NickT

I am at wits end and need some help.

Symptoms:
From a cold start car starts and runs fine. 750RPM solid. Once the car is up to operating temp, the idle fluctuates up and down a few hundred RPM. If the throttle is blipped, the speed will dip below 750, come up and start to bounce, settle down and start to swing in smaller amount. The ARM-1 display goes from full lean to rich corresponding to the idle speed. There is also popping in the exhaust during deceleration.

What I have done.
- Intake R&R. No air leaks, 100000% sure of this.
- Knock sensors replaced.
- Injectors cleaned.
- New fuel regulator, dampers, filter.
- New fuel lines.
- New Temp II
- Rebuilt MAF.
- New O2 sensor.
- Checked TPS at LH connector and it was good.
- Plugs, caps, rotors and air filter (stock, no K&N)
- 5 year old Nology wires. #2 did not ohm out but does carry spark.
- Did I mention no air leaks? (I know this will come up again ).

- Coils, visual inspection shows no scratches or cracks and seem to ohm out fine.
- Slight hesitation when throttle is blipped.

I did not replace the ISV, I cleaned it and was satisfied that it operated correctly before I put the intake back on. Stable idle when cold seems to indicate that it is functional, but maybe a heat issue ?

I have not looked at the LH or EZK yet and fuel pressure is an unknown at this point.

Car starts up with no problems, seems to have plenty of power and pull.

Any thoughts? Smooth idle seems to be something of a holy grail seeing as there are a number of posts about this.

thanks...

Mike Frye

Are you sure you have no air leaks in intake?

Just kidding, how about the throttle cable, is it adjusted properly? I'm not following the whole part where the rpms drop slightly when you hit the gas, but that seems key to me.

edit: 5 year old wires where one is questionable might be a good place to look also.

also possibly timing? Popping during decel and the hesitation could possibly be caused by this I think.

killav

Next time the idle hunts up and down; pop the hood, walk around to the engine, manually close the throttle with your hand. Just push it back towards the fire wall. See if this stops the "hunting". If the idle switch is not closing, the idle will hunt up and down.

NickT

ha ha your funny.. Yea the wire are next on the list.. Going back to Beru for a cleaner look. Throttle cables seem fine and don't bind.
I did pick up the Porkens 32Vr timing tool and plan on checking the timing soon.

Good idea.. I know it engages when manually moving it (hear the click and ohmed at the LH) but I have not done that with the engine running. But as I said above, the cables seem ok.. Maybe I will try a push on the arm for the butterfly valve (if I can see it).

Thanks guys.

John Speake

Use a diagnostic tool to do a an idle adaptation, it can often sort out a rogue car like yours.....

Aatos

Diagnostic tool forcing adaptation didn't work on my '88.

John Speake

>>>>>>>>>>>>>>>>
That depends on whether there was a problem still remaining. So how did you fix it ?

It appears in the above case that most possible causes have been eliminated, so an idle adaptation may help. But it won't mask a underlying problem.

JP Rodkey

What you describe closely matches my own issue, detailed in this thread:

https://rennlist.com/forums/showthre...ight=jp+rodkey

By description and process of elimination, if you can't get your hands on some diagnostic equipment, sounds like the ISV is likely culprit.

Aatos

John, I unplugged battery for the night and then drove around the following day and adaptation happened. It just didn't work via the tool. vilhuer can elaborate on that.

ew928

Emptied about 1 whole bottle of WD-40 over the course of 1-2 weeks into the ISV. That helped a bit.
Swapping in healthier alternator with new voltage regulator definitely helped settle idle.

Hot idle would drop the old alternator to not so healthy voltage levels. Finally culminating in car dying at idle with lights on.
That's my story.

Hi, my name is Ernest and I own a 928 . . .

NickT

Checked my valve timing this weekend.. Not that, both sides were spot on.

Car does have a huge bog when you blip the throttle. Reminds me of the old double pumpers dumping too much gas in.

I do need to recheck the TPS, I did check it at the LH but I want to verify that (my memory is failing). I know John had a failed one that seemed to check out so I might just put a new one in anyway.

ahhh I just want to drive the car.

fabric

Nick- you have somebody nearby who you could swap an LH with? Sounds like you've tried everything but this, and it's the last culprit.

John Speake

Yes, sometimes that works, if everything is in tolerance. But some cars need some help...

worf928

Just to be 100% certain of what works and what doesn't: Both the LH and EZK get signals from the idle/WOT switch. Check both at both ECUs (you wrote that you checked the LH.) Same goes for Temp-II - check resistance at both ECUs.

The ISV can also be tested from the ECU pins.

mj1pate

I noticed that the MAF was rebuilt. I would suggest as a quick test unplugging the MAF and determining if there is any discernible difference. Quite possibly not; but unplugging mine causes some reasonably broad and varying hunting.

Mike