mdkelly1

The suggestions you are getting about checking the tension on your throttle cable is sound advice (and it is easy to do).

safulop

I got this info from a Volkswagen mechanic:
Symptoms of a faulty ISV:

1. Insufficient or no idle boost on start-up
2. Idle too low when cold
3. Idle too high when warm
4. (automatic only) RPM dip-and-rise behavior when throttle is lifted for coasting in high gear
5. Idle lower with AC engaged
6. (auto only) Idle substantially lower in gear versus neutral

There is a constellation of symptoms pointing to ISV; when it's faulty it does all these things.

SeanR

What does your mechanic say?

safulop

^^^^^^^^
Not much beyond just wondering why it's doing this because the intake system has been checked twice, and said that normally the ISV system doesn't need to be "rebooted" on a 928 so he was interested that it helped fix it for so long. Might double-check the throttle cable in future, hoping I can just roll with it being a bit faulty for the time being.

This car is going up for sale in the near future so, naturally it should have one of those 928 "quirks."

SeanR

He's right, there is no "reboot" on an ISV, it's a simple mechanism and it either works or it doesn't. Now the hoses around it can cause all sorts of issues if they are not clamped down correctly. Hell, I had one split on my own car after a few months after it being new, took taking the intake off to find it. The symptoms were exactly what you are having.

safulop

^^^^^^ This is good to know, thanks. Might have to dig under that intake after all.

safulop

In my experience, the ISV can partially fail in interesting ways. Oftentimes, people complain that the car will not hold idle at all. I guess this is a total failure of the ISV, that can sometimes be "fixed" by spraying WD-40 down the little tube.

Before I got this new ISV, the previous one was sort of failing. As my mechanic described it, it was too aggressive when cold, and not aggressive enough when hot. The result was, the car would idle as high as 1800 rpm upon start-up, depending on the ambient temperature. Idle speed would slowly (very slowly) settle down to spec over a period of perhaps 10 minutes. Then once good and warm, it acted up in the reverse. Warm starts would end up in a slightly unsteady low idle, occasionally would nearly stall when I stuck it in Drive. So I don't think these things are as simple as "either works perfectly or not at all."

SeanR

I'll defer to your vast experience on this. If it's sticking, it aint working.

FredR

The shuttle of the ISV is vulnerable to accumulation of debris. Many years ago [15+?] mine was doing strange things and I was told by the main agents it was "likely" shot. Did not accept that premise and barrelled into my first inlet manifold r/r. I found squidgy rubber pipes, knackered knock sensors and push in connectors that looked far from convincing. I pulled the ISV and found what appeared to be a dry powdery substance in and around the shuttle that was reluctant to move. Strange considering the amount of oil that was in the lower manifold section. Spent what seemed like ages cleaning the shuttle with clean cotton cloth until there was no sign of dirt pick up on the cloth and the shuttled then seemed to move freely with no noticeable resistance. I have a feeling I put some light lube oil in there before boxing it up. The thing worked perfectly thereafter and touch wood is still in operation today some 100k km later. I dare say it will pack up one of these days but until it does it stays put.

Interestingly on one manifold removal job I did not make the ISV connector correctly [I do not use the retaining clip to make removal easy] and apart from some difficulty during cold starts, found that the detente position the ISV sits at without any control correction works very well indeed and one could run the car without such being operational if push came to shove and you manipulate the throttle appropriately until it warms up.

Thus I believe either it works or it does not and the detente or failsafe position should not be confused with the thing "partially working". If the shuttle happens to stick somewhere in the open range positions the motor is going to race to some extent.

Rgds

Fred

Mrmerlin

This seems appropriate how the 87 on ISV works


https://rennlist.com/forums/928-foru...+control+valve

safulop

Yeah this thing is really interesting, and frustrating. After working great most of the week, today it kicked into the high idle condition again. It kicked in after the car idles perfectly for about 5 minutes. I'm just sitting there idling and... tick... it just kicks up the idle by about 200 rpm out of nowhere, like flipping a switch. When I rev it up, it first settles down to 600 rpm, then bounces a bit before figuring out the amount of support -- ends up close to 1000 rpm after a few seconds. I don't see how a snagged throttle cable would produce that kind of effect. Oh well, the next owner can maybe figure it out.

SeanR

If you are not getting a consistent idle contact on your Throttle Position Sensor this can happen.

safulop

^^^^^^^^^^^^
OK would this also explain why resetting the battery never fails to fix it... for a while? I've got a new TPS in there, so what is it that needs adjusting to get a consistent contact?

Mrmerlin

the TPS needs to be turned a bit more in the off direction or opposite to the opening direction of the throttle plate

safulop

^^^^^^^^^^
OK now, thanks, I have a track to run on. We've come a long way in this thread, started from "it's a vacuum leak" and went from there.