No, I'm not suggesting an injector o-ring in your case. You have too much evidence pointing to something in the MAF/LH. If the car runs better with the MAF disconnected, that is what needs to be investigated. Of course, make sure you have good vacuum at idle (16" at sea level).

The 4 cylinder mode operation is stated by Porsche to break TT's in 928s with automatic transmissions when run at 1K RPM.

 

The O2 sensor has nothing to do with tripping 4 cylinder mode (although the O2 sensor is disabled when running in 4 cylinder mode). This is a separate system. There are thermo sensors installed in the heads at the exhaust port of two cylinders from MY89 on that monitor the exhaust gas temperature of those cylinders. In 89 it was cylinders 4 and 8. In MY91 it was switched to cylinders 3 and 7. You can see them easily from underneath the car, and you will note unused plugged bungs under the exhaust port for all cylinders. The exhaust gas temperature monitoring is being used as a proxy for detecting ignition failure. One of the monitored cylinders is served by the left side coil while the other is on the right side coil.

left side coil: 1-4-6-7
right side coil: 2-3-5-8

The sensors produce a small voltage signal related to exhaust temperature. If a difference in the voltage output of the 2 sensors is greater than 6 mv (which equates to about 300F temperature difference), the system trips the relay which shuts of the fuel injectors to the cylinders served by the coil that serves the monitored cylinder and lighting an indicator LED. This shows which cylinder was determined to be colder and therefore which coil is suspect and which corresponding cylinders had their fuel injector signals disabled. We enter 4 cylinder mode. There is a delay of 15-20 seconds before the system is active on start-up.

left side: Red
right side: Green

So, a green LED indicates that the exhaust gas temperature cylinder 8 (3 in MY91+) was determined to be at least 300F colder than cylinder 4 (7 in MY91+).

One other thing - if the voltage difference is considerably greater than 6 mv, the system treats this as a defective sensor and trips the right side (Green) consistently. Pulling the relay and measuring resistance across the harness leads to and from the sensors may find an open circuit condition (bad sensor).

 

Bill, you are awesome!

So - if mine sporadically goes into 4-cyl mode (meaning sometimes for no good reason, like when it's still cold, and other times like it means it - like yesterday all the way home from Lime Rock), then it seems to me likely to be a bad sensor or loose connection to it. Is this a sound assumption?

I looked through the parts list, but could not find this sensor anywhere. Can you help me find it so I know what to look for?

Also, how exactly would I measure sensor resistance from the relay? Which relay? Which poles should I compare?

Thanks so much for your help!

 

Sporadic tripping can still be a real ignition problem. It could be something as simple as a corroded coil wire connector. I have found that on my car. Also, since only 2 cylinders are monitored, it might just be a single spark plug fouling or a bad plug wire to one of the monitored cylinders. It might even be a single bad fuel injector. Anything that might cause a cylinder to run cold. So, you need to rule out a real problem. There are probably a hundred ways you could do this. To rule out the coils, you could swap them and see whether the problem swaps to the other side.

To check the sensors, first get under the car and make sure they are still snug where they screw into the head (see picture below). Then pull the plug off the relay. Look at the relay base and you will see the terminals are labelled. Moving over to the plug, test resistance between the corresponding terminals for E1 and E2. It should be about 2.5 ohms. If it is much higher (usually infinite if bad), one of the elements is broken or possibly disconnected. To figure out which one, you'd have to find where they plug in to the harness, which is behind the engine. Here's a picture of the driver (right) side sensor. You can feel along the sheathed lead and feel the plug, but it's not accessible from below. Below that pciture is a screen capture from the PET. Note these sensors were well over $200 each last time I checked.

 

3.4 Ohm E1 to E2. I got a couple of fingers on the sensor, but that's about it - my big Popeye forearms won't let me trace the lead back, and I can't get to it from behind the engine.

There's always the bypass relay...

 

Scratch that - I was able to trace the sensor wire up the back of the head where it clips into an anchor. Then it goes into the harness right next to the MAF - never to be seen again. I was able to see the same lead on the right side but could not trace it anywhere.

I disconnected the left-side sensor at that retainer clip and measured about 3 Ohm across its two wires. I could not get at the right-side sensor wire connection - too much in the way for Popeye.

 

And another thing...

I am thinking that it's the relay or the sensors, not a real ignition problem. The car runs perfectly until it switches into 4-cyl mode. If there were a real problem causing the temp of one bank of cylinders to fail, I think I'd be aware of it. Could cylinder #8 failing by itself could go undetected? I'm new to V8s so I don't want to assume.

 

Only one O2 sensor that I found on my 90 GT, it is center located at the front of the cat assembly.

There are two EGT sensors, one on each side, they could be your problem. I didn't get to reviewing those, but there are some notes in this topic from others suggesting how to do so.

 

Hello Bill and other members i had the limp mode sporadic like you guys about a year ago .I had replaced everything in the ignition system and the problem was still there,until i traced it to the bosch o2 sensor .I paid 150.00 for the o2 and the problem went away and if you had not replaced one its a good chance that is the problem......

 

Hey Bill,

While my resolutions is not confirmed enough to know for sure (as the car is still on jack stands). It does seems possible that replacing my O2 sensor correct my poor running issue and the resultant relay tripping. So, isn't it possible that a bad O2 sensor results in a poor mixture from the LH, and that may result in the IMR tripping (presumably from an over rich condition that gets recognized on one bank before the other)?

I think Dave Chamberland mentioned something similar earlier as well.

 

Yeah. I think he did...

And the ECUs will store a 'dead 02 sensor' error if the o2 sensor is KIA. (If you have an '89 or newer with the diag port and the means to use it.)

 

Well, sure I guess it's possible a bad O2 sensor might somehow cause the ignition monitoring relay to trip (although I'm not sure how). I just meant to indicate that it is not part of the monitoring system itself.

 

The intent of the ignition monitoring system is to pick up a significant ignition failure where one of the 2 coils is not doing its job and a lot of raw gas gets into the exhaust and could start a cat fire. You'd think that certainly would be no problem at all to detect. Well......

The first time my ignition monitoring system tripped, I sensed a loss of power and the car downshifted strangely, but I kept going for 5 or 6 miles until it dawned on me. I actually had the carpet out of my car at the time, and I finally looked over and there was the green LED glowing brightly in the dark. I pulled over and killed the motor. On restart I had absolutely no problem with the car for about a month. Then it tripped again one day when I hit a bump on the way to work. It did it again a week later. Each of those times, I instantly knew what it was.

So, loss of power from 4 cylinders, which is pretty obvious to say the least, didn't stop me immediately...the first time. Hence, I fully understand why Porsche put this system in. Although dual coil 928s before 89 didn't have the monitoring system and we rarely hear about cat fires, they did and still do happen. One of our members had one on an 86 a few years ago when a coil shorted and he drove on.

After a series of 3 transient malfunctions, where the relay tripped, and upon stopping and restarting the car there was no problem at all for weeks, I disabled the system in my car. That was 3 years ago. I'm banking that I would recognize the ignition failure this system was designed to detect after the experience I've had.

 

I've had 3 experiences with the IMR tripping.

1: I had a bad coil wire ont he passenger-side that would get wet after washing the car. It would dry out and be OKAY. I've since replaced.

2: Coolant got into Cyl #7 becasue of a failed head. Pulled engine and fixed.

3: Just recently, I noticed upon cold start it will sometimes go into 4-cyl mode. I've since determined that the replacement O2 sensor I just put in must be bad as I've received the same engine code twice now, "Fuel mixture too rich." It was fine before this, just replaced the O2 sensor as I think it was original to the car.

 

Okie-doke...

After monkeying around with the left-bank sensor wire and checking the ignition monitoring relay, I went out for a long, spirited test drive and had no problems at all with tripping 4-cyl mode.

The next thing I'll do is check for any codes, especially O2 sensor codes (tomorrow AM on the way to work).

Bill, when I said I think I would notice a true ignition problem, I don't think I made myself clear... I meant that if there was a legitimate ignition problem, I would notice the change in performance it would cause before the IMR kicks me into 4-cyl mode (it's quite obvious that it's in that mode after it trips). My point being that I would notice something was "off" before the IMR would notice something is off and switch to 4-cyl mode. Therefore, I think I have a phantom problem with the IMR or EGT sensors rather than a real problem with the ignition system.

I'll also re-read the info on the exhaust gas temp sensors and see if there is anything else I can do to check them. Unfortunately, the engine will be waaaaaay too hot to test them if it were to trip into 4-cyl mode.

Thanks again everyone!