1990 928 GT with chronic 4 cylinder mode issue

Stan,
I feel your pain!
I had a similar situation with my S4, I tried everything you have done, but still intermittent 4 cyl mode, or sometimes it just quit. Mine happened every 3-400 miles, or 5 miles, really frustrating.
After literally replacing and checking most components, I dismissed the LH computer, as I had already replaced it with a remanufactured one.
After being stranded on I75 south of ATL, I trailered it home and again replaced the LH computer as a last resort, and it has not failed since (almost 6 months). This is my DD so it is driven plenty.
It may not be your issue, but I read you replaced the EZK, but I did not notice the LH status.
Good Luck,
Dave

Outbackgeorgia;6024 wrote: Stan,
It may not be your issue, but I read you replaced the EZK, but I did not notice the LH status.
Good Luck,
Dave

During downtime in the winter, I sent both modules to Rich Andrade for review. He didn't find any issue and used them in a car there.

Thank you for the thought, it was one that I was hoping for as well

I replaced the ground wire from the block to the chassis on the passenger side, cable used was 4 gauge battery wire from auto parts store. I then also cleaned all the battery connections with a wire brush.

I started the 928, and it immediately went into 4 cylinder mode with the IMS relay showing a green light.

I used a timing belt to see if the cylinders connected to the driver side distributor were getting spark. I found I have spark, or at least the timing light thinks so. I moved it between a few different plugs and they all have spark.

The IMS relay showing a green light is in error. I would suggest that my problem is the IMS relay turns off fuel in error *except* the symptom occurs when the bypass relay (eliminating of the IMS relay) is in place as well.

Perhaps the green led can show the opposite, when the fuel injectors are shutdown. If so, I need to determine why.

Hi Stan I'm just reading this post and i have trouble shot this issue before on a 91.
Is you car an Automatic?

If so remove the Trans Kick Down relay the car will function w/o it.

This relay has a smart circuit in it that senses when the car is at cranking speed to start and also the Kick down switch is enguaged, it kills the ground to the injectors to clear the fuel out of the eng. It can fail causing this same issue.

Also if this does not work, remove the CAT protection relay (the one w/the red and green LEDs), put in the three wire jumper to by-pass the relay.

Now when the car goes into 4 cly mode (CAT protection Mode), check if you have 12 volts to both Fuel Injector harnesses.

This comes from the battery by the extra red wires on the + connector, you should have 12v w/ key on.
As the computer switches the ground so 12v should be at both harnesses this can checked right at the injector cyl (1,6,7,4) is one harness the other is cyl (5,2,3,.

If that is not the problem then move to the wire harness inside the car (no power) and ohm the harness from inside to the injectors (with the injectors disconnected) you do not need to cut any wires or boots to do thes tests.

If the wiring checks out, lets go from there.

Hope this helps and was not already covered.

Dave

N2O-928;6036 wrote: Hi Stan I'm just reading this post and i have trouble shot this issue before on a 91.
Is you car an Automatic?
If so remove the Trans Kick Down relay the car will function w/o it.Dave

This relay has a smart circuit in it that senses when the car is at cranking speed to start and also the Kick down switch is enguaged, it kills the ground to the injectors to clear the fuel out of the eng. It can fail causing this same issue.

Not an automatic, a GT, but thanks for those thoughts.

N2O-928;6036 wrote: Also if this does not work, remove the CAT protection relay (the one w/the red and green LEDs), put in the three wire jumper to by-pass the relay.

Now when the car goes into 4 cly mode (CAT protection Mode), check if you have 12 volts to both Fuel Injector harnesses.

This comes from the battery by the extra red wires on the + connector, you should have 12v w/ key on.
As the computer switches the ground so 12v should be at both harnesses this can checked right at the injector cyl (1,6,7,4) is one harness the other is cyl (5,2,3,.

Good information, this is where I am heading next, as I don't believe the problem is in the ignition circuit.

I think I understand, as I just cleaned the battery terminals, there are three extra small bolts on the positive cable end. You are referring to two of these. I am not sure where on the injector harness, etc. you mean. I haven't pulled the covers to see what is under there, but I will do so. If you have a photo or better description, that would be appreciated.

N2O-928;6036 wrote: If that is not the problem then move to the wire harness inside the car (no power) and ohm the harness from inside to the injectors (with the injectors disconnected) you do not need to cut any wires or boots to do thes tests.

If the wiring checks out, lets go from there.
Hope this helps and was not already covered.
Dave

I haven't seen the fuel injector diagnosis covered before, so this will be very helpful, thanks.

Progress report:

Per Dave Roberts of 928 Specialists suggestion, I removed both exhaust temperature sensors, cleaned them, and installed them in opposite sides. His guidance is that if the problem switches sides, then it is a sensor issue, if it the issue stays on the same side, it is a real temperature issue or perhaps a wiring issue, or if the IMS issue does not re-occur it was a dirty sensor…..

A few drives later and no faults yet. Wouldn’t it be too ironic if this could have resolved this so quickly by cleaning the sensors, perhaps a couple of hours for them….

I found that my bypass relay is no longer around, the shop lost at least one other part, provided no diagnosis information, so I guess I should not be surprised. I now have 3 IMS relays…

Thanks to David Roberts, regardless of the outcome on this.