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Discussion Starter #1
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.
 

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I would check for intake leaks and check the map sensor that's on top of the intake manifold for leaks and its wiring. Had that problem on a recent LS swap.
 

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Discussion Starter #3
Drove the truck tonight and the p0300 has left for the time being but was replaced by p0172. So it appears both banks are now rich. Truck is truely gutless below 3000 rpm and wants to backfire I presume due to rich condition. Any thoughts about this? Could it be the tune?

I will check the map sensor tomorrow.
 

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Intake leak( check around the throttle body and intake gaskets) map, maf, O2s, bad injectors or the tune. I've seen all cause rich conditions.
 

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Just throwing this out there as I had this issue a while back when I did this. I simply re-tuned the computer. With a fresh tune it fixed all of these mysterious bugs that I was having. Maybe something with your computer tune went funny and it just needs to be reset.
 

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Discussion Starter #6
New MAP sensor going in tomorrow, although tuner said today the old one is fine. New tune also being loaded tomorrow. We'll see what happens.
 

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check your converters, my dad just had the same problem,removed cats,runs like a bat outta hell now.
 

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Discussion Starter #9
New MAP and Tune cured the P0101 and the truck running rich, but now the random misfire is back. Misfire is not isolated to one cylinder or bank and seems to jump around primary from 4, 7, and 8. Misfire is still isolated to light throttle. Above 3000RPM it runs great.

The cats have less than 5,000 miles on them and I would think if it were the cats it would run like crap at all RPM.

Sent the tuner a new Misfire data log to see what he can tell me from that.

Any new thoughts? O2 sensors? Throttle position sensor?

I'm strongly considering taking it to a shop for a vacuum leak test and fuel pressure test (although tuner doesn't think fuel delivery is the problem). I think I need a fresh set of eyes on this thing.
 

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When i did my motor swap, i too had this random misfire and rich condition. It didnt have any low end power and would backfire through the exhuast. I had new plugs and wires but didnt have new coil packs. I replaced ALL of the injectors with new ones, and that solved my missfire and backfire issues. But i still have a code for a rich condition. I dont have cats on mine though i have an o/r y pipe. And it seems i have an issue with both bank 1 & 2 o2 sensors. Bank 2 was suppose to be turned of in my tune so i still need to get with my tuner on that. Anyway, needless to say the injectors solved 1/2 the problems and i think the rest falls on the o2 sensors.

My motor had been sitting for a while before i installed it and we found that the ethonal gunked up the injectors from sitting. They were almost completely blocked off from the white gunk. I ran all kinds of cleaners through the fuel system before replacing all of them with a new set. So if thats your problem your only option is to have your current set cleaned or buy a new set. Hope this may help
 

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Check the timing? The motor was just rebuilt, timing chain could be off a tooth. Had a similar problem with the same symptoms on a 5.7, different setups but same concept.




Sent from my iPhone using Tapatalk
 

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It runs pretty good in the higher rpms, but i still think i have an issue as it just doesnt pull like i think it should. I mean i think the 5 speed trans kind of takes away from it a little bit, maybe its my tune. But the truck pulls nothing like my extended cab 03 does with a 4.8, or my 2014 crew cab with a 5.3 . But after the injectors it definantly helped out. No misfire or anything anymore. I ordered new plugs, wires, and coil packs a few days ago. So i will be adding all that this weekend. All AC Delco stuff. I also bought a compression checker. Going to do that while im changing the plugs. Since i did buy my motor from a junk yard. Doubt thats my issue, but does hurt to see.
 

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Be sure to gap the new plugs at .40 not .60 gm changed gap specs.
 

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yeah my current one are gapped at .40 ,Thanks for the heads up though
 

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Discussion Starter #16
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?
 

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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?
Are you sure there's no vacuum leaks? Check around the throttle body, and where the intake meets the heads. Around the map sensor where it slides in the intake. And all vacuum lines. Other than that is cut the cats out just to see if those are partially clogged which could hurt the low rpm flow. Seen it before
 
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