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Q: Rpms won't drop.

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I recently had an aftermarket header installed along with a high flow cat. I had to replace the o2 sensor since the original was seized in the exhaust manifold. I drove about a week without the upstream o2 sensor installed and the same day I got it put in the rpms won't drop when shifting, almost all the time, occasionally they drop like normal, and just today I noticed an occasional surging idle upon startup. I've also noticed I'm getting very poor fuel mileage. When my original clutch was going out, the fuel mileage was still pretty good. Could it be the clutch? Iac? Was driving without the o2 sensor for that long capable of damaging anything? Is it safe to drive until I can get the issue addressed?

My car has 158000 miles. Current clutch and flywheel have about 10K miles. My car has a manual transmission.

A: It is possible you have a large vacuum leak...

It is possible you have a large vacuum leak or the idle air control valve is stuck in a position best suited for colder weather. A vacuum leak will allow unmetered air into the engine causing the computer will add fuel to compensate for the lean condition. If the idle air control valve is stuck open it can cause a similar situation. Consider hiring an experienced technician like one from YourMechanic who can come out and take a closer look at the high engine speed concern and offer a more personal diagnosis.

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