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My 928 today started exhibiting strange behaviour. Going down the road I noticed it wouldn’t slow when i backed out of the throttle. Pushed the clutch in and saw the engine speed bounce between 1200 rpm and 1800 rpm every 2 or 3 seconds. Got to my destination ok and parked. Upon getting back in the car an hour later, same problem. It will sit there parked and the engine will go from 1200 to 1800 rpm. Normally it idles around 750 rpm. A month ago, i was driving around and noticed it had an elevated idle, 1200 rpm. The problem disappeared the next day and has run normally till now.
looking at threads that relate to fluctuating idle, I don’t see any with exactly my symptoms. There is mention ISV, LH, EZK, MAF, vacuum leaks. I had a rebuilt MAF installed a couple of years ago and also new rubber hoses under the intake. I replaced the air pump a few months ago with a rebuilt unit from Mark A as it was making noise.
The car has 85k miles and has had much care from Greg Brown, unfortunately , I moved to NY from LA 5 years ago so no longer able to take it to Greg’s shop.
The odd thing is the fluctuation in engine speed without throttle input is very regular between 1200 and 1800 rpm. Seems unlikely that a vacuum leak could cause that behavior. But that leaves a lot of other possiblities like ISV and the ignition and fuel computers.The car continues to run well with part or full throttle.
just went out to the garage. It had sat for an hour and it started up as normal but went to 1600 rpm and held, after a few seconds it dipped to 1200 rpm and then back to1600. I can hear some mild popping sort of misfire noise from the exhaust.
Any idea what might cause these symptoms and how to go about trouble shooting?
First see if you can hear the TPS click when you move the throttle quadrant. engine off.
if not or its in question
I would turn in the throttle body cable adjuster one turn,this to reduce the tension on the cable.
then retest .
Also see if the wheel is binding ,
put a drop of oil on the pin that the wheel rides on.
inspect for hoses or other things touching the cable or wheel.
Also did you replace the plastic vent hose junctions that plug into the MAF boot?
These are known to crack and crumble
I do not hear a click when moving the throttle linkage. I am Un sure which cable is the “throttle body cable” to try and loosen nor do see a wheel to lubricate.
Here is a photo of my throttle linkage.
There are three cables, am I looking in the right place, as I do not see a wheel or pin?
The wheel Stan refers is behind the Vee- the throttle cable rides on this wheel and fastens onto the throttle lever arm just inside the vee on the passenger side
ok, I found the wheel and cable that is driven by the top cable. I also now hear the click when I open the throttle, if anything the cable maybe a tad loose, as see about 1/4" of linear cable travel before I hear the click. The plastic Y junction into the MAF boot seems to be intact.
I found a DIY by Dwayne that details a test procedure using an ohm meter at the EZK connector. I found the following:
between pins 8 ands 18 (idle), resistance measured with throttle closed is 17 ohms, when throttle is opened it jumps to 3200 ohms.
between pins 18 and 26 (WOT) resistance measured with throttle closed is 3000 ohms, with throttle approximately 90% open it drops to16 ohms.
Spec is under 10 ohms for idle throttle closed and open circuit for throttle open. My TPS seems to be somewhat out of spec at 16 or so ohms and 3200 ohms, while not an open circuit, is a big jump from the idle value. My problem seems more related to the idle behavior of the switch not the WOT. I don't know the if sensitivity of the circuit in the ESK that reads the switch that it will reliably operate with these somewhat out of spec values. I would appreciate opinions on that. I also am curious if I should tighten up the cable since the movement to actuate the idle switch seems excessive and the WOT transition seems to occur at 90% of full throttle. I would appreciate opinions on that before resorting to replacing the TPS which is a big job. I guess no harm in trying that in any case.
I would not put too much signficance to your meter readings given the difference you actually report. If the idle contact switch was not working you would have no idle at all and the motor would simply stop running unless the throttle was fethered. That you report similar numbers on both contacts is a strong clue that both are working OK and when such switches fail it is usually the full throttle contact that fails.
The above suggests to me that you probably have a significant vacuum leak and the only way to evaluate such is either remove the inlet manifold to examine each and every component carefully or carry out a smoke test which is probably a better approach.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Ignore whether the TPS clicks when off the gas. The TPS will click when the throttle is at idle regardless of whether the TPS actually closes the idle circuit (0 Ohms) You have basically random idle circuit resistance - it should be 0 or infinite Ohms anything else means the internal connections in the TPS are failing. Combine this with a sticking Idle Speed Valve will lead to random idle speed. First check is to see if the MPG gauge always without fail indicates 90MPG when off the throttle in gear and over about 2000rpm.
ok, drove it just now. The MPG readout does consistently show 90 after a maybe a couple of seconds of coasting when above 2000 rpm. what does this indicate in terms of diagnosis?
11-09-2022, 11:59 AM
Fluctuating high idle on a 90GT
