Hunting/Pulsating Idle - ICV?
 

Yeah...tons of threads on this topic - but I cannot find any thread with these exact symptoms: starting with an occasional "hunting idle" - usually after the car was warmed up and after parking for a few minutes, hunted rapidly between about 900 and 2000 rpm. But only occasionally.

Now it does this hunting/pulsating from startup - engine cold.

My thought? Go right to the ICV, take it out and clean/test it.

Other thoughts? AFM? Dirty/defective temp sensor? MAF?

But I keep coming back to the ICV - thinking over the history of this issue and realizing that its been progressive.

Any other thoughts/approaches? Thanks!

PS - car is an 85.5 NA

Remove and clean the aux air regulator (found under the intake manifold) and see if she idles better. That was my issue; I ended up replacing the unit which cured the problem.

If you lift the intake have 4 new intake gaskets on hand. ~$10
There is a very short 2" hose from the intake to the Venturi Tube that may be cracked due to age.
The AAR has 2 hoses that may have cracks. If you remove the AAR, keep track of the arrow. It has to point towards the intake. This is confusing because the arrow will point away from the intake but the "S" hose redirects it toward the intake.

Have a new seal between the TB and the intake on hand ~$4.

Clarks garage says it could be the "Throttle position switch" $~70
Throttle Position Switch - Information, Troubleshooting, Replacement, and Adjustment

Test the AAR for motion with your battery----Search. I think.....the center pin is 12V and you test each side by touching Negative and watch the inside move L to R.. Verify!

EDIT: Hunting between 900 and 2000 is a vacuum leak.

Just a couple of ideas.
J_AZ

Yes, most common cause of hunting idle is a vacuum-leak. The intake-tract after the AFM is sucking in extra air somewhere and that increases idle-speed. Then the ISV closes up to compensate, but too much. Then it opens up again, etc.

Disconnect the ISV's electrical connector and you'll notice that idle will be stable, but too high. Find source of leak and fix it, then idle will drop to normal-levels. Then re-connect ISV.

UPDATE: Tested with ISV disconnected - and still no change. Idle still cycles between about 1000 and 2000K. So my best guess is that the ISV is either is not functioning properly - or its not receiving a proper signal.

I did find a vacuum hose to the evaporative system which had been disconnected at the rear of the intake manifold - but reconnecting this makes no difference.

Anything else I could have missed here? Thanks!

Disconnect O2-sensor and ISV as next test

UPDATE: as my 85.5’s ISV proved to be, even after cleaning it out, mostly unresponsive to electrical stimulation, I went ahead and took it apart (first scribing for alignment, then unbending the metal tabs and pulling out the commutator assembly) discovering, as I’d suspected, that the commutator spindle was pretty gouged out…but more so than I would have suspected, even for a 32 year old car. (makes me wonder even more about the car’s actual mileage!)

At any rate, the three stator pins were pretty worn also…but instead of choosing wisely to trash the unit, I decided to throw a Hail Mary pass by bending these pins inwards and reassembling the ISV - and, lo and behold, the thing now responds very strongly and consistently. Note that I did not say that it actually works! I’ll get things back together and see how how the car actually responds. I have a strong sense that if it does work - this will only be temporary.

Too bad that these ISV’s are now so very expensive new…because my findings leave me feeling that purchasing such a unit used (prices are all over the map) might not be a very wise choice!

probably more accurate to refer to the "stator pins" as brushes.

You do not need the Porsche part to replace these, there are similar parts on Saab, Volvo, BMW, etc. which will work.

OK-944,

I did see your new thread and I do not have experience with the $~70 import aftermarket or the OEM ICV/AAR (auxiliary air valve) on other makes.

Another old thread:
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/767906-icv-idle-control-valve-isv-idle-stabilizer-valve-o-ring.html.

GL
J_AZ

Just put my re-habbed ISV back in - and my 85.5 has not idled this well in years! But I still want to have a replacement on hand at some point, as I'm guessing that my current one will call it quits again...and perhaps before too long.

I'm seriously thinking that when I replace the ISV again...I'll try to reroute its hoses and electrical connection so I can locate it on the outside of the air intake! Especially as I'll likely go with a used ISV - and may need to replace it yet again. Anybody else done this kind of ISV-rerouting?

I have not bothered, I think it'll go 100K before needing to be cleaned and at some point before then I will be rummaging around under the intake for something else.