Fluctuating RPM's only when A/C is off

MIDHUN

MEMBER
2001 HONDA PRELUDE
4 CYL
AUTOMATIC
345,000 MILES

I have an issue with my car, whenever after I drive and put if in park, the rpm's goes up to 2,000 and then come back to 1,000 and then again back to 2,000 and so on. But sometimes when my A/C is on it stays fine and when I turn it off it comes back again. Please can you tell a solution for this.

3 Replies

KASEKENNY

MECHANIC
18,907 POSTS

This doesn't have anything to do with the A/C itself. When you turn the /AC on it puts a load on the engine and smooths out the idle. I suspect you have a vacuum leak which is basically allowing air to come in and it revs the engine.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If you test for this and don't find any leaks then we need to move onto the idle control.

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

https://www.2carpros.com/articles/idle-air-control-valve-service

Basically this is responsible for allowing air to come in the engine and if it starts allowing too much air it will match that with fuel and the RPM will come up.

Take a look at this material and let me know what you find. Thanks

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Monday, June 8th, 2020 AT 5:00 PM

MIDHUN

MEMBER
13 POSTS

I want to know if changing the idle air control valve will do any help to this problem? My RPM doesn't stay up, it just goes up and comes down again and again when I park the car.

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Monday, June 8th, 2020 AT 11:18 PM

KASEKENNY

MECHANIC
18,907 POSTS

IAC's can fail in different ways. It is definitely one thing that can cause this. It controls the air that comes in when idling so while it is more common for it to allow too little air and cause a stall but I would remove it and at least clean it as laid out in the guide above to see if it changes anything.

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Tuesday, June 9th, 2020 AT 6:30 PM