My Car’s Rpm moves up and down during idle. What would cause this?

Hi, I have a Toyota Corolla 98 LE and today, my car started this thing: when My foot is off the gas, the RPM automatically goes up and down. It Idles around 800 RPM and then goes up to 1400 RPM, then comes back again to 800, then goes back up and down and do it again and again every second……

It does this…

Vacuum leak !!!
When vacuum leak it will cause the engine speed to go hight. Then the ECU see that it will shut the gas. Then when the RPM go low again it let gas in and the RPM go high again.

There are many things that can cause this. It could very well be an O2 sensor. That’s what it was on my Civic. There’s no real way for anybody to answer your question over the internet. The best we can do is say “It could be this, or it could be that….”. The only real way to determine they actual cause is to physically look at your car.

Are there any codes (Check Engine light on)? If there is, take it to a major parts store and they most likely can read the code, and then that would tell you what the problems are.

i would check the level of the coolant in the car.

could be a faulty idle air control valve.

more then likely this condition will set a check engine light soon and point you in the right direction.

a small vacuum leak can cause this condition also, but would set a code quicker then low coolant or an iac valve.

corroded spark plug wires and possible worned or soaked spark plugs… check them and if corroded or wet replace all wires and plugs… if the problem persists try a compression test, but first go with the easy stuff replace old plugs and wires…=) also dump a bottle of injector cleaner into gas tank every now and then..

Vacum leak, o2 sensor or even egr valve. Try a code reader to start with.

Sounds like a vacuum leak.

You likely have a P0300 code: ramdom misfire

I second vacuum leak. Good luck chasing it down.

I would have to say that you bought some bad gas or your plugs are going bad

idle sensor might be going out.

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