Ryan9551s

Hi everyone,

This is my first 928 post, but have gained a lot of info from the forum.

Anyway, I picked up my ‘87 S4 a year ago. It had been sitting for the better part of 5 years at the mechanic shop when we struck up a deal and he had it delivered to my garage.

At the start, it didn’t even pretend to start. After hours with a multimeter, I determined they had installed an incorrect temp 2 sensor. $9 later, and the car was really trying to start, but wouldn’t. Bit more playing around, and I sent the injectors to get cleaned and tested. 7 cleaned and 1 rebuilt injector later, the car fired right up. Oh YEAH!!

But it wouldn’t idle, and would stall immediately. After trying the WD40 trick to try to loosen up a seized idle control valve, I got a used one, installed it, and now it starts AND idles. I started feeling like I’m getting greedy here!

The car runs and drives really nicely. The mechanic had just installed a new Bosch MAF, new fuel pump, and I did all the fluids and filters before starting to drive it.

The last remaining drivability challenge I’m facing is a pretty massive rpm surge. The car will surge in park from a normal idle to about 2,500rpm. Sometimes it will hunt back and forth, sometimes wit will stay at 2,500 for a minute, sometimes it will idle perfectly for 5 minutes. It happens cold, hot, medium, driving, idling ... there’s really no rhyme or reason to it. When I’m driving it will accelerate/decelerate as it hunts, and if it’s idling high at stop light I can hold it with the brakes but it’ll be idling at 1,200rpm instead of about 700. It seems as though something is just telling the ICV to go full open and creating a massive vacuum leak, because with the air filter off it just generates a huge sucking sound through the MAF.

I changed the throttle position sensor thinking that might be it, but no change. I tried a different EZ-K, with no change. I just had the LH looked at by an electrical engineer friend thinking there might be a crack in a solder joint, but he looked very carefully and spent lots of time cleaning, but no change.

Honestly, I’m now at a bit of a loss where to go next so looking for advice. It has a new Temp 2 sensor, MAF, injectors are good, ICV is known to work, I’ve had 2 TPSs on it. When it doesn’t hunt, it runs great. Pulls to redline, idles perfectly, drivability is just fine.

To me it seems like something electrical is telling the ICV to go full open, but I can’t think of what it could be. Of course, it could be something else as well, but it just seems as though all of a sudden a massive vacuum leak appears which causes the rpm to spike.

Any thoughts from anyone on where to go next?

Thanks for any tips folks, I really appreciate it!

FredR

By the sound of it you have had the inlet manifold off twice- once when you did the ISV and then when you did the throtlle posiotion switch.

Given what you have described, the first suspect I would be looking at would be the ISV outlet hose. Changing the ISV is one thing but carefully inspecting the hoses for a crack is another- if you did a careful examination and pushed and pulled the thing and found nothing then we need to look elsewhere but if you did not follow such protocol then that is where my initial focus would be.

Ryan9551s

Thanks Fred,

i was trying to avoid removing the intake again, but I’m leaning towards doing that. I’ll hunt for a vacuum leak the best I can with the intake on the car but if I don’t get success, I’ll pop it off and installed a new ICV and vacuum hoses to be able to rule that out once and for all.

The ICV I installed was a used one, so perhaps it’s failing.

Thanks for the advice, I really appreciate it.

worf928

Download my inspection guide (URL is in my signature block).

It describes a number of tests that you can perform with a multi-meter at the ECU pins. These tests will help you rule-in or rule-out a number of items.

From your thread-opening post, it seems like you are going about this 'one part at a time' which is fine but not generally how "we" approach systems like the intake. Nevertheless, here's what I suggest:

1)
- Test the TEMP-II sensor at the ECU pins (see my inspection guide) Yes, I know it's new. I don't care and neither do the ECUs. They only care about the NTC curve they see at their end of the engine harness.
- Assuming it tests well, fabricate a ~2-foot-long ~14 gauge lead with alligator clips at both ends. Provoke the RPM surge and then attach one clip to a ground (engine brace bolt will work if not rusted) and then touch the other clip to the (hex) base of the Temp-II sender and see what happens.

2) Do a smoke test of the intake and see if you have 'vacuum leaks.' Smoke testers - that actually work - are available for a couple of 100. Or, pay a shop and watch the test being performed so that you see exactly what happens (or not.)

The Forgotten On

I would run the car and spray some ether around the intake to see if it raises the idle. If it does, you've found your problem.

I had pretty much the exact same issue and it was broken and cracked hoses under the intake.

Petza914

+1 on the smoke test. Best way to find even very small intake leaks.

Shark2626

Disconnect the O2 sensor (located on the passenger footwell in front of the CE panel) and then disconnect the battery for a few minutes to reset things.

Unplugging the O2 sensor eliminates any extra adjustment being made by the LH. If it still goes crazy, then it's more than likely a vacuum leak (although it could also be a bad fuel regulator or damper - but those are pretty low on the list of common failures).

If you have an air compressor, you can go to your local Home Depot and get a PVC end-cap. I forget what size, but if you take your MAF with you you can match it up. 4" I think. Then drill and tap it for an air fitting. Apply 15 psi of air and listen for leaks. You will find your air leak pretty quick and you can then fix the issue. Minor leaks around the throttle body are not of concern. You want to find BIG air leaks.

jschiller

Dwayne to the rescue:
https://rennlist.com/forums/928/4826...re-w-pics.html