I have a 96 chevy blazer with a 4.3 vortec rough start usually backfires through the intake then starts and runes for 2 to 3 min changed fuel filter an checked fuel pressure and it is good
Do a compression test on all of your cylinders when it is cold. Backfiring is a sign of a worn valve seat allowing mixed fuel and air (lit by spark) to be forced back into the intake. You will probably see compression loss on one or more cylinders.
Also... there is a chance that the EGR valve is not totally seated when the engine is cold with just a small blow by. The EGR valve seats coke up over time with black gunk.
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Would scan for codes.May be crank sensor,ignition controll mod.
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