Q: Just rebuilt the motor and had an issue with a misfiring, rough running engine when warmed up. It idles great at start up.

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On start up it is strong and stable idle at about 800-1000 RPMS. But after it warms up (maybe 5 minutes) it begins to lose RPMS, runs rough, sounds like a misfire and loss of power... then stalls after another few minutes running.

My car has 218000 miles.
My car has an automatic transmission.

My best guess would be that this involves a crankshaft position sensor. They can do strange things when they are cracked or broken. Temperature has been known to affect the signal strength and clarity to the PCM. It is mounted on the top of the transmission bell housing on the passenger side. It is retained by two bolts and can be a little challenging to get to. If you did not remove it before you pulled the engine, there’s a good chance you bumped it with the engine’s flex plate. Finally, check for codes. If there are not any, I would still replace the sensor. They are known to fail around 150K. If you would like help having this done, a certified technician from YourMechanic can come to your home or office to diagnose your vehicle’s loss of power and assist you with any repairs.

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