RPMs drops when letting off gas only at 35-40 MPH

RISNER08

MEMBER
2005 SCION TC
2.4L
4 CYL
2WD
AUTOMATIC
193,000 MILES

When I let off the gas at 35-40 MPH only the RPMs will drop below where it would idle then come back to idle. Example: if I let off the gas and it should drop down to 1,500 RPMs and idle it drops down to about 1,100 RPMs then comes back up to 1,500 RPMs and stays there.

1 Reply

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi,

This could be a few things. Actually, the idle speed should be approximately 700 RPMs. If it is going very low, the first thing to check for is an engine vacuum leak. An engine vacuum leak can cause a rough idle, stalling, or lower than normal RPMs. Here is a link that shows how it's done:

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

If there are no leaks and the check engine light isn't staying on with the engine running, it could also be related to something as simple as the throttle body needing to be serviced. Here is a link showing how that's done:

https://www.2carpros.com/articles/throttle-actuator-service

Let me know the idle speed the engine runs at when sitting still. Also, let me know if the check engine light is staying on with the engine running.

Take care,

Joe