Fluctuating rpm even while driving

hey guys, I really need your help!

the Supra is acting weird lately.it's a 93 jdm-spec 5 gear manual NA. so here's what happened:

the car is having trouble since a couple of months with idle (normally idles between 850 and 1000rpm, but stays at the rpm), I don't mind since I wanted to take the engine and everything apart over this winter.

unfortunately things got worse. at some point it fluctuated between 1000 and 1500rpm at stops.....then it got even worse and the car couldn't find idle at all. it was all over the place at some point it even stalled.

took the battery off and let it off over the night next morning car runs like a charm, even the idle was spot on at 750 rpm! after some weeks it all began again.

now it got to a point where it's even acting more funny than before.

at the weekend I took it out for a spin, everything was ok except high idle (950 rpm or so), drove about 60 miles and at a stop I noticed the rpm where fluctuating at 1000 rpm just about 50 or 100 rpm up and down.

I visited a friend and let the car cool down for about 1 hour or so, came back to drive home and I still had this strange fluctuations! (of course!!) but I noticed that this is not just happening at idle! it also happens during the driving.

I noticed it within the next town where I had to drive 30km/h which is around 2000rpm or so in second gear. the car was shaking, like someone blipped the gas pedal just for a second and then let it go. I didn't noticed it at higher speeds, but this could be because of the bumpy roads outside towns.

already searched this forum and supraforums.com, even googled this symptoms, but couldn't find this fluctuation thing while driving....only at idle.

didn't check yet for codes, but I did previously and there where no codes, except the speed sensor code which sometimes comes on.

could this really be because of a bad TPS or IACV? I unplugged the IACV while the engine was running and it got a bit better in terms of idle rpm (dropped about 150rpm to 1000rpm) but it was still fluctuating.

When you blip the throttle does it return to around 700rpms? Same symptoms i had. Throttle body was shot

Might be worth checking the closed oil breather circuit non return valve?

Might be worth checking the closed oil breather circuit non return valve?

Yes, common issue when the engine gets tired. Clamp the hose off unless it's as hard as a rock, see if the idle magically stabilises. Otherwise check signals to idle control valve withn a scope (garage job) and if OK test the idle control valve itself with a tester. It may be seized. Could be a worn or incorrectly set TPS. Could be umpteen things.

I had this, at first it was erratic idle, some days I'd start it and it would fluctuate like crazy between 1000-1500rpm constantly and some days would idle low ~ 500RPM close to stalling. I thought it would stop when driving but exactly as you describe it, it was like someone was hitting the throttle then letting go constantly.

Turned out my IAVC had seized up and stopped moving altogether, as Chris suggested. First thing I would do is take it off, give it a good clean and check movement.

When you blip the throttle does it return to around 700rpms? Same symptoms i had. Throttle body was shot

nope at the Moment it will stick to 1000rpm no mather what!

thx guys for all the Kind help!!!! also tried to get some more infos over at suprafourms.com....no one replied . i know it could be the TPS or the IACV, but i wasn't sure if this could also be the problem while driving!

i totally forgot to mention that i had some issues with the AC before....could it be that the AC might be a Problem too? i know that if you press the AC button the rpm will raise a little bit by demand of the ECU.

so here are the symptons i had with the AC:

it went on and off automaticly without pushing the button at all, i noticed this only because the bearing of the AC failed and i heard this strange noice coming from the engine bay. i also have to say that the "defrost windshield" button was always engaged and as far as i know this means that the AC will always be some sort of "on".

what i did then was i removed the plug for the magnetic rotor so the AC won't work anymore (i'll prob. ditch it anyways since i hardly use it at all) and everything was good for some time.

yesterday i checked if the AC still tries to activate itself. so i plugged in the connector to the magnetic rotor and nothing happened!

even tried to push the AC button and turned the AC up fully......still nothing happened.....the rotor didn't engage!?!?!

my question now is, is there a possibility that the AC ECU is shot and always tries to engage the compressor, giving a signal to the engine ECU to raise the rpm????

oh one more Thing just popped into my mind.....i'm always getting random Speed sensor Signal codes from time to time. last year there was again the CEL on so i checked and it throw me the TPS code, but after disconnecting the battery over night that code never came up again!?!?! car ran like normal!

so guys the most mystirious thing happened looks like the problem solved it self????? i don't know why!!!

herre's what i did yesterday to the car: just wobbled on some of the vacuum lines, unplugged them here and there, replugged them....

i noticed the water level in the tank was almost not at zero?!?!?! don't know why.....so i refilled it to full and let it run for a couple of minutes.

it was still fluctuacting like carzy afterwards!

so here's what i did today: i measured all the pins on both the TPS and the IACV. IACV cold and hot! all in specs, except one was close to out 30 ohms (hot condition) which is the upper limit.also measured the TPS as of manual....also here everything in specs except the third value which should be "no continuity" had continuity!! the .60mm feller gauge didn't do anything....i needed a 3.6mm feeler gauge until the "no continuity" status was achieved!

am i doing something wrong? since it's a NA j-spec supra i'm not 100% sure if the values are the same as the US NA ones.

i'm also not getting this : "apply vacuum to throttle opener before testing TP sensor"

IMO there is none on the NA throttle body.....or?

Reset the TPS *BUT SCRIBE IT'S CURRENT POSITION FIRST* tom meet the specs. Some markets may use a vacuum operated throttle closer, but I have never seen one, ignore that.

so here is an update on what happened next (for future references):

took the car out for a quick spin about 15 miles and left it sitting for about 30-45min. came back started the engine and same thing as before!

idle gets stuck at 1000 rpm and fluctuates up and down about 50 or 100 rpm! all the symptoms where back, also during driving condition.

so i drove home and had a look under the hood. again umplugged some vacuum lines and repluged them without any change.

then i took the ohm meter out and measured the IACV terminals all over again.......and guess what! 2 terminals where way out of spec!

they showed 40-50 ohms, the upper limit hot should be around 30 ohms! the other 2 terminals showed values around 25 ohms which is absolutly fine.

so i guess my IACV is gone.....at least once it's hot.

so what i did next was to cool down the valve and see what happens....and the idle got a bit better (i guess it would have been fine if i tried cooling it down more).

but still is might be also a problem of the TPS.

i have never adjusted a TPS, so i took the spare throttle body i have laying around to try this.

i also noticed here that i have continuity between the 2 terminals with the .60mm feeler gauge!

but i also noticed that the throttle plate has some Play in it until it really opens. this is about 2.3mm!!

so i opened the stop screw and turned it all the way until it was firm with the point at which the throttle plate is just about to open.

then i opened the TPS and adjusted it to the point where with the .40mm feeler gauge i still have 2 Ohms (Workshop Manual tells u to be within 500 or less ohms?!?!?!), and the point where i have no continuity with the .60mm feeler gauge.

all the other values where also in spec.

can anyone tell me if this is ok? or is it wrong to adjust the TPS like this?

from what i read on the Internet is that there is a value which tells the ECU "you're not idling now, we are driving", which is read from the TPS......could this be the point with the .60mm feeler gauge?

i'm not an expert on this so any kind of help would be awesome

some update here:

so i bought a new TPS and IAC (planing to keep the car, so no reason not to change out some expensive parts IMO) installed the TB with the new components and well it got a bit better. so what it does now is cold start idles up to 2000rpm and then drops it very slowly over a long time to 1300rpm where it stays forever. so i took the car out for a quick spin and the car felt def. better then before, but still idles at around 1300 rpm all the time.

i know u guys mentioned about getting another ECU, but Supras are pretty rare over here in Germany, yet to find a correct JDM ECU! I could prop. buy one from the UK, but than again i have to spent Money, just to try......well in the end i might have to,but for now i'm still trying several other things.

what i noticed yesterday was that the IAC is "stuck" open at warm idle?!?!? i took the intake tube off and let the car run without it....same thing as before, idle rpm at 1300

and once i put something on the hole for the IAC the car stalls??? another funny thing is that even if i unplug the IAC the car stays at the same idle rpm (1300) should this not effect the cars idle? so i took my ohmmeter out and measured the pins on the IAC plug (harness side) while the car was running and had IMO some strange readings....

all i got was around 78 ohms constant on all pins if i used B1 and S1-S4 . If i used the B2 pin and S1-S4 i didn't get any Signal/ohms ????

that's as far as i got last night. i'm just trying to keep this thread updated for the future . i ran across sooooo many started threads with the same problem, but once the seemed to be close to be solved the thread ends.

really start to think it's the ECU ?!?!!?

I think there was someone selling a 5 speed NA ecu on here, not a bad thing to have anyway if you are thinking of long term ownership. Whole NA throttle bodies sensors and all used to be almost given away too, so might be worth seeing if you can get one of those too.

To me it sounds like you have one or more of the following:

- Massive vacuum leak (get a vacuum gauge on the car and report back with what pressure you are getting at idle, also cross check all your vac lines for leaks etc, there are a few hiding under the Y intake piping)

- I've seen a few NA throttle bodies that get worn over time and lead to a sticky throttle which leads to revs sitting higher. Try rotating your throttle the opposite way when the car is idling high, can you get it to drop revs this way?

- Check ignition timing

I know this is old thread. I also have some issues with high idle. Is this been solved? If it was, what did you do?

To idle high, unless ignition timing is abnormally advanced, the engine needs excess air for the desired RPM, either through the ECU commanding the idle air control valve to open, or the valve to have stuck and be unable to close enough to obey the ecu commands. Otherwise the throttle valve itself is too far open at idle or worn, or there's an external air leak post the throttle valve. I see PCV valve issues causing high idle on N/A's quite often, that's worth checking (block it off).

Thanks Chris I'll try it with blocking the PCV and see if the rpms will lower.

I already changed all the vacuum hoses and cleaned the IAC.

Cheers,

Edvard

Ok, update. I checked all the hoses and PCV. All seems ok. Than I started the car and all worked normal. The idle gradually dropped to 750rpms and stayed there. I'll go on a longer drive on the weekend and see if the high idle and fluctuation will come back.

Also I noticed that the rubber grommet for the PCV is hard as plastic so the PCV is wobbling around. I'll order a new one and see if that will cure the problem.

If that grommet leaks, or the dipstick seal, or any external air can enter the engine then PCV valve may cause a higher idle speed that the IAC valve will try to reduce. I bin PCV valves, they are a bloody nuisance!