Causes for MisFire Code
 

OK I have a 2000 Chevrolet Silverado w/ a 4.3. It has a Random MisFire Code for cylinder #3 (P0303). I have installed a new cap and rotor, fuel filter, spider injection, converters, swapped plug wire after clearing code - still shows #3 misfire, compression is good on that cylinder @ 165 PSI, the fuel pump is right at 60PSI with key on engine off and drops to around 55 running - the pump is extremely loud though, the plugs were installed a few months ago (I am still going to swap out 2 plugs to see if it stays #3 misfire)...what am I missing???? The loud pump concerns me but I think I would get a random misfire not just #3 if the pump was bad right? Oh yeah, that head just had a valve job done too. This thing is driving me insane!!!!!!

Hey take your valve covers off and run it like that. I think it was you who had that problem with the rocker arm seat broken.

The only other thing I can think of is a worn out lobe on the cam.

Yeah that was me, the weird thing is that #1 was the affected cylinder by the broken seat. I had zero compression on that cylinder. I now have plenty of compression on all cylinders. The bad thing with this situation is I bought the truck with the compression issue and have never heard this truck run right. So I am sure it has/had multiple issues. The truck idles great and if you take off slow on a flat road it runs decent. The moment you try to either take off fast or especially climb any kind of incline it starts running like crap. Yesterday I also got a #4 misfire code and a random misfire code, but after clearing them they haven't come back. The #3 misfire has been there and keeps coming back. I don't know why but I think it is fuel related (I guess the SCREAMING pump has something to do with it) and it is just barely within spec. It is supposed to be between 60 and 68 with key on and engine off and it barely makes it to 60 after a few cycles of the key. Am I right that if it was the pump I should get more than a consistant #3 misfire code though? I am going to try your idea of running it with the valve cover off today and see what happens there. Thanks for the input!

The fuel pump making noise is not really a bad sign.....if it has been replaced then the "whine" is normal. I would also run a couple of cans of injector cleaner through it. if this was a fuel related issue you would get a lean condition and not a misfire. The misfire indicates it is an ignition problem, you said the head just had a valve job right? Then check your compression and change the plug wires.

OK it is now running great...A spark plug that was only a couple months old took a crap. I swapped #3 (where I was getting the misfire code) with #1 and the code went to #1 misfire. So I went and bought one new plug and BAM truck runs like a champ! The weird thing is that it idled real decent with the bad plug in the #3 hole, but when I swapped it to #1 it had a definite miss at idle. Thanks to everyone for your input!

Sweet find!!! Sometimes it just takes a little patience. Im still learning that. :)

That's awesome. That truck is slowly coming together!