Murray

Well, I had a thread going earlier about my 89 S4 AT exhibiting a low idle. It was idling around 600 but was smooth. That has now changed dramatically.

Now the car is surging at idle and will stall. It is not dropping really low in the rpms but surges from 500 to 800 at idle. It continues to surge even at light throttle openings. It smells really rich when it is doing this.

So I guess my questions are: is it...

ISV, MAF, LH, relay, something else? and how best to diagnose it?

Andrew, I would take you up on your offer to put it on the ST; but it really isn't safe to drive at the moment.

Thanks for feedback.

Murray

ltdan12a

I'd start with checking for vacuum leaks....

AO

Unplug the O2 sensor and run open loop. See if that makes a difference.

auzivision

Searching for a source of unmetered air is a great place to start.

After that, Swapping MAF with known good one, or just get it rebuilt if you don't know the history IIRC about $150.

ISV, not expensive, but a bitch to get out because it's buried under the intake. I believe once the throttle is moved off idle, the ISV is irrelavent. If that's true, surgeing would cease at light throttle.

LH diagnostic is simple if you have a friend to loan you a known good one.

I hope you get it sorted out soon. Good luck

vsalvato

I did struggle with idle issues and engine stalling on my '88 S4 for literally over one year and ended up changing randomply thousands of US$ worth of parts (rebuilt MAF, LH check, alternator, O2 sensor, fuel pump... among others) and ended up with Hammer Bosch diagnostic pointing at the knock sensors; got the air intake removed, seals replaced and found rodents to have enjoyed wires and cables and the knock sensors to replace. That was the end of an Oddissey... No more idle issues and engine stall since then.

AO

I'm sorry, but the knock sensor has absolutely nothing to do with idle. The knock sensor detects if the engine is pinging and will pull back timing in an attempt to prevent pinging or knock - hence the name. If the knock sensor is "kaput: then the engine will always run at 9�* retarted ignition timing which is a loss of approximately 30 HP. Same with the Hall sensor.

I suspect that your pulling the intake and addressing the issue cured a vacuum leak or a mis-adjusted throttle position switch.

Back to Murray's problem. There are three positions for the throttle position switch (TPS): idle, cruise, and wide open throttle (WOT). As Auzivision points out the ISV will cut out as soon as the you come off idle (that is the throttle position switch reads cruise.) This is why it is critical to make sure your TPS is functioning. With the engine off, you need to make sure you hear a faint "click" when the throttle returns to it's resting stop. This is idle position. Despite hearing the click, you can sometimes have a bad switch that clicks, but doesn't register. A sharktuner or Hammer, or 928 Diagnostic tool can validate this very quickly.

Assuming the TPS is good, you have an issue where the ISV is fighting some issue - either air or fuel. Hence the wild idle swing at idle.

It's most likely, an air leak somewhere, but it's easiest to eliminate fuel issues. I've had (actually still do) a rouge O2 sensor that threw my car into a tizzy. It would cause the car to go crazy. The O2 adjust would go bonkers and it would try to compensate by leaning things out to the point where the car would want to stall. I couldn't figure it out. I kept adding fuel and the car still wouldn't work right. Finally I was maxing things out and it was running better, but that didn't make sense. I unplugged the O2 sensor and everything went back to "normal" I've been running open loop for the past 2 years and couldn't be happier. It's on my list of things to fix, but it's WAY down on the list.

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. The 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.

Good luck.

Murray

Thanks for all the input. I will do a battery disconnect to reset the brains and then see what happens if I unplug the O2 sensor. I have a small compressor, so I can try to pressurize the intake per AO's instructions. I'll keep you posted on the outcomes.

thanks

Lizard928

Andrew is correct on most things. Except that if the knock sensor(s) are dead, or the hall sensor it only pulls 6 deg of timing out.

Now on to your issue, I would be looking at intake refresh, MAF, and O2 sensor.

Livio928

Hi Murray,

I have the DT999 tool that is similar to the Bosch Hammer. I can come and we can hook it up to get a diagnositc report.

Unfortunately, I can't do it until the weekend after labour day. If I can make an evening before then, I will give you a buzz. I know that your car is not running well, but If you can bring your car to my shop before then, I would be more than happy to hook it up for you.

Bill Ball

If the ISV is stuck closed, the idle will be quite low, and the car will stall. If the ISV is partially opened but restricted in movement, it will idle somewhat low, like the 600 you observed. Once on the gas, if the ISV is the only problem, the engine will run fine.

You can try the WD-40 treatment for a sticky ISV, if you haven't already. The rear arm of the large vacuum line on the left side of the intake manifold leads down to the ISV under the manifold. You can unclamp that hose from the fitting and squirt WD-40 down for 10-15 seconds, start the car so it is sucked down into the ISV. Repeat a couple of times. Reattach the vacuum line and see if that helps the idle at all. Worth a try. If you had a Sharktuner, it shows the ISV position signal, which if everything is good, should be about 50%. A stuck closed unit will show a much higher percentage as the LH tries to open it and get more air to bypass the closed throttle plate and maintain the idle.

Now, you report you symptoms have changed and you call the condition "undriveable". Does the engine misbehave when driving, other than at idle? Then it could be an aged/defective (lean) MAF, as well as the ISV. Swapping the MAF from another decent running S4+ can be helpful. Checking the wiring to the MAF plug, looking for brittle, frayed, exposed wires would be a good idea. If the MAF is defective, the LH goes into a "limp home" mode, which only provides a couple of RPM based fuel injector timings, and the car is barely driveable, requiring careful use of the throttle. I've driven some 928s in limp home mode and it is difficult. In that case, unplugging the MAF temporarily makes no difference to how the car runs. With a good MAF, unplugging it should make things far worse.

SeanR

This can happen if either of the hoses to the ISV have split.

PHIL928

Just had this on my gts.

Surging idle, stalling easily. The split in the hose was so big that you could hear the vacuum leak from a mile away. Like a really loud sucking noise.

westija

For more details as how to test the throtle position and the air leak device mentioned above by ao, look under Dwayne's garage - intake refresh - great write up with lots of pix. The device to check for leaks is cheap to build ($25 or so). The throtle tests are easy and quick.
If problem goes beyond idle, yes, you have something else.
Good luck and keep us posted

Livio928

Hey Murray,

If you build one of these devices, I could use it on my intake refresh that I am doing. I am sure there are others in the area who would be interested in using it as well.

AO

I will try to post pics of what I mean tonight. It's pretty simple actually.