One bank misfiring on 2001 Tahoe 5.3 LM7

Hi all,
I've recently changed power steering fluid on my 2001 Tahoe and since then, I get cylinders 2 & 6 misfiring (right bank), and only those two. Firstly I thought I may have tampered the ground wiring from the battery, but I re-checked the closest ground location and cables and all seem fine. Ground wire is tightened good to the (-) terminal. The strange thing is that the misfires slowly disappear once during warm up and don't show at all on the warm engine in any driving condition - that should elimitate electrical issue (grounding)

This is what I've done so far:

• Fuel Injector Balance test - all injectors were pulsing and showed identical pressure drop
• Checked the spark using grounded spark tester on #2, #4, #6 - all showed good and consistent spark
• Swapped coil from #2 to #5 (the easiest to reach) - no change (2 & 6 still misfiring)
• Swapped wire and plug from #2 to #5 - no change (2 & 6 still misfiring)
• Checked both bank O2 sensors using live plot - each one starts pulsing normally shortly after start
• Tested the right side of the intake for vaccuum leaks using water spray - no effect
• Checked vaccuum hoses
• CKP variation relearn
• Cylinder Powe Balance test - hard to read from Tech2, but the RPM-drop numbers seemed very similar

I didn't check for blocked exhaust, but that's very unlikely to have happened as the problem appeared all of sudden. The engine really misfires as it shakes a little on a cold start.

Here are freeze frame data for a typical P0300 occurence. Sometimes the CEL is even flashing, but other times it just lights up. Probably depends on the engine temperature.

I hope I didn't miss anything in the write up but will be thankful for any hint!

What are your short and long term fuel trims on bank 1 and 2?

These the ones with fuel pressure regulator leaking issues? Does fuel pressure hold ??

your engine is in open loop... did not see engine temp. provide LTFT for both banks with engine at operating temp and in closed loop. bank specific codes will not be a vacuum leak.

you checked spark at the end of the plug wire but not #2 & 6 plugs. if the problem stays on 2&6 when you moved coils that shows the coils and wires are ok, the plugs would be suspect.

Tutti57 wrote: What are your short and long term fuel trims on bank 1 and 2?

When in closed loop, they're pretty good - STFT is always around 0% (when idling), LTFT is -1% for bank 1, -2% for bank 2. I may take some live plot of those if desired.

scannerjohn wrote: your engine is in open loop... did not see engine temp. provide LTFT for both banks with engine at operating temp and in closed loop. bank specific codes will not be a vacuum leak.

you checked spark at the end of the plug wire but not #2 & 6 plugs. if the problem stays on 2&6 when you moved coils that shows the coils and wires are ok, the plugs would be suspect.

I moved the plugs too. Actually, I moved only one - from #2 to #5. There are no misfires on #5 after the swap. So essentially all secondary ignition components were moved from one bank to another with no effect. Since the problem appeared all of sudden on both banks

Cheryl wrote: These the ones with fuel pressure regulator leaking issues? Does fuel pressure hold ??

Yes, the pressure holds fine - this was also confirmed during the fuel injector balance test. I changed fuel pump recently, so I checked the regulator thoroughly.

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I've yet tried to disconnect Bank 2 O2 sensor, but as I expected it had no effect since the problem appears on open loop. Today, I will try to re-check voltage and grounds using a test light.

Hmm, I would have thought you had an intake vacuum leak with these symptoms. Vacuum leaks can show as single bank codes depending on where the leak is or if one banks fuel trims hit the magic number to throw the code and the other bank may be very close but not there yet. Remember that look at those fuel trim numbers after it's warmed up, could be after the leak "sealed" itself up a little after warming. I had a baby LS 4.8 once that sounded just like this and I didn't find anything with the water test but smoking showed the intake gasket was shot. I still think it's a possibility but don't want to get tunnel vision on it and get you stuck in the weeds.

are you getting any other codes? the P0300 is a general random misfire, what led you to #2 & 6.?

constant in this is the wiring to coils #2 and #6. If you changed coil, plug and plug wire positions and no change you may have a bad wire/wire connection or pin not making constant contact in connector . 2,4,6,&8 share a ground. Do a wiggle test on each and inspect connector wire integrity. Corrosion at a junction can cause intermittent problems.

Sorry for the silence, I couldn't reply here with my original account for some strange reason...

It's bit of a mystery as it happened all of sudden during the power steering fluid change. I remember only a little move of the fat (+) battery cable aside. But I also believe that the electrical-related issue wouldn't disappear after warm up. But maybe, the warmer engine is, the lesser current is needed for a spark to generate... (wild guess)
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Lastly only the P0300, the car will drive few hundred miles this weekend so I will see if there any more codes pop up. I watch misfire current monitors and it always shows numbers raising on #2 and #6. The engine is little shaky too.
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Yesterday I rechecked all leads, both to coils and injectors using a test light and all look fine, there's a good ground, good (+) from ignition and good injector signal. Then I tried a home-made smoke generator and I think there was some little smoke coming out of the driver-side of the intake, maybe it was some rubber hose for propane (the car was converted to LPG several years back). I will focus more on that area in the next days.

Yet it bugs me, that the smoke comes from the other side than the actual misfires...

If I had any smoke coming out of the intake I would change the gasket. v engines can have internal vacuum leaks from the gasket too that can't be seen with the smoke machine. It's possible that it's bad on the side with the misfires too and you can't see the smoke. It also could be totally unrelated!

So the smoke was coming from an injector o-ring on the opposite bank. I took the fuel rail off and replaced all o-ring. After that, no leak so far. The intake gets pressurized quite a bit by the smoke "machine" so it looks like there's no other leak, at least not any major that would cause the misfire, I believe.

The misfires are still on #2 and #6. After I started the engine, I unplugged wire from the #2 coil and checked the spark with a test light, it was strong and jumped for almost .5in. Same as on cylinder (coil) #1. When looking at the live graph, it can be seen that the misfires are slowly disappearing as the engine warms up.

Next will be a compression test, but I'm bit pesimistic about that too...

Don't focus on #2 and #6,specific cylinder misfire can be sometimes misleading.I have a P0300 too at idle intermittently and still looking for the cause.
I have a LTFT around -10% with upstream O2 fluctuating normally but today i show that the downstream 02 follows the waveform of the upstream,which is something not good,but this happened only while driving the car.While at idle the upstream was OK again and the downstream around 0.6 volts.
At first i though of bad downstream O2 but now really don't know cause i would expect that it would fluctuate as well at idle but it didn't happen.

Does your GM 5.3L have the CSFI or does it have the upgraded MFI Fuel Injection system?

If I'm not wrong, it should be MPFI (single injectors on a fuel rail, not spider injectors). Pulled out spark plugs yesterday but the compression tester is not holding pressure, need to get a new one probably by tomorrow.

Here are the spark plugs, not the best pics but I tried my best
Keeep in mind that they're burning LPG (propane), so reddish coloration is normal. From what I can tell, they all look almost identical, except #7 and #8 looking way different, maybe too lean. I know this was common in o'le Chevy SBCs, but with sequential fuel injection in an LS Vortec, it looks a bit strange or at least interesting.


I couldn't do compression test as the gauge failed, need to get new one, hopefully by tomorrow.

Ok, did the compression test and results are bad


The number two numbers are dry / wet.

Considering that the misfires slowly vanish when engine warms up, I'd say there's problem with piston rings on cylinder #2.

Is there other way to confirm the piston rings problem?

After thinking about it for some time, I will yet test some of the good cylinders in "wet mode" to see how much psi they gain (don't think too much). Then I will also check if the failing cylinder blows by through the crank case and PCV valve subsequently.

Could be also lifter sticking probably, but not sure now.

Just for the update - I filled the cylinder #2 with compressed air (closed valves) and it noticeably leaked into intake. There was audible hiss from the airbox. It's a time for cylinder head service.