1990 928 GT with chronic 4 cylinder mode issue

I am planning on picking up my 1990 GT from a service shop tomorrow. It has been there for 4 months. A number of problems have been resolved, but there is a nagging and disheartening problem still with the car. While I appreciate any thoughts, I ask that any potential respondents read carefully what has been checked, or changed, on the car, in an attempt to resolve this issue.

I will review, and revise the below description as my memory and testing clarifies the issue.

The Ignition Monitoring System (IMS) relay triggers putting the car into 4 cylinder mode ( by cutting fuel as I recall). This happens often and without knowledge as to why at this time.

There is a IMS bypass relay. I have one, and have used it. The symptom of 4 cylinder mode still occurs just less often.

Prior to the problem, I had a complete intake refresh done, with every part replaced except one vacuum hose. Since then the vacuum hose has failed and been replaced (twice).

I have tested much, but as it is a 20+ year old car, I have also replaced many parts in an effort to resolve this issue. Including the ignition modules and the ignition coils.

The shop doing the recent service did find a problem wire near the EZK, and thought it resolved the problem when repaired, but optimism was misplaced.

I did have a known good EZK sent from Rich Andrade swapped in, and it did not resolve the issue.

I have owned this car for 4 years, and driven it just a few thousand miles due to these issues.

Hi Stan;
Here is the way I would go after this problem.
The Ignition Monitoring System (IMS) will have values and tolerances that need to be met or maintained.

Here is a link
www.landsharkoz.com/tech/rich/ticmr.htm

Other then following the test,
What I see in this link is the response to the ignition is given by the Exhaust prob.
I would disconnect the probe to stop the shutdown issue,
But, as you can see from the doc, the ign. over heats and fails, then the exhaust cools and the fuel is turned off by the prob.

Soo either the ign. is over heating or the probe is malfunctioning.
I am not using my probes if you want them?
of coarse I will need to cap the ports. Right now my probes are just tied out of the way.

Is this possible that this is the problem Stan or did I miss something.
Brad

When the bypass relay is in place, the IMS is not monitoring exhaust temperature, so even if there was a different in temperature it would no longer be recognized and acted on.

Essentially with the bypass relay in place the car would function as a pre0IMS 928.

Have you went through this document to see if it sheads any light on your issue

www.porsche928forums.com/download/928EngineControl.pdf

I provided that document to the repair shop that has my car. They used it to identify the bad wire. The author of the document, Rich, is who sent the known good EZK unit to test in my car. I anticipate starting at the begining when I get the car back, but it is distressing.

I know this is frustrating and really hard to track down especially since most everything most of us can think of has been covered. For now, I'd keep the relay jumper in place.

Once again, I'd go and pull the boots back on the Temp II, MAF and O2 and make sure that the wires are in no way touching each other. The O2 won't have a boot per se, but you can check the grommet where it enters the car for misplacement or fraying. If you don't have an X-pipe, you might need a mirror to see the wires going in to the O2.

I'll keep looking at this thread, and if I can come up with any thoughts, I'll add them.