Hard starting when engine is warm only.
2003 Chey. When engine is cold engine starts and runs just great. Only when engine is
has been running and I leave it off for about 5 to 10 minutes does it have a hard time
starting. After starting it runs a little rough but then it runs fine. When I shut it off I can
smell unburned fuel/hydrocarbons.
I would check your fuel pump to begin prognosis, be sure it is fully priming the fuel line and does not make any unusual sounds. Next listen how your flywheel on your starter spins as you turn the key to start your engine (is there hesitancy, grinding, etc?). From your description you mentioned starting, rough running once started, and smell of unburned fuel, after the engine has been warmed up; which is the identifying factor here. Valves are an important part in the distribution of fuel into the engine block, after the engine warms there maybe some blockage or expansion from heat. Be sure to check your distributor cap and other electrical connections, sometimes the slightest electrical sensor misread tells the vehicle it's not working correctly when it really is. I hope this helps or at least gets you started in the right direction
Try cleaning the MAF and O2 sensors. It seems like it's a flooded condition, when starting after a brief shutdown. Like the injector driver from the computer is holding the injectors open. The PCM can't tell if the engine is hot or cold. Cleaning the throttle body may help, as well as air and coolant temp sensors too.
What fixed the problem was replacing the fuel presure regulator.
You must check the alternator under full load as well as check the ground wire both where it connects to the battery and the transmission. Also check the coolant level. If it is alright then you must visit car service station. https://www.yourmechanic.com/cars/chevrolet
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