Q: It's runs really bad upon start up, but if allowed to idle for 30-45 min it will run perfectly.

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This is an old truck yes, but it's been constantly overhauled. Nearly new transmission, good compression, and almost every part underneath has been replaced. I recently had a cam and crank sensor issue that I mistook for a misfire. So I replaced everything in the ignition system. Coil, control module, entire distributor( all quality Delco or top dollar parts), wires, plugs... etc etc.. Which also replaces the cam position. Well it ran great for awhile, but then the thing died going down the road and I had to put a crank pos Sen on it to get it running again. Now the problem I'm having is weird and with all components new it's hard to diagnose. Basically it runs like crap when you start it up cold. It has to warm up for 30- 45 minutes and then shut it off and restart it. After that it will run like a champ. Also if your going down the highway on a cold night it will start stalling and running bad again. Maybe a closed fuel loop issue?, but even after it closes upon warm up it runs bad.

My car has 298000 miles.
My car has an automatic transmission.

Hi there. What seems to be happening is your engine is being leaned out when running cold until the engine reaches operating temperature. Check the coolant temperature sensor or the engine temperature sensor. These sensors could have a fault in them causing the computer to think that the engine is warm and at operating temperature. When this happens, the computer leans out the fuel and advances the timing. If you need further assistance with your engine running rough while its cold, then seek out a professional, such as one from Your Mechanic, to help you.

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