Q: My car idles up and down after it warms up--rpms jumping up and down about every second between, about, 1600 and 1900.

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This up and down jumping of the idle happened once before (though that time it happened when the car was cold and stopped when it warmed up) and was fixed after simply replacing the idle air control valve with a used one from pick-n-pull. So I tried the same thing again, but two different used IACVs changed nothing. Next when I pulled off the temperature sensor connection the car idled smooth but at high RPMs. I replaced the temperature sensor with a brand new one, but this changed nothing. I'm trying to figure out what to check next. Any ideas? Thanks a bunch.

My car has 250000 miles.
My car has a manual transmission.

This may indicate a faulty fuel pressure regulator or dirty fuel injectors causing a fluctuation in the fuel supplied to the motor and thereby causing the RPM’s to fluctuate. As you know, when you turn the key to on, the fuel pump is activated and the fuel system is primed with fuel at a specific pressure in order to supply the fuel injectors with enough fuel to start the motor. When this is less than what is needed in order for the motor to fire or in some cases higher than normal, you may notice an erratic fluctuation in engine RPMs.

This may also indicate a vacuum leak. A vacuum leak can be one of the most frustrating and difficult problems to diagnose. High idle speed is a common sign and stalling is another. Confirmation of an engine vacuum leak can be a very loud hissing sound coming from the engine area. When a vacuum leak is present in an engine, this causes an excessive imbalance in the air/fuel ratio which creates a loss of power and causes the motor to run at a much higher RPM as a result of the increased oxygen present in the fuel charge being injected into the motor. I would recommend having an expert from YourMechanic come to your location to diagnose your vehicle’s running condition.

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