rjc1629

iTrader: (8)

So I finally got my longtubes installed and I took the car for a 30 minute drive and everything seemed fine. Two days later when I started it up I noticed a rough idle and it was throwing a P0300 random missfire code. I read the first thing to check was the fuel pressure, so here is what I found...

20-22psi with the key on and engine off
34psi engine on at idle with FPR vacuum hooked up
43psi engine on without FPR vacuum hooked up

The car has been hard to cold start for a while now. I'm guessing the low priming pressure doesn't help. What could cause this and would that be enough to throw the P0300 code? BTW I replaced the opti and wires with new AC Delco replacements a few months ago.

Nostang

iTrader: (4)

Does not sound like a fuel pressure issue as low fuel pressure would likely cause it to stall. Your psi's that you measured seem normal for these cars. I would check or change you spark plugs and make sure all your wires are on correctly. You could have damaged one of the plugs when you did the headers and not known it (if you drop a spark plug and crack the ceramic this could cause misfires). If cylinder #1 is misfiring, check the EGR valve for hanging open if you still have it on the car. Did you hook the car up to a scanner to see which cylinders are misfiring? This could help isolate the cause.

rjc1629

iTrader: (8)

All the plugs looked good and the wires are on tight but I will check again. I only have the little bluetooth scanner and it doesn't have misfire counts. I would think that an individual wire or plug would throw a cylinder specific or bank specific code, but maybe I'm wrong.

rjc1629

iTrader: (8)

Ok, so I figured out how to do misfire counts and it looks to be on cylinder 5 only. Once it cools down I'll check the plug and wire again.

Edit: #5 plug looked fine, good color and not wet. #5 wire looked fine, no arching or burn marks. The O2 sensor (bank 1, sensor 1) voltage drops every time the misfire occurs.

Nostang

iTrader: (4)

Sometimes a plug can look good but still be faulty due to small unseen cracks. I would replace the one plug and see if that helps. Also, it could be a fuel injector on that cylinder. Makes sure you can feel it clicking when the car is running. It is hard to diagnose the fuel injectors with taking them off and flowing them so I would focus on the plug first. If a new plug doesn't fix it, I would swap the injector to another cylinder and see if the misfire moves to that cylinder. That should help you narrow it down. I am thinking it is a combo of the plug and/or wire. Let us know what you find.

The o2 drop voltage is normal during the misfire as you are getting a rich spike which will cause the voltage to go lower.

atlantadan

Normal key on engine off fuel pressure for an LT1 is 41-47 psi. Mine runs 43.5 PSI with the FPR disconnected and engine running. Something is wrong with your fuel delivery.