Never ending P0300
03-03-2019, 09:54 AM
#1
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Never ending P0300 Alright everyone, need some help/sanity check here.
‘99 firebird LS1 swapped. Maybe 200-250 miles on this build. Long tube headers, LS6 PCV conversion, EGR/AIR/rear O2 delete.Getting P0300 misfire code. I have a diablosport trinity 1000 standard tune on my ECU with the above mentioned deletes tuned out. Nothing else done with that tune yet, waiting to get this issue fixed first. When I run a log on my laptop through the trinity looking at cylinder misfire count, cylinders 4&7 are the culprits.I have all new engine sensors (crank and cam position sensors, IAC, TPS, IAT, MAP, MAF), new NGK plugs, new wires and running Bosch 13111 front O2s. I have moved coils/wires and injectors around and problem stayed with cylinders 4&7. ECU to coil wires checked good, grounds are good. Fuel pressure is 58-60 and steady. I did do a compression test on all cylinders. I kinda forgot to block the TB open when doing it so my numbers were low (140-150 across the board vs the 180+ most people report), but they were all close and held steady on the gauge....Can always redo of necessary. Car pretty much idles fine, but there’s some hesitation/stumbling when you get on the gas initially. Checked for vacuum leaks, and unless it’s actually leaking at my valley cover, we appear to be leak free.Racking my head on this and not sure where to go next. Appreciate any/all help thrown my way, banging my head against the wall at the moment, at a total loss here. Thanks!
03-03-2019, 10:10 AM
#2
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Most of the stuff you listed has nothing to do with a specific misfire. Let's make it a step-by-step process.
Misfire on only 4&7 means the fuel and ignition is generally ok which rules out most of the engine controls and narrows it down to a couple of cylinder specific problems.
First you pull those plugs and look for damage or fowling or any other signs of the problem. If they're soaked in full for instance it'll narrow down the issue.
You swapped coils and wires but the problem stays with those cylinders. Which means that either the wiring harness leading to those coils is bad (faulty connector possibly) or the ignition is fine and the problem is elsewhere.
You swapped injectors and the problem stayed with those cylinders which rules out clogged or bad injectors. It could be an issue in the wiring harness leading to those injectors (faulty connector possibly)
If the problem stays with the cylinders after swapping coils/wires, plugs and injectors then it's a valvetrain/piston issue (your compression check seems to rule that out) or it's time to open up the harness and look for chafed wires.
Misfire on only 4&7 means the fuel and ignition is generally ok which rules out most of the engine controls and narrows it down to a couple of cylinder specific problems.
First you pull those plugs and look for damage or fowling or any other signs of the problem. If they're soaked in full for instance it'll narrow down the issue.
You swapped coils and wires but the problem stays with those cylinders. Which means that either the wiring harness leading to those coils is bad (faulty connector possibly) or the ignition is fine and the problem is elsewhere.
You swapped injectors and the problem stayed with those cylinders which rules out clogged or bad injectors. It could be an issue in the wiring harness leading to those injectors (faulty connector possibly)
If the problem stays with the cylinders after swapping coils/wires, plugs and injectors then it's a valvetrain/piston issue (your compression check seems to rule that out) or it's time to open up the harness and look for chafed wires.
03-03-2019, 12:01 PM
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I should mention that most of my sensor replacing was fixing some different issues a few weeks ago......
I could pull the main harness and physically check the wires and connectors. They all ohmed good, and I swapped the coil harnesses with no change. I had someone else mention either alternator issues or maybe crank position relearn needs to be done. My engine and ECU are from different vehicles, so I could potentially see the crank sensor relearn, although I’m not throwing any of those codes.
I could pull the main harness and physically check the wires and connectors. They all ohmed good, and I swapped the coil harnesses with no change. I had someone else mention either alternator issues or maybe crank position relearn needs to be done. My engine and ECU are from different vehicles, so I could potentially see the crank sensor relearn, although I’m not throwing any of those codes.
03-03-2019, 02:58 PM
#5
P0300 is random multiple misfire.
Usually with a 300 you have a common source issue. Bad fuel, low pressure, bad MAF or MAP. In other words not usually a single cylinder issue like A plug or A wire.
If it stumbles on tip in it's likely has a fueling issue. May be a timing issue as well.
If all cylinders are low on compression, I would suspect cam timing may be off a tick.
Usually with a 300 you have a common source issue. Bad fuel, low pressure, bad MAF or MAP. In other words not usually a single cylinder issue like A plug or A wire.
If it stumbles on tip in it's likely has a fueling issue. May be a timing issue as well.
If all cylinders are low on compression, I would suspect cam timing may be off a tick.
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03-03-2019, 07:30 PM
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03-03-2019, 07:33 PM
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Thats what I’ve been hearing. I’ve checked most areas that I could think of. The only thing I haven’t checked is the valley cover gasket, really hoping I don’t have to get into that again anytime soon!
