fbgh2o

I continue to chase an idle issue with the car. The idle will spike to 1500 to 1600 rpm from time to time. This almost always occurs when the engine has gotten quite hot - i.e. around town driving and usually after (but not always) the high speed fans have come on. Sometimes it will return to normal almost immediately, other times not until after I have shut off the car and restarted it.

I have removed and thoroughly cleaned the ISV (and it was replace about 4 years ago). The TPS seems to be properly calibrated. I am thinking that perhaps that the TPS is failing when hot due to lowered resistance or perhaps it is the harness itself.

Would appreciate any thoughts.

crooster

I'm chasing a similar issue. Idle climbing up to 1800 - 2000 only when engine is hotter usually in stop and go traffic. I have checked for vacuum leaks (nothing obvious), replaced throttle body seals, checked the TPS, disconnected the cruise control cable. I even jumpered the diagnostic plug to disable the ISV and still experienced the problem.

Dougs951S

I once had a similar issue and finally found that the cause was the metal in the throttle cable and throttle body expanding when hot, and pulling the throttle open enough to raise the idle. I had the throttle set VERY tight so that when the engine was cold, the tps would click if you looked at it funny. The car drove great and had razor sharp throttle response, but the aforementioned idle issue when it got hot. One day I decided to test my theory ans after a 2 hour drive I got out of the car and pressed gently on the throttle cam. At once I heard the TPS click and the idle dropped from about 1400 rpm to 950. Its worth checking when the idle goes high to see if your tps is clicking on the closed throttle stop. You'd be surprised how much the idle can soar if the throttle blade is just .5mm off its seat.

crooster

Thanks - in my case the cable has plenty of slack. Since it still occurred for me with the isv disabled I am suspicious of the throttle valve itself hanging up when hot. I made a small adjustment to the hard stop on the throttle to see if that helps.

Question - does disabling the ISV using the diagnostic plug as described in the setting the idle procedure truly close down the ISV valve? Would it be more conclusive to clamp closed the idle bypass circuit hoses?

kent426

Follow the idle set

http://www.lindseyracing.com/LR/Parts/IDLE944T.html

Dan951Man

I created the same problem with my car.
I was trying to ensure all the basics were correct, while searching for the cause of a hiccup during initial throttle input. Testing the TPS-
Both microswitches tested ok, but the potentiometer resistance was not very linear. I tested a couple others in my garage til I found one that increased smoothly, and the microswitches were working.
After I installed it, when the car got warm its idle would act crazy like you say. It would go from 900- 1600+ intermittently. I would find if I played with the throttle (blipping and letting close quickly) it would sometimes temporarily correct the issue.
After getting it **** hot I confirmed a failing idle microswitch with the multimeter. Also double checked all mechanical adjustments were ok.
So yes, the TPS could do this

crooster

I have just ordered a new TPS. Although the TPS on the car bench tests ok and I'm fairly confident it is calibrated right, I have noticed when I get this hot high idle condition that blipping the throttle will usually bring it back down similar to Dan951Man. It's an easy swap at least. Will post results...

crooster

New TPS is in. Initial results look good but I need to drive more in stop and go traffic to be sure it's fixed. The car is now idling below 1000 rpm - it always seemed to idle about 1200 before.

Van

I've seen TPSs not "click" the idle switch when they're hot... but they seem to work when cold. That's pretty easy to get to, so you might want to start there.

fbgh2o

Thanks Van.

I have slacked off the throttle cable a bit. We will see what that brings. Then it will be on to the TPS.

Tom M'Guinn

I'd would:

Reset base idle, with diagnostic port jumper in place.

Check and clean bell housing grounds. (Never hurts.)

Triple check for vacuum leaks. If you already have done that, then quadruple check.

Confirm TPS throttle-closed contact at DME connector when engine is running fast. (Testing when engine running correctly won't help much.)

Physically block off ISV hose (baggy under installed hose to test) if TPS checks out ok.

Check ISV signal (automotive multi-meter with duty cycle) and harness, and/or swap DME if suspect. There was a good thread recently showing which components to change in the DME if the ISV circuit goes bad.

Lex_GTX

So did the replaced TPS fix this permanently?

crooster

Yes - in my case the new TPS totally solved the issue.

TheLlamaGod

replaced dme temp sensor and tps and same time and that solved the problem for me

Chris White

Here is a little info on how the ISV works that might help.
The ISV does not have a 'standard' position like other systems. Many idle air valves default to closed when power is removed. The 944 ISV stays wherever it is until it receives a new signal. The ecu doe not really know how much the ISV is opened or closed, it only reacts to the RPM and tried to open or close the ISV to get to the right RPM. If the ISV is sticking (and it can get gummed up) RPMs may hang either low or high, if there is an issue with the wiring the same thing can happen.

All that being said if the ECU does not see an idle position signal from the TPS it won't try and stay at idle speeds.