similar to rotating, which seems likely) when working near the right side drain plug (see the thread Fumoto drain valve F109 does work as block drain plug replacement). In fact, I even mentioned it in that first post:

So, I'm now guessing (and hoping, since it would likely be easy to address) that things are now a little miscalibrated.

I've prepared the car for measuring that voltage difference (between E1 and E2 on the ignition monitor relay). For the record, here are some photos that might be helpful to others with IMS issues to diagnose.


Side (front, when mounted) view of ignition monitor relay 928.618.175.00. Of particular interest is the location of the green and red LEDs, which you can see (even unlit) through the translucent case.




Bottom view of ignition monitor relay 928.618.175.00, so you can see the terminal labels. The relevant terminals for measuring the voltage difference mentioned in the TSB diagnostic procedure are E1 and E2. When the relay is mounted in the car (at least in my '90), the E1 and E2 terminals are the two corner ones on the front, with E1 on the left and E2 on the right (when facing the relay from the passenger seat).




I just inserted two tinned wires into those terminals and made sure they held firm) ...



... and connected the other ends to a multimeter. The measurement shown on the multimeter is insignificant, as the ignition was off.




Now it will be easy to monitor what's going on as I use the car over the next few days. I'd just like to have good numbers before I go to the trouble of getting the car up on ramps, removing the belly pans, etc.

the flyin' scotsman

Ed, the issue has not recurred since I last posted and thats after many thousands of kms some at full revs

Stan, the MAF in this car is what was in the car when I bought it and IIRC I believe it has a JDS sticker on it that I saw when doing the intakes refresh. The MAF on my S4 is a known JDS rebuild I could swap over to see if theres any differences.

dr bob

Fixed, thanks Wally!


Ed-- I suspect that rotating the TC's offers different physical contact for the probe within the little thermowell in the manifold. Those are likely type K thermocouples, so the 2.5mV delta between the two can be related to temperature in degrees, I suspect. Seems awfully sensitive, as TC's are not exactly low impedance devices.

From your pics it looks like they used decent terminations for the TC leads at the relay, a plus (so to speak...)