IHeartE30s

Hey, new member here. Always have lurked for info but finally have an issue that I cant figure out! I am not the owner of this '90 928 GT 5speed, Im just the poor tech who has to figure out this issue.

So the customer said it stalled on the freeway, car towed in and it drove fine. Then the issue came, it would get to 3000RPM and fall flat on its face. Once in the garage, I could rev to 3000RPM and hear the fuel pump relay cut-out, drop to idle, click and rev back up. All while my foot it on the gas. I pulled a code 12-Throttle Position Sensor so we pulled the manifold and changed it with all the vacuum lines and fuel lines associated. . .many hours later and the issue is still there. Sometimes doesnt even want to run...

I've done a lot of testing but before I loose my mind I figured I would ask the experts on RennList if theres any common issues or where to aim my sights.

Thanks,
Skylar S.

John Speake

When it won't start, check if there is a spark or not. That will help diagnosis.

Stromius

Skylar - Can you check the Throttle Position Sensor (TPS) for proper function? Here's a procedure write up https://rennlist.com/forums/928-foru...re-w-pics.html. About halfway down post #1 to check operation with Multitester.

IHeartE30s

It seems to be operating fine, we have the factory scan tool and it reads correctly on Idle and WOT. Plus we just replaced it with a new Bo$ch unit.

worf928

The fuel pump relay is switched by the LH computer. The LH computer will switch the fuel pump relay on only at the command of the EZK computer. The EZK computer will only command the LH computer to turn on the fuel pump relay if the EZK computer believes that the engine is spinning. The signal the EZK listens to for determining if the engine is spinning comes from the impulse sender that "counts" teeth on the flywheel. If the EZK believes that the engine has stopped turning it will command the LH to turn off the fuel pump relay and will also cease activating the two ignition amplifiers that fire the coils.

Based upon the symptoms you describe, site unseen and with no knowledge of the car's history, if I was going to throw parts at it I would:

Replace the impulse sender. The sender side connector has a lifetime about about 10-15 years. Vibration from the engine at higher RPMs transmitted through the bad connector could cause the normally very noisy signal from the sender to get so noisy that the EZK can't find the impulses from the flywheel. There is a 944 part that is much cheaper than the 928 part. Same guts but the 944 part has a longer lead.

If I was troubleshooting - rather than throwing parts - I would wrap a length of safety wire around the sender-side lead near the harness connector leaving enough length to be able to wiggle the connector once the airbox is back on. With the engine idling, you wiggle/tug the connector with the wire and see if you can make the engine die/sputter/etc. If you can, then the odds are that replacing the sender will fix it. The improbable cause is an internal break in the engine harness branch to the sender's harness-side connector.

I've seen both and either can cause the symptoms you have described.

The connector for the speed sensor was factory mounted in a bracket integral with the throttle cable pulley console at the rear of the engine. Interestingly enough, every bad speed sensor I've ever replaced had at one time had its connector removed from the bracket. If the connector for the sender on the car you describe is free of the bracket then....

IHeartE30s

worf928 - Wow, thanks a million. That helps me understand why the relay is clicking off. I just checked spec for the crank sensor at terminals 7 & 19, should be 600-1600 Ohms (according to AllData) and I get 9 Ohms. . .thats not good. My guess, it has enough inductive capabilities to fire up but when it spins fast enough it looses track and therefore killing the relay.

Hoping new sensor fixes it.

AO

You might also find the LH/EZK diagnostics link in my signature helpful.

Stromius

Missed you replaced the TPS already.

+1 on the CPS. I replaced mine with the 944 part and had to replaced the female end of the AMP/Bosch connector as well due to deterioriation. 928srus.com, 928intl.com, 928gt.com are great sources for those. Simple with the intake off, not so with it in . YMMV

worf928

Good luck with it.

My guess is that when you go to disengage the connector the sender-side connector will crumble to dust.

IHeartE30s

Conclusion: So I got the new sensor in today (shop closed Fri's), and it runs great! Like worf suspected the connector end stayed in the harness, fished it out pretty easily. Now we can all breathe easier at the shop knowing its fixed.

Thanks again for all the help, now we know these sensors fail and what to look for.