Can egr valve make 4.3 egine miss fire

100 PSI is way to low, I think you have a burnt valve. Check the cylinder compression on all cylinders, no cylinder should be 5% lower than the highest cylinder pressure measured or the engine will run rough or misfire.

Pontiac Grand


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Disable the EGR valve and test. If issue is resolved you have 3 plugged EGR ports and all the EGR flow is being routed to # 4 cylinder resulting in misfire or the EGR valve is sticking open at times though you probably would not feel a misfire except @ idle & take-off.

P0303....Cylinder 3 Misfire Detected

P0304....Cylinder 4 Misfire Detected

sounds like a new set of HT leads and plugs wouldnt go amiss(fire wires) weird name that fire wires HT stands for high tension leads in proper english .

Recheck the firing order; 1-6-5-4-3-2

On the distributor passenger side radiator to fire wall. 2-4-6-coil
On the distributor drivers side radiator to fire wall. 3-1-5

Cylinder passenger side radiator to fire wall. 2-4-6
Cylinder drivers side radiator to fire wall. 1-3-5

Good luck and also check the distributor cap again for proper seating.

that should fix it,unless you have a burnt valve. hope not

The po301 is a misfire code. The backfire can be caused by a excessive amount of fuel, bad valve timing etc.
My guess is you have damage to an exhaust valve that may have occurred when you had the timing belt problem.A compression test will indicate if you have a leaking exhaust valve.The reason for the backfire is the exhaust valve is still partially open when the spark plug fires.

Pin point the dead hole and check that injector.