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P0300 Random Misfire Code

(1999 Chevrolet Suburban 1500)
in Sugar Land, TX on April 18, 2011
Ok, I'm helping a friend out with the running issue on his suburban, this all started when he ran out of fuel. He filled up the tank only to find that his fuel pump had crapped out on him. So that has been changed along with the fuel filter. Once we got it started, it seemed like it was running on only 4-6 cylinders. The next weekend, I helped him with a tune-up/diagnostic. Changed out the plugs (gapped at .060), wires, cap, rotor, PCV & air filter. I had also spotted/replaced a couple of bad vacuum lines/elbows and cleaned the MAF & Throttle Body. We also tested the fuel pressure (56psi at prime) which dropped slowly. Upon further inspection (removing the upper intake plenum) we discovered that the fuel regulator was leaking. We removed the fuel injection "spider" and replaced the fuel regulator and cleaned the injector nozzles. After reassembly (no nuts/bolts were left over, hehehe), we cranked it up and it improved quite a bit, but it still ran with a mis. I've got a Snap-On Deluxe Scanner so we hooked that up to see how random the misfire was. We discovered that only cylinders #2 & #5 are misfiring. We double checked the plugs/wires and were still getting the misfire. Just FYI, the running fuel pressure was steady at 56psi at idle. I don't find any fuel in the oil/crank case and I know the EGR, IAC is working. I'm kinda stumped. The only thing I can think of is either the injection "spider" went bad in those two cylinders or maybe the ignition control module crapped out. There is no mechanical knock or any metal/metal grinding sounds or valve chatter that I can hear. Any input on this would be appreciated. Thank You.
Displaying 1 answers & 0 comments
Popular Answer
on April 20, 2011
I'm suspicious of the injectors in #2 and #5. I think this is not uncommon for these systems.
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