when i am idling, my rpms jump up and down sometimes as if the car is going to shut off- rpms as low as 200-300 and can shoot straight to 1500 then back down. normal is 600. i tried some Seafoam cleaner and it kinda helped, but it start to do it again, even while coasting at 2,000 rpms. my next guess would be misfires, thus concerning me about the head gaskets. i had a "major tuneup" done recently; spark plugs, wiring, distributor cap and rotor. also have a fairly new O2 sensor. what's next? fuel lines? (i thought a whole can of seafoam on a half tank of gas would've helped). so should i give Blue Devil sealant a shot? Again, i don't even know the exact problem, but would it hurt my engine to try it?
My car has 240000 miles.
My car has an automatic transmission.
You can try the Blue Devil head gasket repair fluid. The only thing is if the cylinder head has a crack, the gasket repair will not effect aluminum or cast iron. For the conditions that your described, either the Idle Air Control valve is sticking or the Mass Air Flow sensor is dirty or malfunctioning. Check the IAC and see if its clogged or not working.
If the IAC is new or cleaned and you are still having the same problem, then the MAF sensor would need to be cleaned or replaced. Engine idle is controlled by the IAC and the MAF. The head gasket being burned will have no effect on the engine idle jumping around and not being steady. The head gasket will either make the engine have a misfire or cause the temperature to rise up fast. If you need further assistance with your engine idle being erratic, then seek out a professional, such as one from Your Mechanic, to help you.
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