LS swap Random misfire. Need advise

[Steve l.]

The back story. I recently swapped a 2001 5.3 LM7 into my 1998 K1500. I rebuilt the engine but reused a lot of the peripheral components. Since initial fire up I have been experiencing a menacing P0300 code. I have an Autocal and have been able to monitor individual misfires on all cylinders. Initially the primary cylinder that consistently misfired was number 4. Now all seem to misfire randomly and 4 is no longer the primarily culprit. 7 has its fair share but again, all seem to now be misfiring at one time or another.

The conundrum: At initial start up the engine runs fine. However, once I get about a half mile down the road the misfire begins. The misfire seems to happen primarily between 1300 and 2500 RPM. If I put my foot in it, the misfire goes away and the truck revs to redline really well. Then back down to cruising RPM and it begins to misfire again. The engine revs fine in neutral and park. If I shut the engine off and immediately restart the misfire goes away, but comes back shortly after.

Corrective actions I’ve taken with no positive results. I have changed the plugs twice. It currently has AC Delco Iridium plugs. I have changed the plug wires. I have swapped the coils on cylinder four and the fuel injector on cylinder four. I have checked for vacuum leaks and found none. I checked the fuel pressure regulator and it is not leaking into the vacuum tube. My tuner said fuel pressure looks good. The fuel filter is new (AC delco part). I tried running the truck with the MAF unplugged and it helped for a little bit but then the truck started running even worse than before. The catalytic convertors are relatively new Magnflow parts that functioned just fine with my 5.7 L31.

The truck does not have EGR and when I had the engine apart the intake was thoroughly cleaned as well as the injectors. Truck also has a brand new Camshaft position sensor. I am assuming that if the PCM was bad my tuner would tell me. I’ve exhausted everything I can think of to remedy this misfire and I’ve searched for hours on the internet trying to find a solution so I’m here asking for advice.

Things I haven’t checked: Crank positions sensor, O2 sensors. Again, I am assuming if either of these were causing a problem the tuner would let me know.

I cannot understand why the engine does not misfire at all RPM, not all the time and not when revved in neutral and park. Truck also seems to idle ok. I would think if it was a problem with the harness it would be more consistent and would not subside just by turning the truck off and then back on.

Yesterday I replaced the MAF with a brand new Delphi part. I also replaced the fuel pressure regulator with a new unit. I'm now getting, P0300, P0101 and P0175. Truck is also backfiring through the exhaust under light acceleration. Seems like I'm going in the wrong direction. Misfire is worse rather than better. Also, I've noticed when I hit 3000RPM the truck has a surge of power up to nearly 6000, the feeling is almost like that of turbo lag, then a surge when boost comes on. Of course this truck is not turbocharged so I'm unsure what is causing it.

[crzy8mnky]

Have you hooked a gauge up to fuel pressure port on the engine? I don't believe the computer monitors fuel pressure as it's vacuum operated. Also, which MAF did you use? The calibrations are different between the 01 and 98 MAFs.

[Steve l.]

The gen3 Maf was used. Fuel pressure has not been checked with a gauge. I'm going off what the tuner has told me regarding fuel delivery.

[Steve l.]

Update: Problem still not resolved. Time, money and parts thrown at this thing with no success.

New plugs, wires, MAF sensor, MAP sensor, O2 sensors, fuel filter, fuel pressure regulator, new grounds for the coil packs, swapped coil packs around, swapped fuel injectors around, New tune, crank relearn, additional grounds run to the ECM and additional ground strap run from the block to the chassis.

Still having the same problem. Low RPM misfire from idle to 3000RPM. When the truck hits 3000RPM the misfire stops and it takes off like a rocket. It's like flipping a switch at 3000rpm. What would explain this?

[crzy8mnky]

The surge of power you feel around 3000 is normal, that's when the ls makes power. What wiring harness did you use with the swap? Are you using a stock tune? Verified 12 volts at the pink wire for each injector? Checked continuity between the ground wires for the injectors and their respective places on the pcm connectors?

[Steve l.]

The surge of power you feel around 3000 is normal, that's when the ls makes power. What wiring harness did you use with the swap? Are you using a stock tune? Verified 12 volts at the pink wire for each injector? Checked continuity between the ground wires for the injectors and their respective places on the pcm connectors?

The power surge is MUCH more dramatic then the engine just coming on the cam. It goes from pulling like a 4 cylinder in a tank to a V8 with a warmed up cam. Injector wiring is not the issue as the misfire does not occur at cold start or above 3000 rpm.

[Chevydillan]

I'm leaning towards the tune as well. I read that had it retuned but is your tuner a reputable guy? What are the codes? You've replaced everything so I can't see how it could be much else. Did you ever verify fuel pressure? Is this a completely stock engine. Is base tune for a non flex fuel engine and are your injectors the factory ones from the lm7?

[Steve l.]

I'm leaning towards the tune as well. I read that had it retuned but is your tuner a reputable guy? What are the codes? You've replaced everything so I can't see how it could be much else. Did you ever verify fuel pressure? Is this a completely stock engine. Is base tune for a non flex fuel engine and are your injectors the factory ones from the lm7?

Code is p0300. Tuner is very well know and I gather well respected in the GM truck community. Tunes are via Autocal. Injectors are stock lm7 units. Have not verified fuel pressure with a gauge but have been told it is fine by tuner via data log.

[Chevydillan]

I didn't know the ECM was able to monitor fuel pressure. It would still be nice to verify exactly what the pressure is.
Only other thing I could think of is a bad coil that is breaking up when hot.

[Steve l.]

Discovered something new today. Tuner requested that I record a data log while driving the truck in second gear. It seems the misfire is related to throttle position. I previously though that at 3000rpm it smoothed out, but when running in second gear it misfired at all RPM under light throttle. Once I throttled past about 40% the misfire stopped and truck ran great.

Anyone have experience with a misfire that is related to throttle position?