Chevy 4.2L Vortec Code P0300- Engine Misfire

1997 Chevy Pickup 4.3 Vortec v6 service engine soon light on. OBDII Codes = P0300 Work done prior = customer installed intake manifold gaskets. Done to repair = new spark plugs, plug wires, cap, rotor, cam sensor, crank sensor, mass air flow sensor, fuel filter, fuel press regulator. None of this work has helped.

Question:
I need to know what i could be missing or what needs to be checked next. This truck has good compression, good fuel pressure. It will run fine for a while. You can drive it for 8 miles then it backfires and starts misfiring. I'm at my last wits on this one and any help will be greatfull. We need to get this truck back to the customer as soon as possbile.

Answer: Well, we really need to know what cylinder(s) are missing first so to concentrate on those. Some things that could be causing this problem are- sticking fuel injector after they warm up, carbon floating around in the EGR valve /passage, possibly the engine timing is off. Check to see if the ignition coil is breaking down when hot by having someone brake torque it and spray some water around coil.

If the ignition coil is failing, spark will jump around it. I am Curious as to why the CAM, crank and MAF sensor was replaced. Was there codes stored in the PCM for those?

VISITOR: In regards to the last info i gave you this truck shows to be missfiring on all six cylinders. Number 5 and 2 show to be missing allot more. I slotted the distributer hold down to try to adjust cam retard to 0 BTDC and it still misfires on all six cylinders. Engine has a good vacuum reading and i replaced all the injector poppets with a known good ones. Please help.

Answer:
Don't start messing with the timing like you did. Put the distributor back to where is should be. With all cylinders misfiring, 2 of them more than the others, I would suspect a plugged catalytic converter or poor fuel quality. Did the customer install the lower intake gaskets properly? Maybe he put in the wrong gaskets. I would remove a couple of the spark plugs and inspect them. Make sure they are not burning coolant. You woould see orange depsots if they are.

If there is signs of spark plugs burning coolant, then i would next check that the engine does not have a blown head gasket or leaking intake manifold gaskets. Worst case for this engine is a cracked cylinder head. That is not very common on your truck with the V6, but it needs to be checked to be sure.

The 4.3L Vortec does not really have catalytic converter problems like the V8 engine. So, i would concentrate on an ignition problem. Check if there is slop in the rotor of the distributor. This would indicate a worn timing chain or worn out distributor itself. That would usually cause problems all the time, not just after the engine warms up, but it is another thing to check.

The rotor and shaft should turn slightly by hand. The specification for too much play is about 10 degrees. If there is too much then a stretched timing chain may be the problem, or even a worn out distributor.

Let us know what you find and if you need more help.