PO300 / PO307 Misfire, (Catalyst Damaging type), Can't track it down
03-22-2006, 08:57 AM
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PO300 / PO307 Misfire, (Catalyst Damaging type), Can't track it down I have searched a couple of forums, can't find any information that seems particularly useful, but this is such a "could be anything" SES code so that doesn't surprise me. This car was just purchased, light has been on the whole time. 1997 LT1 Ws6, stock, 85k. Since it wasn't on my dime I took it to the local pontiac dealer right after I purchased it, they told me it was PO300 and that 4 of my injectors needed to be replaced (1,3,5,7) because they failed a balance test. They wanted about $1,000 to do the job and replace my spark plug wires, which looked original.
I bought a used fuel rail and injectors (stock, 8k miles on them) from someone on this board, installed it, car seemed to run a little smoother, but it didn't get rid of my SES light. I still had a little bit of a stumble in low RPMs with a surge at about 2500. I replaced all of my spark plugs one weekend with Champion Platinums, gapped at .055, the old spark plugs looked ok, one of them (maybe even the #7) had a little bit of carbon build up on one side. No improvement. I ran out of time so the next weekend I replaced the spark plug wires, MSD 8.5s, car ran great afterwards, stumble and surge both gone. SES light didn't go off by itself, but I didn't clear it manually.
I don't know how far I really drove it, say 10 miles that day, the next morning I start it up and I'm driving down the road and it starts to miss pretty bad, my SES light starts blinking (which means that I have a misfire severe enough to damage the catalytic converter). So I borrow an OBDII scanner from Autozone, it reads out PO307 and PO113 (IAT high voltage). The IAT problem is just that the sensor is unplugged, I plugged it back in and reset the codes, it doesn't come back.
The PO307 reading I get when the engine is on, if I turn the engine off (key in on position), it reads PO300. That part has me confused. I checked the plug and wire, they're not loose. I haven't pulled the plug back out yet to check it / regap it, but since the car ran fine immediately after the plug/wire change I don't think that's going to be the problem.
So, engine misfire, may or may not be isolated to one cylinder, replaced fuel rail/injectors, plug/wires, no stumble or surge anymore, just runs real rough and I would say there's a loss of power. Suggestions anyone?
I bought a used fuel rail and injectors (stock, 8k miles on them) from someone on this board, installed it, car seemed to run a little smoother, but it didn't get rid of my SES light. I still had a little bit of a stumble in low RPMs with a surge at about 2500. I replaced all of my spark plugs one weekend with Champion Platinums, gapped at .055, the old spark plugs looked ok, one of them (maybe even the #7) had a little bit of carbon build up on one side. No improvement. I ran out of time so the next weekend I replaced the spark plug wires, MSD 8.5s, car ran great afterwards, stumble and surge both gone. SES light didn't go off by itself, but I didn't clear it manually.
I don't know how far I really drove it, say 10 miles that day, the next morning I start it up and I'm driving down the road and it starts to miss pretty bad, my SES light starts blinking (which means that I have a misfire severe enough to damage the catalytic converter). So I borrow an OBDII scanner from Autozone, it reads out PO307 and PO113 (IAT high voltage). The IAT problem is just that the sensor is unplugged, I plugged it back in and reset the codes, it doesn't come back.
The PO307 reading I get when the engine is on, if I turn the engine off (key in on position), it reads PO300. That part has me confused. I checked the plug and wire, they're not loose. I haven't pulled the plug back out yet to check it / regap it, but since the car ran fine immediately after the plug/wire change I don't think that's going to be the problem.
So, engine misfire, may or may not be isolated to one cylinder, replaced fuel rail/injectors, plug/wires, no stumble or surge anymore, just runs real rough and I would say there's a loss of power. Suggestions anyone?
03-23-2006, 11:57 PM
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This is an LT1.. no coil packs. I pulled the plug and wire tonight. The spark plug wire was melted/cracked already (only been on there for... 3 or 4 days) and the spark plug had a lot of carbon on it/very blackened. The electrode wasn't blown off or anything. The way the #7 wire is routed is really retarded on the LT1s. I replaced the plug and wrapped the wire in heat shield, and re-routed it to the outside of the exhaust manifold instead of running between the motor and the manifold. I cleared the codes and drove about 5 or 6 miles, it hasn't come back up yet. I'll take it out on the highway tomorrow for a few miles, I don't know if the problem with the wire is that it's so hot in its normal location or if there's a problem with the spark plug fouling out that causes the wire to overheat and the the insulation fails... sort of inside out. I don't even know if that's possible. The spark plug was gapped properly.
