SOLVED! Misfire at higher load

[Mooseman]

For.my son's 04 Silvy 4.8, approximately 180k km. It starts and runs fine until you give it a bit of gas, I'd say just above 1/3 pedal, and it will have misfires in the odd side bank. It will flash the CEL as it's happening and will eventually set P0300. Things that have been done so far:
- swapped coil packs from bank to bank
- swapped plugs
- swapped injectors
- checked for vacuum leaks with Carb cleaner, nothing
- compression test, all 160-170psi
- fuel pressure good at 60psi and holds it even under load

My son monitored the misfire data using the Tech 2 and it would misfire on 1, 3 and 5. Funny thing though is that he swapped in a spare PCM and it was now misfiring 1,3 and 7.

The plugs all looked good, if anything, slightly cleanish with a partially white/brown porcelain.

Next things I told.my son to check is wiring and grounds. He's very technical and is currently studying heavy diesel tech as well as following my footsteps. I'm also at a loss.

Any ideas of other avenues to follow?

 

[I_Shoot_Back]

cat plugged?

 

[Mooseman]

I've never seen a plugged cat cause a misfire. Certainly worth a look. I do have a backpressure tester we can explore this with.

 

[mrrsm]

This video describes similar symptoms on a similar truck with the same power plant and includes a "How To Make" an inexpensive Home Made Back-Pressure Gauge... and the diagnosis and solution after using it are definitive:

 

[littleblazer]

I mean carbon build up could but like... it'd be a lot and you would think both banks would show... the cat sounds promising as each bank has it's own. I'd start with the back pressure test...

 

[m.mcmillen]

I’ve had the same symptoms and it was the valve springs on higher mileage 6.0 engines. They get weak over time. I went through all of the things that you have mentioned. I even went as far as changing the intake gaskets.

What I end up doing is replacing the springs on the cylinders that have the highest number of misfires. The dealership in my small town keeps them in stock which says a lot. Once done, I do a CASE relearn.

 

[NJTB]

When the inner valve spring breaks it causes problems like you describe. Old school, try a running compression test. I've never seen it on multiple cylinders though.

 

[Sparky]

Make sure the O2 sensor isn't being stupid on that bank also. Coworker's 04 5.3 had a dud O2 on the passenger side that caused all sorts of chaos. In his case, it would run great until it warmed up, then when it hit closed loop it would pig the gas, foul the plugs, and misfire like crazy. Now it sounds like his isn't doing something like that, but if it only does this when warmed up it could still be bad data from the O2 sensor. If it does it even when cold then it is not O2 sensor related and ignore what I said.

 

[Mooseman]

This video describes similar symptoms on a similar truck with the same power plant and includes a "How To Make" an inexpensive Home Made Back-Pressure Gauge... and the diagnosis and solution after using it are definitive:

Looked at that video and it sure does seem to have the exact same issue as mine. Another option for making your own backpressure tester would be to use a hollowed out gasket type spark plug with a welded on bung. I already have an actual tester I bought from Amazon at a reasonable price.

Make sure the O2 sensor isn't being stupid on that bank also. Coworker's 04 5.3 had a dud O2 on the passenger side that caused all sorts of chaos. In his case, it would run great until it warmed up, then when it hit closed loop it would pig the gas, foul the plugs, and misfire like crazy. Now it sounds like his isn't doing something like that, but if it only does this when warmed up it could still be bad data from the O2 sensor. If it does it even when cold then it is not O2 sensor related and ignore what I said.

If I unplug that O2 sensor, it would basically stay in open loop and run on pre-programmed parameters? That should eliminate that sensor as the cause if it runs without misfires. Or it just won't be able to detect the misfires without it and it will just be by feel. I'll give this a go if the backpressure test is negative.

Valve springs would be the last thing I'd be looking at but I'd find it kinda strange that only one bank would be affected.

Right now, my money is on the plugged cat. I should be looking at this in the afternoon with my son. Thanks all!

 

[Sparky]

Correct, if you unplug it, it will never hit closed loop and just run safe (but a little rich) parameters. If it still misfires, then it isn't the O2 causing it.

 

[Mooseman]

Here's the latest from yesterday. Backpressure test showed 7psi at 3000rpm so as a temporary test, we punched out the cat. The front honeycomb was broken and breaking up with pieces blocking the rear honeycomb. Problem solved? Nope! Still getting misfires but here's the kicker. Now it's also showing more misfires in 3 and some in 1. And at high RPM, now getting some in 6 and 8. What the ????

When we took out the O2 sensor for the backpressure test, its threads were destroyed. replaced it with a used one I had kicking around. That eliminates that sensor as a possible issue. fuel trims are actually quite good with STFT and LTFT between 0 and -5.

With the Tech 2, did an injector balance test. All did 60 to 40 psi drop so they're good.

With 3 being the biggest offender misfiring the most, we first swapped the plug wire, same. Then the coil with #2, same. Then put a spark tester on #3, sparks great and regularly but here's another kicker, misfires improved! When the compression test was done, noticed it had NGK iridium plugs. Don't remember the part number.

I know the 4.2 is very sensitive to the brand of plugs used but haven't heard that for LS engines. Since the engine was hot, we didn't want to chance it taking out the plugs so we ended the day. Since it was a holiday yesterday, we couldn't have gotten new plugs anyway. Will get some 41-110 ACDelco plugs and maybe new wires at the same time. A new Y pipe with cats is $580 . Might get a local shop to just weld in one.

 

[littleblazer]

Here's the latest from yesterday. Backpressure test showed 7psi at 3000rpm so as a temporary test, we punched out the cat. The front honeycomb was broken and breaking up with pieces blocking the rear honeycomb. Problem solved? Nope! Still getting misfires but here's the kicker. Now it's also showing more misfires in 3 and some in 1. And at high RPM, now getting some in 6 and 8. What the ????

When we took out the O2 sensor for the backpressure test, its threads were destroyed. replaced it with a used one I had kicking around. That eliminates that sensor as a possible issue. fuel trims are actually quite good with STFT and LTFT between 0 and -5.

With the Tech 2, did an injector balance test. All did 60 to 40 psi drop so they're good.

With 3 being the biggest offender misfiring the most, we first swapped the plug wire, same. Then the coil with #2, same. Then put a spark tester on #3, sparks great and regularly but here's another kicker, misfires improved! When the compression test was done, noticed it had NGK iridium plugs. Don't remember the part number.

I know the 4.2 is very sensitive to the brand of plugs used but haven't heard that for LS engines. Since the engine was hot, we didn't want to chance it taking out the plugs so we ended the day. Since it was a holiday yesterday, we couldn't have gotten new plugs anyway. Will get some 41-110 ACDelco plugs and maybe new wires at the same time. A new Y pipe with cats is $580 . Might get a local shop to just weld in one.

That's the problem we have with the escalade, just doing the cats myself is 700$ for the pipes. I'm thinking universal. @brianlibby791 has a welder.

 

[Mooseman]

Local price on plugs was $11.05 discounted (F that!). Amazon.ca was even more ludicrous at $16 each! Ordered a set on RA for total of $72.59. Just have to wait for delivery from the US via Fedex. Why do we get so hosed here?!?

Stay tuned.

 

[Sparky]

Ouch. Can get them for $6.90 each on Amazon or $42 for 8.

 

[Mooseman]

SOLVED! It was the spark plugs. The ones in there were NGK TR5GP Platinum. Put in a set of NGK TR5IX Iridium. Runs mint, no misfires.

Decided not to wait and just get these at Crappy Tire at $10 a plug. I had read online that these engines don't like platinum or multi-electrode plugs.

Now to get some cats. Regardless, they needed to be replaced.

Thanks guys!

 

[littleblazer]

As soon as you brought up the plugs I thought they were a little sus... glad it was straight forward and simple.