NEED HELP 5.3L Multiple Cylinder Misfire/ Catalyst Below Threshold Bank 2

[Finally]

New to the site. Looking to get an idea of where my time is best spent diagnosing the problem with the 5.3L LH6 engine on my 9-7x. Back around the holidays, it started with a DTC code for Catalyst Efficiency below Threshold Bank 2. I had seen this months before, so I did a reset to see if it showed up again. It did and stays on. But my emissions test isn’t for another year so I figured I’d wait for warmer weather before looking at addressing that.

Around that time I started experiencing what sounded like lifter noise about 10 seconds after startup that would last for less than a minute and go away. This would only happen on cold starts. My engine does have the oil consumption issue, and it was down about a quart at the time, so I added oil. It didn’t get any better. In the past, I have had some noise immediately after startup that would go away after a few seconds and go away completely after a fresh oil change. Usually lower oil levels seemed to be the culprit. No issues with engine power at this time.

Over the last few weeks, the noise has persisted for longer durations to the point where I had to let it sit idling for a couple minutes before it would dissipate. It also got louder and sounded like more than one.

Yesterday the noise was sticking around longer after a cold start. I let it sit for a couple minutes and drove away with hearing a faint noise. On the way home, I noticed that the power seemed sluggish. When I stepped on the gas to let it down shift, the engine sounded very throaty as if I had an aftermarket exhaust system and definitely had reduced power. As it got to a higher RPM around 3k the engine light started flashing and I felt the misfiring. DTC code for multiple cylinder misfire. Looking a little more in depth with my scan tool showed the misfire counts all happening on 2, 4, 6, 8 (bank 2).

The weird thing is now after a cold startup, I don’t hear that same noise anymore. The engine sounds relatively nice and quiet, but I notice the idle sounds a little off as if there’s more of a pause between the V8 burble sound at the tailpipe. The engine light will flash under high loads or higher rpm and you really feel the hesitation. No black smoke, no blue smoke. Still that new throaty sound under load is there.

Brought it in my garage last night and listening underneath the car I can year a tap tap tap, but listening over the engine you don’t notice anything.

I’m planning on getting into this over the weekend, but I’m wondering where my time is best spent first. Sounds like more than a coincidence with the catalyst code on the same engine bank. Is there an easy way to check that a clogged cat is the issue? Doesn’t sound like a bad coil pack if all cylinders in that bank have some issue.

Plugs and wires were changed 15k miles ago. Truck has 115k miles. I realize there are plenty of threads about misfires, but I thought the history of this truck is rather unique for figuring out the problem. Any pointers would help.

Thanks

 

[Mooseman]

Welcome to the Nation!

Sounds more like a flaky lifter, which are known to fail on these early DoD engines. I disabled DoD on mine before that would happen and it also helped my oil use. Still, I get valve clatter at 2500-3000 RPM. Might be too late for you. The Cat low efficiency could be caused by the unburnt fuel passing through it and may also be plugged by it. The leakage could be so gross that the PCM can't decide which cylinder is misfiring but at least which bank.

You could do a compression test on all cylinders and confirm this. If that's all good, check the coils. The cylinder that's showing as misfiring (either by a code or fouled plug), swap the coil with another cylinder to confirm.

 

[Finally]

Thanks for the response. I did some more tinkering tonight. Pulled the bank 1 drivers side valve cover to check for any excessive rocker arm play, but it all looked fine. Bank 2 passenger side is the side with the issues right now, but bank 1 was easy to get to so why not rule it out.

Put it back together and decided to start it up before pulling off the bank 2 cover and this is where it got interesting. Very rough idle, lots of misfires and shuttering, engine light blinks. Misfires are all on 2,4,6,8 bank 2 cylinders. Smelled like unburned gasoline coming out of the tailpipe. I used a laser thermometer to check bank 1 and bank 2 cats. bank 1 got warm to about 220 degrees as expected after a couple minutes but not hot, and the exhaust manifold was around 220 also. bank 2 was cold....like very cold as in 45 degrees. the exhaust manifold on that problem side measured in the 80s, so it's as if no cylinders are firing correctly.

Now for the codes:
P0300 multiple cylinder misfire all on bank 2 cylinders 2 4 6 8
P0352,4,6,8 for Ignition Coil Primary Secondary Ckt for cylinders 2 4 6 8.
P0430 Catalyst efficiency below threshold bank 2.

O2 sensors on bank 2 reading .020 before cat and .055 volts after cat.
O2 sensors on bank 1 reading .855 and .915.

 

[Mooseman]

OK, obvious question, any spark on any of those cylinders? Lifter failure usually affects just one cylinder. I'm now thinking a wiring issue not firing all those coils.

 

[Finally]

Update:
Yeah so it was late friday when I was tinkering with the valve covers and coil packs. I had already removed the coil plugs from bank 2 before i was finished putting bank 1 back together. Realized Saturday morning that not having any of the coil plugs connected on bank 2 MIGHT be the reason why nothing was firing on that side....just a good guess. oy.

So back together everything went. started it up and it was back to normal quiet idle with the engine having that pause I mentioned between the exhaust pipe burble. My father in law helped me out with some diagnostics. We have spark, we have fuel pressure, we have injectors pushing fuel, and no apparent vacuum leaks. A few misfire counts show up during idling for cylinders 2,4,6,8 with most of them in cyclinder 4. for the sake of it i swapped coils between 4 and 5 and nothing changed.

When you rev the engine up to 2k, the misfires increase and the engine light flashes. and the engine sounds louder in the intake area under those higher revs. It really sounds like a clogged cat on the passenger side because it's not that bad during idle, but the engine struggles to increase in rpm under load. just sounds like it's hard to breath. I think if it was a lifter problem i would hear a lifter rattling around, but we don't hear anything.

nothing rattles around the cat when we hit it with a dead blow. No appreciable increase in cat temp. We wanted to remove the O2 sensor on the passenger side before the cat or the flange bolts at the y pipe on that side to see if flow improved, but couldn't break them loose without feeling like they would break off. Is there another way to test the cat or is there another test worth trying?

 

[Mooseman]

The fact that the cat doesn't warm up would be indicative of a dead cat. Even one that's partially clogged could still be working enough to not throw a code.

 

[Finally]

Wanted to follow up with the ending. The cat was in fact plugged. Replaced it with oem y pipe and new O2s. $1500 later runs like a champ. No strange noises of any kind and sounds like it's breathing easy. What a difference in power. You don't notice the decline when you drive it every day.