Multiple Misfire P0300
 

So.....truck got back on the road on Monday after Pacesetter LT's, catless Y, 3200 RPM Circle D stall, PCMforless wires, NGK TR6 plugs, heat socks, trans cooler added to truck. Also have IEATSRT8 intake, e-fans, 160 tstat, tuned by Alvin, Magnaflow 11229 no res.......

Anyway, today, got off work and headed to the Wadmalaw to put boat in water and go to restaraunt for some drinks. (Boat was at parents house so not hooked to truck cuz i know you will ask). Anyway, I had my friends new Gtech SS and wanted to see what it said would happen in the quarter.

So I got to straight stretch of road, stopped, hit record on Gtech, and flashed TC to 3000-ish and off I went. First gear, 5500-ish RPM and it started breaking up real bad, shifted, and I backed off. CEL was flashing and speedo said "0" which I figured threw the code. Pulled code on Aeroforce and was P0300, which I knew instantly was multiple misfire.

Didn't do any more runs so as not to hurt anything. Got to parents house and was talking to my pops (mechanic for 25+ years but no ECM experience) and he said he was willing to bet it was the plugs......and he wanted to see it happen. So off we went, turned out of dirt road, lined her up, and whaddaya know? P0300 at about 5500-6000 RPMs breaking up bad.

Any advice? Plugs were gapped to .035" as Alvin told me.

Could this be tune related, or something else? Truck runs absolutely perfect otherwise. Even WOT from a stoplight through 1-2 shift, no codes. Only threw the code when from dead stop, stabbing throttle to floor.

The plugs I put in are gapped .035" for sure. I always check plug gaps as they come out of the box. I would recommend to at least go back to the stock plug wires and see what happens for a pull or two. Tracing down a possible secondary ignition problem basically boils down to process of elimination via part substitution.

On the fuel side, are your O2 sensors staying rich with the pedal on the mat?

Sounds like something may have been missed in the tune, a cell or table. If the CEL is flashing it's a missfire from something, has nothing to do with speedo.

pcm4shit wires most likely the issue......

This is when the local tuner thingy would be nice...

like Gerald stated: check plug wires. I've had the same code before but with the LT1 and it turned out a few wires were not pressed on all the way. Or it could be just the PCM4Less wires too.

X2

Sure is, but, well, you know.......

Anyway, how would a wire work all the time, and then only under hard pull it quits? Doesn't makes sense to me, but I'll give it a shot.

...because as the load increases, so does the cylinder pressure and an increase in cylinder pressure causes an increase in resistance across the plug gap. So, if you have a weak ignition component, it will be stressed more under WOT.

I wouldn't feel it's tune related since you don't have a camshaft, unless something got really fat fingered.


The only other thing I would consider is some sort of fuel cut (limiter) or TQ management. Possibly a loss of speed signal causing the cut, but you would normaly get other codes with that.

Set scan gauge to monitor misfires a little while ago on way back home. Got on it and at 1-2 shift, got 160 misfires on cylinder 8. When it cools down, I'll get in there and see of it's anything obvious. If not, then I'll start swapping parts and see if
I can eliminate it.

Another TBSS had same issue and was a bad injector. Time to start diggin around.

I would check #8 plug wire. You can check the plug wire with a digital meter to see if its good, or just move it to another cylinder to see if the problem follows if you dont have a meter.

just swap the number 8 coil pack with another cylinder, the wire with a different cylinder and then go for another drive. see which cylinder it moves to and you found the problem. if it doesnt change cylinders then its the plug, injector, or mechanical issue with that cylinder.

no need to spend money on parts just yet.

Sometimes the core of the wire ohms out good, but the insulation of the boot can be weak. Under light load, the mixture in the cylinder is easy to light and the spark energy has no problem making it all the way to the plug gap. Under heavy load, the mixture in the cylinder is harder to light, and the spark energy can find it easier to breach a weak plug boot and jump to ground instead.

Swap the wire on #8 first. If you still get a misfire staying with #8, then trade plugs with another cylinder. You may eventually even have to switch the coil or injector with another cylinder. When you get the misfire to move, you have found your offender.

Swapped #8 and #1 wires.....no change, still missing on #8 under heavy load. Will swap a coil tonight I guess.

Of course it's got to be #8, the hardest one to get to.

Swapped #8 coil and same thing. Just picked up a new plug, gonna throw it in there and see. I don't know what else it could be.

Swapped plug. Same issue. WTF?

Might be an injector or injector driver in the ECM. A misfire can be caused by spark, fuel, or compression issues. Even a broken valve spring can let a valve float at high rpm but yet be fine at lower rpms.

If your tune in your original ECM is safe enough for a WOT pull, you could try that to see if it is an ECM issue.

That #8 plug is a bitch to get to, ain't it?

Isn't hot rodding fun?

Yes, number 8 is a bitch! Not so bad with wheel off, IF it's cooled down.

Just got of phone with my old man, and he blames it on valve spring too. Just fucking awesome.......

Any way to check short of pulling valve cover and dismantling valvetrain?

wouldn't a compression leak down test show a broken valve spring?

i'd recommend a compression and leakdown test aswell.

i've seen multiple LS injectors fail causing the same problem, so if you're up for it swap the injector with another cylinder....

it could be many things if its mechanical, not just a valve spring, valve seat could be beat up, valve could be hanging up in the guide, or it could be bent (would likely get a misfire at idle with most of those problems though.

i doubt its a plug, honestly i've never seen a plug have an issue at WOT only in N/A applications, but its worth a try before going any further.

Unfortunately, putting air in a cylinder with a leakdown tester just blows the valves shut and keeps them shut, despite low tension from a broken valve spring. Multiple coil valvesprings can even be harder to detect a broken coil by sight, especially if one of the inner springs is broke.

I have seen where a vacuum gauge can be used to diagnose a broken valvespring. However, at 5k+ rpm, any abnormal pulse reflected back into the intake may be too short of a duration for a mechanical gauge to react.

There are piezo transducers that are available that you can hook to a lab scope that can show a quick pressure abnormality at high speeds. Unfortunately, they are about $400 and up for a good one. Here's a link to one: http://www.aeswave.com/products/Prod...343&ci=84&sa=Y