Dear all,
still in my quest to bring my 928 (1983 928s, with a 88 S4 engine swap) back to perfect running conditions. Now the car is presenting a hunting idle, that oscilates from 500 to 900 rpm, in 1 second intervals. It starts ok, cold idle is ok, but when the car reach operating temp a small hunting starts (few rpm's floating in the beginning), oscilating every second, but them it grows to the 500-900 scenario, up to a point that the car eventually stalls. This failure happens ocasionally, like once per day. Noticed that when the AC is on the oscilation gets worse.
Already replaced Temp II, filters, ignition wires, plus, caps, rotors. Connected the vitivac and I have a steady 15 mm of vacuum reading at idle (is that too far from normal? My city is 3000 feet altitute).
Had an intake manifold service done few months ago to replace the knock sensors. The hunting was not detectable before that.
So,could this hunting idle connected to the vaccum leak? Considering the target 21 mm of vaccum at sea level (i believe is the standard, right?), should I look for small leaks at vaccum hoses or could this be a big leak in he manifold? Thank you all and best, GM.

 

It could certainly be a vacuum leak, possibly a large one. I had the same problem on my '85 a couple of years ago that was caused by a large rupture in a vacuum line directly under the manifold ( https://rennlist.com/forums/928-foru...uum-lines.html , see post #11 for a picture).

Of course it could also be other vacuum lines, but a hunting idle seems to be the dominant symptom of a vacuum leak.

 

Start by checking the idle air valve?

 

Try hitting the Idle Stabilizer Valve(ISV) with a few long sprays of WD-40. There are instructions on the forum.

 

NC928S4, tried the ISV WD40 trick, with no change. Actually, the ISV was tested ok during the intake service, but did the trick, because I didnt replace the ISV at the time so might have failed in the last months. But I believe this should be and vacuum issue, not only for the low vacuum reading, but also the transmission shifts are a little more rough (another symptom of vacuum). Tried almost every line, and saw no pressure (0) for the line that goes to the pressure regulator. So I may have found the source of the vacuum leak, but this may also have other implications for the engine. The problem is that this line goes under the intake, so, big service ahead to fix this!

 

I had a hunting idle on my 82 after warming.

I changed the O2 sensor, which was ancient and all is good now.

 

Another possibility is the mass air flow sensor. There are several different things that can cause them to hunt. I did change out my O2 sensor first (simple, fairly cheap), then pulled and tested the MAF, finally did the vacuum/intake refresh.