polcan

Subject 1988 928 S4
I.m chasing low idle problem on my S4. Problem occurred over night , not slow deterioration. When cold engine starts right away but idles very low, sometime idle is very erratic.When bench tested idle control valve worked properly,cleaned and lubricated back in intake and no difference.Still same effect. Did resistance test on Temp 2 sensor in engine bay and at LH ,proper readings ,TPS test ( Dwayne intake refresh ) proper reading.Am i missing something ?
My question is :
With Stethoscope should you be able to hear idle control valve opening and closing at idle speed ? when bench tested it was pretty loud
All i hear is "vacuum air sound " at idle and goes away with throttle open as it should (vacuum lowers with high rpm) or i got this backwards?
There is no clicking in idle control valve( at idle rpm ) ,kind of expected same sound like injectors
Just trying to make sure before ordering new parts .
Thx in advance

FredR

Based on your description I would think you may have a problem with the iSV not responding as it should.

First thing to check might be the cables supplying the control signal and the connector at the ISV if you have reason to believe the ISV is functional. If you can get a stethoscope onto the body of the motor you should be able to hear it whirring.

The system is designed such that if the ISV is non functional the thing passes air to keep the idle going when warm. Thus cold starts in this condition result in the motor firing and dying unless you fether the throttle until it gets warmed up and then it stabilises after a couple of minutes or so.

The motor pulls the deepest vacuum when the throttle is closed and the revs are lowest.

Rgds

Fred

polcan

Hi Fred
My bet is on faulty ISV too ,just trying to make sure,they not cheap. How do I check ISV harness ? Connector is good for sure .thx.

davek9

On an 88 you need to remove the Intake to get to / replace the ISV, what did you replace while you were in there?
I'm asking if all the rubber bits looked sound and flexible?
Also how does the engine idle in park/neutral when hot and what is the RPM?

IMO that is a lot of work to put an old ISV back in, just saying.

Dave

FredR

I always found that trying to fit that connector was a sod of a job and getting it off nigh on impossible so I took the spring clip off taking the acceptable to me risk that it might come undone. One day after refitting the manifold I had the strange idle behaviour I wrote about- cursed thinking the ISV had gone south- was about to remove everything again and at the last minute double checked the connector and felt it push home more solidly- problem solved. Has not moved since.

If you have the original ISV fitted chances are it has failed. There is a test procedure in the WSM that needs an oscilloscope [as I seem to remember]. Presumably if you have had the manifold off recently you will have checked/replaced any soggy or suspect hoses under there. Even then the hoses that carry the ISV may look good but suddenly let go but at the moment based on your input I would think that disgnosis is not likely to have occurred.

Like Dave said- how is the idle behaviour when warm? As I alluded to in my original response on my motor when warm the idle was like normal with the a/c off and with it on the rpm's sank to the point of collapse but I doubt you wil be using the a/c at the moment [incidentally always good to run the a/c in winter time for a few minutes once a week to keep the seal faces lubricated].

Rgds

Fred

polcan

Hey
Idle gets much better when warmed up.I know when ISV works properly just by watching tach. gauge.
Rpm drops fast to about 950 and then goes dawn slower to 650-700 steady .Now most of the time drops dawn fast to 450-500 when i think LH adjust mix to bring it back to 650 ish,engine a bit erratic <---IVS faulty.
Salute

Mrmerlin

FWIW with a good flash light you can see the ISV connector through the intake manifold.
look from the top down through the small hole in the rear of the intake,
its lined up with the ISV connector. Once you can see what your hand is doing it makes working on the connector easier.
I use the long needle nose with the curved tips that will fit a hose,
these grip the connector pretty well and if the MAF is out of the way its easier

Here is the light I use, worth every penny, it comes with two battery's and recharging dock
we have 5 of them in our shop.
I got one for the home, the mag light cant keep up with this.
NOTE dont buy a black handled light you will lose it


https://www.google.com/shopping/prod...Dy4Q8wIIiwQwAQ

worf928

You can test the ISV from the LH pins. Instructions in my inspection guide (link in sig.)

You don't hear the ISV 'click' in normal operation because it gets pulsed with a square wave ground signal that meters the opening according to the duty cycle of the signal. You only hear a click when a constant ground connection drives it to full-open; the click is the sound of it hitting the stop.

worf928

If the idle is steady and in range after the car is warm, and your ISV isn't 'sticky' when cold - you did write that you cleaned it - then that means that the O2 loop is compensating for some other condition. The O2 loop kicks in after a minute or two. Before that the control of idle is "open loop" and depends only upon the amount of fuel and air entering the engine. The ISV is sent a signal that keeps it partially open but at a 'base' setting for an engine getting the proper amount of fuel and air.

polcan

Thx all.
Did one last test. Disconnect battery,disconnect maf,connect battery,start engine. Well It did start but very erratic,very low rpm,as soon as throttle applied will stall. It not supposed to start ,,,,!!! ,but at least I know maf is good.
Salute