Recently rebuilt the engine in my '89 GTUs and have been having some really annoying rough-idle conditions... I've replaced and checked virtually everything that could be associated with a rough-idle and have neared the end of my options.

Well, I had accidentally left the ignition key in the ON position (was checking the voltage at the injector harness plugs and the water-thermosensor just a minute before) when I started unbolting the rats-nest. I had previously bolted the ECU/wiring harness ground to the rats-nest bracket on engine-reinstallation, and when I began unbolting the bracket with the ground attached to it, I got SPARKS. Hmmmm... That didn't seem right, not for a GROUND. As it was loosening the bolt I could hear the fuel-pump relay (?) clicking as the ring-clamp wiggled and sparked. I tested the ring clamp of the "ground" with my multimeter and it was putting out 9V...

This is the ground putting out 9v for those of you who aren't familiar: (AaronCakes picture)



The car has ran OK for about 150 miles, runs and revs fairly smoothly, seems to be low on power and averaged 13mpg.

I'm guessing the multiple ground points for the ECU are offsetting this random 9v signal from *this* ground? (on the s5 there is the additional ground on the UIM and one right next to the ECU, right?)


The question is, what would YOU do after discovering this? Does anyone have any bright ideas as to *where* the voltage could be coming from? Obvious culprits? Is there anywhere else I should be checking with the DMM to find out? Am I totally off-base and missing something here?

 

Whoa... Am I missing something here? This other ground is also seeing 9V:

 

Just pulled up the carpet and checked the ECU pins. Talk about problems!

All of the grounds (3A, 3B, 3C, 3D) are seeing about 0.6V with the ignition OFF and 9.6V with the ignition ON.

I randomly checked pin 3G, which is the crank-angle sensor. The FSM states that it should see below 1V with the ignition ON. It measured 0.6V with the ignition OFF and 9.6V with the ignition ON.

How in the world is this car running and driving? Or again, am I totally missing something here???

 

Do you have high voltage on the ground wires when the key is in the ACC position as well? If so, try removing the 60 amp BTN fuse from the engine fuse box and check for voltage on the same grounds as earlier.

 

Also, did you check the ECU grounds with the plug(s) connected or not? Should be done when disconnected.

 

i think that the 9v is coming from you completing the circuit there for making postive volts come to the ground. i'm sure theres a better way with some diagrams to explain it.

letss see

Battery +---------------(+)load(-)----------------- (-) battery

Battery +---------------(+)load(-)-------- test here u get (+)----------------volt meter--------}This is still ground}(-)--------- (-) battery
i'd hook them(the open grounds) up and then test for continuity(sp?) between the battery's - and +.
You could also just test any ground to any postive for continuity. thats gonna show u signs of trouble.

 

[deleted]

 

Ohhh... Wiring has always been my achilles heel... I just tested what you're saying, and sure enough when I bolted one of the "open grounds" back down, the Voltage dropped from 9.6V to 0V on ALL the ECU "ground" pins...

Wow, I knew I was confused about something... I think the sparks threw me off when I was originally unbolting that rear rotor-housing ground, and made me think there was a problem!

In summary: All "grounds" will see POSITIVE VOLTAGE when UNGROUNDED. Once "grounded" they will register 0V. <-------- I did not know this

So apparently I was totally mistaken about my problem, and this little rabbit-trail into the world of FC wiring was also a mistake. LOL. Why do I always go down it... I was about to go pick up a spare s5 wiring harness at my storage unit.

 

I love happy endings!

 

haha, not *quite* a happy ending as I still haven't discovered why the GTUs is idling like crap. LOL

 

have you set the tps?

then put a wire in the intial set connector and adjust the screw on the top of the intake. or were ever that screw is on s5 N/a's?

Also be sure you have no vacuum leaks ".

 

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Yeah. He had the key on which meant the ECU was getting power and the ground for the ECU is on the top of the engine..............so if you have that gnd unbolted and then touch it to gnd.........you get spark cause you just made the circuit. Normal as apple pie.

 

Just because we all like to hear a happy ending, I solved my idle issue--of which this thread has officially now NOTHING to do with.

I'm not sure exactly what created the fix--but the primary fuel-injectors were replaced, the fuel-injector diffusers were replaced, as was the AIT sensor (engine). The timing was also 10 degrees off, although in fairness to me, for a good portion of my trouble-shooting I had the timing set correctly and the root issue (shaking/vibrating idle) was still very much present.

A note about the timing: Several days ago while in an earlier round of troubleshooting, I was tricked by thinking it was "too advanced" when the *ECU* started retarding it when tightening the idle-adjustment screw on top of the UIM. When I was tightening the screw while watching the main-pulley timing marks with the timing gun, the ECU started pulling the timing. It flickered 10 degrees retarded, and as I continued to tighten it, the timing finally settled, showing the timing I had originally set as being "10 degrees advanced". Therefore I retarded it (adjusting CAS) to match what the ECU was showing me as the baseline. I have always assumed the timing would not change as long as the test-connector was grounded and the idle was below 1000RPM's. Ooops! The ECU definitely keeps working, trying to keep the car running right.

I'm still not convinced that the injector diffusers would make such a significant difference, so the blame goes to the injector(s) for causing the vibrating/rough idle condition and 13MPG--it still seems hard to believe considering they were cleaned/flow tested and unused for over a year, sitting on a shelf in little zip-lock baggies...

Whatever, the GTUs is officially running correctly after almost 2 weeks of troubleshooting... Lesson is, never give up.