59gene

I had a weird thing happen on my early 1986 (5 speed) last week. I drove the car home from work and when I went to back up it stalled. So I restarted it and the idle was hunting around then it stalled again. A few days later I had a look and found the throttle position sensor plug crumbled and came unplugged. It was not at the switch but where the short harness goes into the main harness. So I checked the TPS and it is working. I then stuck the leads back in the female side of the connector just to make sure the idle was OK and now the idle does not hunt around but it appears to want to idle at ~ 500 RPM. The car then stalls again because the idle is so low. I will order the harness but I would like to know why plugging the TPS back in did not restore the idle. I did a search of the forum for “idle” and did not find any help.

Is there something that needs to be done to reset the ECU once the signal is restored? Any help is greatly appreciated.

MainePorsche

gene,
First post.
Welcome to the Tank.

If your wiring and harnesses are OK, I think I remember that disconnecting the ground strap in the rear can/will reset the ECU if that is your next maneuver.

davek9

Sorry to say no, not on 85/86, ECU's don't have a memory learning mode like S4's.
When key is off, they are powered off, unlike the S4's.

It's hard to say what OP'S idle issue is, could still be a bad TPS connection or a bad ISV, we need some maintenance history and info on his car, pics would be nice too

59gene

Thanks for the info guys. I was hoping it was something simple like disconnecting the battery. Sounds like I need to dig a little deeper. I have the WSM so I will order the harness and work on it after I get that installed.

MainePorsche

Right.
I read over that he was an early 86.

davek9

Cool, I do that all to often too here

Just trying to keep those following along straight and didn't want OP thinking a reset was needed.

Dave

928NOOBIE

If the TPS harness is as degraded as you say, it's likely that pushing the connectors back into the engine harness won't completely resolve the issue. I would guess corrosion of the short harness is impacting signal quality.

These cars rely a lot on the quality of an electrical connection. Making sure all ground points are in good condition is a prerequisite to having a well-functioning electrical system.

A new TPS short harness I bet will go a long way towards helping.

John Speake

These are available from Roger at 928sRus

Speedtoys

If you have the proper crimpers, a length of wire and replacement Bosch ends, make your own.

Otto Mechanic

Having exactly the same problem with my '85 right now. Where is the short harness and did this ever resolve?

59gene

Scott, yes the short harness did fix part of the problem but not all. When the harness failed it caused the idle to be erratic. That in turn cause the air bypass hoses to be worked more than normal which caused that old hose crack and leak air. The final fix for my car was an intake refresh where I replaced all of the vacuum hoses, idle switch & harness, and idle bypass valve. If your car has not had a refresh it likely needs one. The harness in question plugs into the idle switch on the throttle body and the other end sit just behind the test connector (to the left of the oil fill when looking at the engine). Reach out to Roger for a list of parts to do any or all of this job. Also search rennlist for intake refresh and you will see the harness in question. I have put a link below which shows the bad harness.

https://rennlist.com/forums/928-foru...-complete.html

Otto Mechanic

Gene -

Thanks. I have done a complete intake refresh on the car and took it all apart again last September to fix a leaking water bridge. Before fixing the leak the car was running perfectly (though with a bad coolant leak) and when I put it back together it wouldn't hold idle when I started it, it just stalled out. I sent the MAF up to Louie in Oregon for a rebuild and just installed it the other day when it started hunting for idle on startup, then settled at a fast idle, between 2100 and 3100 rpm, so I started doing some rennlist research and found your thread, which lead to my question. I appreciate you taking the time to reply.

Last night I read a different thread suggesting I check the idle adjustment on the throttle body. Sure enough, I'd cranked it all the way right, which I believe is open, when I'd moved the car to Wyoming from sea level almost 20 years ago. Over time the MAF had degraded and when I moved it back to sea level I didn't re-adjust the throttle (didn't need to, forgot I'd done it). Having the MAF refurbished caused it to become a problem.

About 1 am today I went down, checked the idle control on the throttle and found it nearly locked in the rightmost position. I broke it loose to the left, no more than maybe an 1/8th of a turn, and the idle settled right down to 500 rpm on a cold start.

So I believe this is now fixed

Regards,