only occurs at 1st start after being off 6 or more hrs
>once it starts - engine runs smoothly, and will restart smoothly (unless engine is off for a prolonged time)
>the problem: engine/starter turns BUT VERY VERY ROUGH.
>usually will start after a nervous 3-4 minutes
Outside temperature not an issue.
The next time it has sat for 6 or more hours, turn the key to on but not all the way until it cranks - just until the dash lights come on. Listen for a 2-3 second hum probably from behind you. Turn the key back to off. Back to on – hum – off. On - hum - off
Do that about 5-8 times and then turn the key.
If that gets it to start immediately then your check valve in the fuel pump is probably bad and the pump itself may be getting weak. Have someone investigate with a fuel pressure gauge.
I wonder if fuel is leaking from 1 or more of your fuel injectors into the cylinder. This creates 2 problems.
You don’t have sufficient fuel pressure immediately when you turn the key. It takes a few seconds of cranking to bring fuel pressure up to where it needs to be.
1 or more cylinders has liquid excess fuel that can foul the spark plug and/or create a flooded condition that causes a rough start that clears itself after the excess fuel is gone.
I’d pull the spark plugs and see if there’s any evidence of a problem in each cylinder.
You could also perform a fuel pressure leak-down test.
Actually, clarify this: “engine/starter turns BUT VERY VERY ROUGH”
Thanks much to all of you