Why is my engine revving up and down at idle?
 

This started a couple weeks ago.I got out of my truck at my sons house and the engine was running,it started revving up and down nonstop so I shut it down and restarted it and it stopped.It didnt do it again for about a week or so later.I thought it might have something to do with my upstream O2 sensor hanging off the exhaust and also my exhaust being in rough shape.Last Friday I bought both upstream and downstream O2 sensors and had a shop install them as they replaced mt exhaust from the Y pipe back.Cost me $600 .Now the thing with the idle revving is still happening.What could it be? 95 4x4 3.0 automatic tranny.

how's your coolant level ?

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Coolant level is good.

slacker may be on to something, that same problem was happening on my 1986 toyota fx and I have read online that there could be air in the coolant system. So I took off the radiator cap and let it idle for a good 30 mins and symptoms were gone. May not be your problem but it's worth a shot.

CEL? Maybe tps could gone bad. Check for codes

Edionne13 whats tps?Throttle position sensor?Where is that located?My cel was onbefore I fixed the exhaust and now its not on so I have no codes to read.Its also running at a high idle,and now at least once a day its doing that fluctuating idle revving thing.I need to fix this asap.I hate when I cant diagnose a problem cuz then I cant fix it.How can I test the tps?

What about a vacuum leak? Watched a video today (either here or pirate4x4) about a vacuum leak near the intake causing idle to fluctuate, and it'd be an intermittent problem.

It could be a vacuum leak when I suspect those I get some carb cleaner and spray it around the suspected areas. If the motor starts to race you found it. Also just because the CEL is not on I think it can still store codes. I wouldn't hurt to put the jumper on the diagnostic port and see. It's an easy way to try find a problem. Also I had a 22re that would do that with low coolant. Try parking it on an incline with the motor up on the high side pull the radiator cap start it and let it run for 30 min or so.

Had the same thing going on and my radiator had a sneaky leak that was getting worse, after getting the coolant level good most of the surge quit but now I have a surge during warm up at 1000rpm, weird thing is it's only with the brakes on? Hmmm weird gonna search a bit on it but the surge at idle after warm up was coolant level:)

Leak in the break booster line?

Kinda my thoughts but I have traced everything and it's all leak free, not sure on the internals of the booster though? Wondering if it could have some sort of internal leak I can't hear?

If it is idling above 1000rpm when it is almost warm, it may be a result of the computer. I had this issue and it turned out that i just needed to adjust my idle down a little. The computer cuts fuel when you are braking and your idle is above 1000 to help you brake, so when you press on the brake and your idle is above 1000rpm, it cuts fuel to drop it, and then adds fuel to not stall out ... rinse and repeat.

Not sure if that made sense, but maybe try adjusting your idle down a touch and see if it helps.

Should be right at 850 afaik

Edit double post

I have heard of break boosters having cracks that can't be seen. Pretty rare but possible.
Here is a thread on diagnosing a break booster problem
https://www.yotatech.com/forums/show...060&styleid=51

Maybe could be your VAFM if your running cold air intake by chance could have messed with it. Doesn't throw codes. It happens since the oil used can screw it up. I've done it to my 3.0 I've read on here throttle body needing cleaning mine will idle high during starts but after will go back down once warm. Doesn't sound like your issue. Coolent level radiator if leak can fix for little time with ground pepper in radiator. Yes it works

Ok guys,I have the day off today cuz its raining.YES.I will check for vacuum leak and I think Im going to just buy a new TPS and install,I will also check throttle body for grime.I will also do a diagnostic test by crossing Te 1 with E 1.I replaced the head gaskets and every other gasket on the way back up last summer,wow did it alone,what a project.Before I did that I ran it frequently low on coolant and never had these symptoms.
Thanks guys,if I forgot something or any other ideas let me know.I hate driving my truck with a flaw.

So far today Ive checked vacuum lines,all good.Topped off coolant.Took a diagnostic reading and got a code 21 and code 27.Those are O2 sensor readings correct?I just replaced those 5 days ago.Makes no sense to me.

Describe a little better what it does??

Now my truck will idle up and down as the AC compressor kicks in when the defrosters and blower motor is turned on.

Could it be this ?? Up when the compressor pulls in comes down when it goes off.

Or is it like some one is just stomping on the throttle ?/

Just how much of a difference in rpm??

Well I have an automatic tranny and no tacometer so I cant give an rpm range,and I have no ac so no compressor load.When idling the throttle oscilates up and down kinda quickly.If i push the accelerater it stops.Another words it just does it when I take my foot off the gas.I think I noticed it doing it on the off ramp coming home yesterday.I think.Been raining all day here and hasent done it yet,dont think that means much.