worf928

Have you electrically tested the ISV from the LH ECU connector pins? (Instructions in my guide linked below.)

safulop

I think my mechanic is pretty on top of these issues, he tested the old TPS and found that, indeed, it was not sending the right signal for the closed position. So now I have a new TPS. I don't know if he tested the new TPS -- yet another possibility for a brand new part that doesn't measure up.

On the other hand, I don't see how the car could idle at 675 rpm when cold, if that was not sending the closed signal.

safulop

On second thought,

a symptom of the car "idling" without the ISV activated is the loping, unsteady, low idle that I was getting when warm-started, before everything was changed out. Now, I am getting an idle that is too high, and this could be due to the ISV being overactive (i.e. sending too much excess air to support idle speed). An idle slightly too high when warm is not consistent with the TPS failing to activate the ISV.

Mrmerlin

I would be checking the throttle cable to quadrant ( the cable that goes around the wheel) it sounds like its just a bit tight

safulop

And this could be slackened, by what method, if I wanted to try different tensions?

Mrmerlin

at the throttle quadrant find the cable that goes to the wheel/ throttle plate,
spin the cable housing adjuster about one full turn into the quadrant, this will reduce tension on the cable,
then see how much cable is used to initiate the click of the TPS by pressing on the ball of the gas pedal cable.

jcorenman

Three key points here:

The TPS contains two switch contacts: An idle switch and a WOT (wide-open throttle) switch. The WOT switch is a couple pieces of bent tin and not reliable, but the idle switch is a proper Microswitch. This is a small switch with two characteristics: It "clicks" when actuated, and it is quite reliable. The chance of a failure (of the idle contact) is low, the chance of an out-of-box failure is right around zero.

However, if the throttle doesn't reliably return to the mechanical idle-stop (because the cable is overly tight) then it won't close and won't click. And the LH won't know the engine is supposed to be idling.

So first, grab the bellcrank on the side of the intake, advance the throttle slightly, and listen for the quiet "click" as the throttle is closed again. Then check for a tiny bit of slack in the cable to the throttle body, and loosen the cable as needed, as Stan explained above. This is critical for a proper idle.

safulop

^^^^^^^^^^^^^
I can play with the throttle and listen for this clicking, with the key in 'on' position without the engine running I figure? Going to be hard to hear the clicking above the roar of a 5 litre hemi.

FredR

You do this with the engine switched off- the click is very easy to hear. that or test at the terminals on the LH plug with a digital multimeter or continuity tester of some kind.

Crazy as it may seem not that long ago someone preemptively replaced an old but still working ISV with a new one and the latter failed to work- Mr Speedtoys if I remember correctly but..?

safulop

Thanks for the tip; I will see about this if the other ideas don't find anything.

safulop

Alrighty now,

what I originally had faith would happen, has finally happened.

The problem has fixed itself. All I did was go away on a trip and disconnect the battery. Today I switched it back on and got it going again, and it now idles perfectly, just as it should. All the previous strange behavior has vanished, for the time being anyway. I drove it around town on errands today in the heat, started it three separate times and it's still good.

You can't fix crazy. I will have to tell my mechanic how this turned out, he will probably not be surprised and tell me to get in to a younger model. These machines are too old for me now, I think.

Adk46

Who knew you were going away? Did you notice anything out of place in your garage? Perhaps you were visited by the 928 Intervention Team.

safulop

One possibility is that my shop actually did put it together all wrong, and then one of the forum contributors took pity on me and fixed it while I was away. To whoever that was, I owe you one.

safulop

OK folks,

after many weeks of trouble-free driving, this ISV issue is acting up again. This morning during my drive to work, the car just suddenly went back to the high idle condition, showing many classic symptoms of a faulty ISV. When I stopped for the day I turned off the battery, since that fixed it the last time.

So this evening I started it up and it was going good for a while, but sure enough once it warmed up it went back into the funky idle. So I'm thinking, I know this brand new ISV is faulty, is it electrical or mechanical? So I'm idleing in neutral at about 1000 rpm and I just gave the gas a little kick up to 3000, just to see how it would settle down again. Lo and behold, that fixed it, and I drove the rest of the way home with a perfect idle.

So I guess that tightens the diagnosis to a mechanical fault in the ISV. I don't see how it could be electrical, with behavior like that.

I'm getting *really* tired of the aftermarket/rebuilt parts quality for this car.

FredR

Your description suggests to me that the throttle cable just might be snagging from time to time so perhaps there is an element in the cable that is starting to fray and randomly snags a little - that or a bit of crud in there. Such would be a bit difficult to diagnose but given the age of our 928's now I guess anything like this is possible any time unlikely as it might seem.

Maybe you can try lubricating the two throttle cable segments with a few drops of light machine oil. As I am aware this does not cause the throttle cable to stick [as can happen with some coated cables]. I used to do this yearly with my late S4 and have not bothered with the GTS given it seems to be working OK.

ISV's on the other hand either work or they do not type of thing.

Rgds

Fred