jesse7flying

I've been having some issues with an erratic idle after coming up to temp. I've had the intake manifold apart twice trying to chase down the problem and all symptoms point to the ISV on my '91 S4. I've cleaned it and tested it and it appears to be working properly, but still, the high idle. I've also followed a couple of the recent threads dealing with this issue and found them helpful. A friend told me about a recent thread where someone used an idle control valve out of a BMW or similar vehicle and had good results, but I confess I did not see that thread. Can someone point me in the direction of that thread so I can see if that helps? Thanks to the group.
Regards==Jesse

FredR

Please expand on your interpretation of "... it appears to be working normally"

The Forgotten On

Did you replace the boot that holds the MAF in place?

It and the hoses attached to it can crack causing a high idle.

Michael Benno

What were the results of your manifold pressure test or smoke test? an air tight manifold should hold 1psi for over a minute.

jesse7flying

Thx for the responses. I tested the ISV by applying 12v from a spare battery and the ISV clicked into the open position. I understand from Michael Benno's thread that the ISV works at lower voltages and opens progressively, but I don't have a suitable power supply so I just applied 12v to it to get a gross measurement. I replaced all hoses when I went into the intake manifold. I did a smoke test on the manifold when I had it out and found some leakage at the top of the ISV where the plastic cap is. I applied some JB Weld and that leakage went away. I also found a slight leak at the throttle shaft, but discounted it, perhaps unwisely.

FredR

The ISV is a PWM device and rather than use a resistor to vary the voltage it uses a process of rapid switching to simulate the effect of variable voltage- as I understand the ISV thus modulates in effect by applying a different voltage to get a different amount of opening and the computer tries to modulate a constant speed of 670 rpms. The GT models about 750 rpms due to the cams.

These things are reasonably reliable but they do not last forever and Roger offers a reasonably priced after market version- FYI.

As for your problem the idle is set by a mix of controlled and uncontrolled metered air and there are limits as to how much modulation can take place. It does not take much of a leak to upset the apple cart as it were and false air leaks are usually but not always the cause of such issues as Michael experienced recently with metered air passing the incorrectly installed throttle butterfly that was subsequently passing more air than it should have done.

There is no need for a different type of ISV - they work perfectly OK when everything is as it should be. These things can benefit from cleaning- BTDT 20 years ago and solved an issue I had at the time- then I had a niggling issue a few years ago and tried the Lowe aftermarket version I stumbled into on EBAY- works just fine and appears to be a very well made item such that I would not be surprised if it is the Bosch item in a non Bosch box!.

Michael Benno

Testing ISV's
The ISV tests conducted in my thread using variable DC voltage proved inconclusive in determining the proper functioning of the ISV. Here were some of the oddities. All three ISV's had inconsistent results for a fixed opening. There were different voltage needs depending on when it opened or closed to a fixed point. These values overlapped from known good ISV's to known bad ISV's. So the only thing I can conclude is the test was not valid. I think you really need to test the ISV with a Pulse With Modulating (PWM) test signal.

Löwe ISV
The aftermarket ISV sold by 928srus is the Löwe unit. The Löwe ISV has a smaller inlet and outlet than the Bosch unit, but that should not affect the performance since the LH will adjust the opening to meet the idle target. There are some people who think the Bosch unit is better calibrated, there are others that feel the Löwe works just fine. I am not sure what to think because I didn't have a reliable way to test them to see if they operated differently. In theory, they should all operate the same because the LH is adjusting the PWM to meet the idle target and the only real limiting factor would be if the Lowe could not be adjusted high or low enough.

My Solution
Fred mentioned one of the problems I had was an incorrectly positioned throttle plate which was resolved and only partially addressed the constant high idle. At the end of my journey, the issue was either ISV malfunction or a vacuum leak at the side covers of the intake manifold. So I cannot say for sure if swapping the ISV back to the Bosh unit fixed the issue, or if sealing a vacuum-only leak solved the issues.

Your Erratic Idle

• Could you elaborate on what you mean by erratic idle? What RPM range is it happening in and how frequently does it change?
• What is the effect of the idle when activating the AC+full fan?
• How big is your vacuum leak, can you pressurize your intake to 1psi, and the air leaks slowly? Or does it not even build pressure? The larger the leak the bigger impact it will have on idle.
• What is the status of your MAF. Is it original or has it been refurbished? The OEM units tended to lose accuracy with age. The refurbished units replace the components that degrade with age with components that do not degrade the signal with age. You can send it in to have it tested for free if needed.

The vacuum leaks from the bearings on the throttle and flappy are small and one would think it would not have a significant impact on idle. However, I know others have found replacing these bearings has fixed the high idle issue. So it may be worthwhile to replace these.

Also, consider there may be a leak that only exists under a vacuum and does not show up in a smoke or pressure test. The intake side covers is a place where this could happen so if you do go back in, it's worth adding some appropriate sealant DC111 or Locktite Loctite 5970

Hopefully, this is useful for you.

FredR

Mchael,

The Lowe unit I have is a perfect fit in the stock hoses but for those hoses I have always used a jubilee clamp lightly nipped up- given they are invisible under the inlet manifold it is of no consequence.

jesse7flying

Thank you gentlemen, I really appreciate your input. I have gone through my intake manifold twice trying to eradicate any vacuum leaks, replaced all the hoses and vacuum lines with Roger's kit. My problems started with a hard to start when hot condition soon after I got the car and developed into a hard start when cold and dying shortly after starting. In the process of tracking down those problems, I found broken/ corroded wires at the Temp II sensor, then the same with my FOE harness and 14 pin connector. I have corrected all those problems. I had the LH and the MAF done by Rich Andrade. After two tries cleaning the ISV with no real success, I found that Roger has the Lowes units back in stock and I have purchased one to replace the Bosch unit currently in the car. The idle bounces gently between 800-1200 rpm when I get to operating temp. No real problem when the car is cold. I will eventually prevail over this problem, but in the meantime, it sure has been fun, lol. Thanks again for the responses. I will update when able, although I am under post-surgical restrictions and won't be able to lift anything for a couple months.
Regards--Jesse