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New 5.3 swap, P0300 code, need advice

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Old 03-18-2017, 11:07 AM   #1
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Default New 5.3 swap, P0300 code, need advice

Ive been lurking here a while but just signed up because I'm stuck here with a problem that I've spent hours on and need all the input I can get

Just got exhaust on my 5.3 swap, started driving it, and now I realize I have a bad miss. Throwing p0300 code on cyls 2, 3, 5.

Things Ive done:

Swapped coils between misfiring cylinder and "good" cylinder
Swapped injector between good/bad cylinder
Compression test on good vs. bad cylinder - 150psi +/- 5
Fuel pressure check (56psi)
Noid night on injector pigtail
New MAF
Brand new AC Delco plugs, wires, and o2 sensors
Crank Relearn

It's a brand new harness. PCM is from donor motor (2000 Silverado) and was flashed (VATS delete/emissions delete) from the place that did the harness.

Truck idles half decent but choppy driving it. Exhaust stinks bad.
Any ideas? I feel like I tried everything.
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Old 03-19-2017, 06:00 PM   #2
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Something stupid, but if using stock fuel pressure regulator on truck rails, pull vacuum hose from regulator and check for presence for fuel. Had a friend with similar misfire code and this was the culprit.
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Old 03-19-2017, 06:13 PM   #3
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Something stupid, but if using stock fuel pressure regulator on truck rails, pull vacuum hose from regulator and check for presence for fuel. Had a friend with similar misfire code and this was the culprit.
I looked at it the other day and it appeared fine, but I just went back outside with a little piece of paper towel rolled up so I could stick it down the FPR hole....pulled it out and it was dry

I added another ground strap to the other side of the engine today and changed the PCV valve, no good.

I've pretty much gone thru everything I can think of. I'm starting to wonder if I have a faulty PCM or maybe a wiring issue.
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Old 03-19-2017, 08:53 PM   #4
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Since it is a new harness make sure it is not built wrong. Check continuity from injector connectors and coil packs from those cylinders to their respective PCM connector pins. Do you have a data logging scanner?
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Old 03-19-2017, 09:04 PM   #5
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Since it is a new harness make sure it is not built wrong. Check continuity from injector connectors and coil packs from those cylinders to their respective PCM connector pins. Do you have a data logging scanner?
I'm going to call the harness people in the morning and see what they think. They are a highly reputable place that's been around for a while, but I guess there's always the chance of an error.

A few weeks ago, when I just got it together, I had a bad engine to frame ground in the very beginning and couldn't get the starter to crank at times. It would just click rapidly like the battery was dead. The times it did crank was because the steel braided transmission lines were touching the block and the cooler is bolted to the radiator support. I figured this out when I noticed burn marks on the lines in 3 different places. I fixed it with a new ground wire, but I wonder if I blew up the computer....

I have access to a Snap-On scanner that has all the bells and whistles, so I assume I can datalog. I also have a cheap USB->OBDII scanner with software called ODBwiz (www.scantool.net) not sure if that does.
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Old 03-19-2017, 10:59 PM   #6
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A screen shot of my cheap scanner program. Any clues?

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Old 03-20-2017, 07:22 PM   #7
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Add your long term and short term fuel trims for each bank and that is how much more %fuel the O2s are telling the PCM to add to run at the stoich A/F ratio. 19.53%+25%, 16.41%+25%. Real misfires will dump a bunch of unburned oxygen in the exhaust and cause that if everything else is sound, ie good fuel pressure, no vacc leaks, no exhaust leaks. My bet is on harness wiring errors if it has never run right. I've seen wiring errors on even the best harness vendor products. Just get a ohm meter or buzz tester and check the injector and coil pack control wires for the right locations.
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Old 03-20-2017, 07:35 PM   #8
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I think I found the problem.

Evidently 2+2 isnt 100% to show a vacuum leak. I sprayed ALOT all around the intake and didnt notice any engine changes.

But today I hooked it up to a smoke machine, and it was coming straight out of the intake base...guess where? Right at the 3/5 cylinder area - the cylinders the computer was reporting a misfire

thats why its adding fuel, it all makes sense now

I'm doing intake gaskets tonight, I'll update if you're interested
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Old 03-20-2017, 08:01 PM   #9
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Old 03-21-2017, 07:20 PM   #10
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Well, the intake gaskets did not fix it.

It runs just as poor. It doesn't appear to be dumping as much fuel according to the fuel trims.

Still misfiring on the same 3 cylinders. 2, 3 and 5. Did another smoke test, no more leaks, but damn...this is killing me.

I feel like the cylinders are getting fuel & spark, but the timing is off. Maybe crank or cam sensor going bad?

The only code I'm getting is the P0300. And the misfire count on #3 increases like a slot machine.
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Old 03-26-2017, 04:04 PM   #11
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I'll just finish up this thread with the solution just in case anybody comes across this thread with the same issue.

It was the #4 injector. I replaced it with a brand new one and the truck runs perfectly.

Not #2, not #3, not #5 - which is what the Snap-On scanner was reporting misfires, #4, that reported ZERO misfires.

Moral of the story: Pull all the spark plugs. That's what I did and #4 was different than the rest.
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