Need help. 5.3L Sierra P0300.

[JC316]

It's not often that I get stumped, but this one is kicking my ass. Here is the story.

I have an 00 Sierra 4WD with 200K miles on it and a 5.3L engine. It was low on power, had crap MPG, and I have the codes P0300, and P0303. Random multiple misfire and cylinder 3 misfire detected. When I first got it, the plugs and wires were old, so I changed them out. Also did compression tests and I am seeing 150ish PSI. Spark tested the coils and they are firing.

The fuel pressure was reading 43 psi at load, so I changed the filter and the pump, now priming to 55PSI and idling at 50 PSI. Some people say this is too low, but I have done a lot of research into this and it appears to be normal. FPR has no fuel in the line and the PSI shoots up to 58 when the line is unplugged. Clamp the return and fuel pressure is at 70lbs. Checked the LT trim levels on fuel and they were up around 20.

I checked for vacuum leaks, didn't find any, but I decided to change out the intake gaskets anyway, just to be sure. They looked like they were leaking pretty good, got it all back together and the LT trims are sitting at 5% now. Much better, but the misfire remains. O2 sensor voltage is jumping from lean to rich, so it should be working right.

Unplugging the MAF causes it to stop misfiring, but the MAF is reading .70lbs/m and a Hz test on it showed nothing irregular. This makes me think that I'm still having a vacuum leak somewhere, but I can't find it and the fuel trims are fine.

Cliff notes:
Multiple Misfire. Checked/changed:
Compression
Coils
Wires/plugs
Fuel Pump/Filter
Intake gaskets
Fuel Trims
Ohm checked injectors.
O2 sensors
MAF unplugged stops misfire, but tests OK.

[shumpertdavid]

Odd that the misfire goes away with the maf unplugged. Have you checked the maf's sensors for buildup or cleaned it. I generally look at maf in Grams I believe rather than lb. It should be 4.5-6 at idle and probably near 140 at wide open and upper rpms.

[JC316]

Odd that the misfire goes away with the maf unplugged. Have you checked the maf's sensors for buildup or cleaned it. I generally look at maf in Grams I believe rather than lb. It should be 4.5-6 at idle and probably near 140 at wide open and upper rpms.

No buildup on the MAF that I can see, but it's the sealed kind. The MAF is in that range. I took some freeze frame data and every time it sets the p0303, the LT fuel trim on bank 1, is 14.1. Every single time. Bank 2 looks normal, but bank 1 is lean for some damned reason. Thinking about swapping the O2 sensors around, see if the code moves from bank 1 to bank 2.

[shumpertdavid]

I would think the O2 would be unlikely but they're easy enough to swap and eliminate. If it were the sensor causing it, it should affect all 4 cylinders not just the one. You might swap the coils between 3 and another bank and see if it follows by chance. It it a hard misfire, only at idle, or under load.


I've had one 99 that had issues clogging the screens on a couple injectors. It took me a little while to narrow it down. I would pull the injector and clean the inlet screen with b-12 spray and a few hits from the blower and all would be good until it picked up enough trash. On that particular application I could only assume it was an internal line issue causing the debris. It had always had regular fuel filter maintenance but the truck spent a lot of time sitting.

[asphaltjunkie]

Most likely not the problem, but I'll ask anyways.

What brand/type of plugs/wires did you use?

[JC316]

I would think the O2 would be unlikely but they're easy enough to swap and eliminate. If it were the sensor causing it, it should affect all 4 cylinders not just the one. You might swap the coils between 3 and another bank and see if it follows by chance. It it a hard misfire, only at idle, or under load.


I've had one 99 that had issues clogging the screens on a couple injectors. It took me a little while to narrow it down. I would pull the injector and clean the inlet screen with b-12 spray and a few hits from the blower and all would be good until it picked up enough trash. On that particular application I could only assume it was an internal line issue causing the debris. It had always had regular fuel filter maintenance but the truck spent a lot of time sitting.

Definitely not the O2 sensor. It's a hard misfire, both at idle and at load. It's weird, I did a balance test on it, and several cylinders didn't seem to affect the idle, cylinder 3 being the most. Rechecked spark, and compression, both were fine, so I suspected injectors, popped them off and all of they seem to be spraying fine.

Most likely not the problem, but I'll ask anyways.

What brand/type of plugs/wires did you use?

Autolite/Duralast, gapped at .060.

[asphaltjunkie]

I only ask because I've heard people complain that using anything other than the Delco/factory plugs on the 5.3 (and other late model v8's) causes them to run like crap, miss, and act underpowered. Just my .02

[JC316]

Coil. Even though the fucker ohm tested and spark tested fine, the P0303 left for cylinder 1 when I swapped them.

[shumpertdavid]

Did you use a HEI spark tester and let it run on it a little while. I've had Ford COP's that would spark good for a few cycles and then begin to miss a cycle intermittently as they warmed up.

[JC316]

Did you use a HEI spark tester and let it run on it a little while. I've had Ford COP's that would spark good for a few cycles and then begin to miss a cycle intermittently as they warmed up.

Nope, I just tested it for a few seconds, saw spark and figured it was OK. Got it swapped out and it's running like a champ.

[dee]

Check tge converters. I've had several that I've changed due to being stopped up.

[shumpertdavid]

Check tge converters. I've had several that I've changed due to being stopped up.

I've diagnosed quiet a number of those failed.

JC316, if you don't have a HEI spark tester pick one up, they're pretty cheap. They look like a spark plug with the end cut off. It forces the coil to make max output.

[JC316]

I've diagnosed quiet a number of those failed.

JC316, if you don't have a HEI spark tester pick one up, they're pretty cheap. They look like a spark plug with the end cut off. It forces the coil to make max output.

Yeah, I have one, just didn't test it for very long.