Persistent P0307 (also a P0420)

[turbojimmy]

Hi all - this is my first post here. I have an '07 Avalanche 5.3L. Same drivetrain most of you have. The Avalanche forum isn't very active and I noticed this one is. So I figured I'd try my luck with a solution here. It has 178K miles on it and I've really done nothing but routine maintenance to it the entire time I've owned it. Due to the fact it's starting to rot away pretty quickly, I only use it as a backup vehicle. Despite this, I really need to keep it in good running condition. It won't pass state inspection with the CEL lit.

So it's a #7 cylinder misfire. I'm not too worried about the P0420 because that could be the result of a lean #7 cylinder (hopefully - if not I'll need to replace the cats).

I swapped the #7 coil, wire and plug with the #5 cylinder. Didn't help. I saw there was a TSB on sediment accumulating in the back of the rail that can clog the injectors and cause a misfire in the rear cylinders. So I replaced the #7 injector. It's definitely improved, but there's still a misfire that I can feel at idle, in gear with my foot on the brake. Curiously, when in park and idling I can't hear or feel it. It doesn't stumble or stutter off idle or during acceleration. The CEL light hasn't come back on but I'm sure it's just a matter of time.

I have the revised valve cover so I know it's not oil fouling, plus the plugs looked good.

I haven't done a compression check or leakdown test yet but I think those might be next steps.

The TSB says to run some top end cleaner through it after draining the fuel rail and replacing any clogged injectors. I haven't done that. Will that really do any good? I don't have the kit so I'd have to pay someone to do it. If it's not likely to help I don't want to spend the $$.

 

[Buzzcut]

Check this intake gasket by #7. I had persistent problem with the 420 lean. With engine running spray carb cleaner and look to se if it’s sucked in at the gasket


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[turbojimmy]

Check this intake gasket by #7. I had persistent problem with the 420 lean. With engine running spray carb cleaner and look to se if it’s sucked in at the gasket


Sent from my iPhone using Tapatalk

Thanks for the reply. After reading your post I tried that and it's not sucking in carb cleaner around the intake gasket. That's somewhat of a relief since I don't want to have to replace that gasket. Although, i could still be looking at a mechanical problem.

I'm going try some SeaFoam or other additive in the fuel to see if that helps. Like I said, the new injector definitely helped. I might have some carbon build up there from when it was running lean.

 

[kbuskill]

Definitely need to do a compression check... cylinder 7 is an AFM cylinder so it could be a problem with a lifter.

 

[Doubeleive]

how is it rotting away?

 

[turbojimmy]

Stabilizer bar tore away from the frame. Both gas tank straps rotted through. Both 1/4s, all the doors, tail gate, fenders, etc. all are rotted through. I've been replacing stuff as it goes but I couldn't keep up anymore. I got another vehicle as a daily driver and kept the Avalanche for winter/hauling duty. I'm in NJ but I bought the truck used from upstate NY - lots of salt. It's served me well - no complaints.

 

[swathdiver]

Check this intake gasket by #7. I had persistent problem with the 420 lean. With engine running spray carb cleaner and look to se if it’s sucked in at the gasket

P0420 is for the Bank 1 catalytic converter. Whatever is/was ailing cylinder #7 may have take out the cat on that side. There are some ways of reviving a cat like driving around in 3rd gear to get them really hot and burn off any contaminants and if those don't work, you'll have to replace it. Do check the Fuel Trims to see if the intake gaskets are leaking as Buzzcut suggests.

 

[turbojimmy]

Thanks for the replies. I think the 420 preceded the 307. The light's been on a LONG time. There was a brief period of time when it went out and I raced over to the state inspection station and got a 2-year sticker. So I'm good to 2020. But...I need to fix it.

I think the cats are probably clogged. It seems down on power and starts with a stumble. That wouldn't explain the misfire though, right? In just one cylinder?

I'll do a compression test within the next couple of days. The check engine light came back on with the 420 today, but not the misfire code. It's definitely misfiring though - I can feel it. The 307 is just a matter of time. I don't have a scan tool that will tell me fuel trim, but it might be time to make that investment.

 

[swathdiver]

Well, we've seen it before here where a weak cylinder was misfiring because of a bad cat's back pressure. It could also be a bad lobe or lifter.

 

[turbojimmy]

Okay - compression test results on the left bank are:
1 - 180 psi
3 - 155 psi (!)
5 - 180 psi
7 - 160 psi

So compression is okay in the #7. And, the insulator on the #7 spark plug is cracked. I know I swapped that from the #5 a couple of months ago and it did not correct the problem. I'm going to change the plugs anyway. They all looked good with no real difference in color among them.