LostWS6

Hello guys I am having problems with my car for the passed 6 months with a misfire on cylinder #6 it’ll come and go and sometimes stay hidden for a month. It brings up Po306 code. I’ve put a new injector, new plugs, injector plug, checked and tested coil packs they’re all getting fire. Tried to make it jump to a different cylinder but it’s staying on 6. It’s a mail order tune 5.7 LS1 with headers, no rear o2s, egr/air. Should I be checking the PCM?

ChopperDoc

Misfires are almost always a mechanical issue, beyond bad fueling or some other badly set parameter in the tune. If that were the case though, it wouldn't be on one cylinder. My guess is the wiring, since you have checked everything else. Have you tried swapping the harness to the other side to see if it follows? #1 and #8 are the purple wire. So make sure you flip it if you switch them. Also check into the rest of the wiring all the way to the PCM. Especially if its an intermittent problem.

LostWS6

Would any sensor like the crank angle sensor cause it to misfire as well?

ChopperDoc

Not on one cylinder. If you are getting spark on the rest of your cylinders, RPM signal, and it only misfires on the one, then I highly doubt it would be the sensor to blame. If the sensor is jacked up, then you won't get spark at all most of the time, and if you do it will be on random cylinders, and others won't get any at all.

LostWS6

Any recommendations on what else to check?

ChopperDoc

Yes, pull that plug out and compare it to others on the same bank. It may be getting oil fowled from blowby or a bad valve seal. Look for black specs on it and discoloration. If it's bad enough, it will have gunk on it. It should be dry, and relatively clean. I highly doubt it's in the tune if it's only happening on one cylinder. I highly doubt it's a sensor issue for the same reason. If it were random, then that would change things. But as you describe, it's just the one cylinder. My money is on a bad harness or ground somewhere. Or a bad coil pack. I know you said you tested them, but that doesn't mean it will work under all conditions.

Two free checks you can do to narrow it down:

1. Swap the coil harnesses, keeping the purple wire opposite, as in #1 and #8 cylinder. The order is the same. On bank two, the purple starts at the back, bank one it's in the front.
2. Switch your coil pack out to another location.

If it's not the harness, it doesn't mean it couldn't be elsewhere in the engine harness. Check your grounds, and check your signal and power wires to the PCM. The spark is fired by grounding the circuit. So at all other times you should see an open circuit. Also, the injectors fire before the coils do. Also easy to check with a noid light, if all the wiring checks out, seals, coils, etc.

It's pretty hard to think of more things at the moment to check, but start with the free checks, and the easier ones to do. Personally I would have already had the harnesses swapped. If that doesn't change it, then the coil, and then after that, I'd start digging deeper.

Darth_V8r

iTrader: (4)

Think it might be worth a quick compression check on 6?

LilJayV10

iTrader: (39)

It would be a slightly bent valve, weak or broken valve spring, weak lifter, cam lobe starting to go or the cylinder is low on compression.

As Darth stated I would start with a compression test, then a leak down. I'd pull the valve cover off, inspect the springs, rocker arms and pushrods.

Depending on what scanner you have you can check the misfire counter and see how much that cylinder is actually misfiring. It has to hit a certain number of misfires before it will actually kick a p0306 dtc. Lets say the threshold is 1000 misfires within a certain amount of time and it misses 999, it won't kick the CEL on.

LilJayV10

iTrader: (39)

Can you actually feel it misfiring. If not it may be worth doing a crank relearn on it. You shouldn't have to unless you've replaced certain things.

LostWS6

When it misfires it will not move but it will move if you full throttle it. I can shut if off and turn it back on and it’ll be fine for 5 minutes before misfiring again. We swapped coil packs and it’s still on #6. Will be doing a compression test and leak down tonight. But what would cause it to go away and then come back days or weeks later? All wiring looks good also.

LilJayV10

iTrader: (39)

what do you mean it will not move? As in the car will physically not move unless you WOT?

LostWS6

It will barely move if I give it throttle correction * sometimes it misfires so bad it’ll shut off or come to an idle and not pick up. But at full throttle the motor is completely fine and not backfiring.

bamabob

any luck on figuring this out? I'm having a similar issue and hoping to see what you ended up doing to solve it.

slowpoke96z28

iTrader: (2)

Have you done the compression test yet? I know it’s a bit of a pain in the ***, but it’s not too bad, and it will give you definitive answers. Do you have a vacuum gauge? You can take the vacuum source going to the A.I.R. diverter valve control solenoid (close to where the fuel line connects to the rail) and see if it has a smooth needle at idle and run a couple other tests. People are getting away from the vacuum gauge as a driveability diagnostic tool, but it’s just as reliable now as it would be on any carbureted car.