Single Cylinder Misfire in my Avalanche, can't figure it out

[Nitrousbird]

2002 Chevy Avalanche, 5.3L LM7 motor.

 

Misfires only under load above 3500-4000 RPM. Throws a flashing SES light, my code scanner reveals it to be a P0302, which is a Cylinder misfire detected - Cylinder #2, which assuming OBD-II codes don't have their own weird cylinder numbering, that should be the front passenger cylinder.

 

After letting off the gas, it may run rough for a bit, or may run perfectly fine. Light goes off soon after. At lower RPM's, runs great, gas mileage has been fine.

 

I've swapped parts, cleared the code, and re-scanned, with the same code every time:

 

- Swapped #2 and #4 plug/wire

- Swapped #2 and #4 coil pack

- Swapped #2 and #4 injector

 

Code is still for #2 cylinder every time. Problem is reproducable 100% of the time.

 

If it were a fuel pressure problem, I'd assume it would be on multiple cylinders. The rail pours a lot of fuel out at the #2 port when I pulled the injector, so I can assume no clog there.

 

At this point I'm stumped...any ideas?

[Guest Removed]

# Inspect O2 sensor connections on the misfiring bank for corrosion or water intrusion. If water intrusion is found on the right bank, it may be due to the AC Evaporator Condensation dripping onto the O2 sensor harness. If this condition is found, reposition and shield the harness to prevent a repeat concern and repair the connections.

# Check for excessive exhaust backpressure using the restricted exhaust diagnosis.

# Swap the Position 1 O2 sensors side to side to see if the misfires move to the other bank of the engine. If so, replace the O2 sensor.

[boostedgtp]

Compression check maybe, what did the plug look like?

[Nitrousbird]

Plug looked clean and like the others.

 

Now I'm much more familiar with OBD-I vs. OBD-II, as I've done tuning with OBD-I, but at WOT, isn't the O2 sensor just giving a reference voltage, and not actually used in changing a/f ratio at all. Further, even if the O2 sensor can change the A/F ratio at WOT, why would it only impact cylinder #2, and not 4, 6, and 8? I'm asking, not saying the O2 isn't the cause, and can easily swap the two and re-test.

 

Now the exhaust backpressure, that is something that could be an issue. The truck was throwing a P0430, Catalyst Efficiency low-bank 2.

 

I cleared that code, and it hasn't came back in the several minutes I've driven the truck. It still may, but I haven't had the chance to drive it long enough yet. When I banged on the Y-pipe, it get a rattling inside of it, but I can't tell which side is rattling (almost sounds like both sides, as if something is in the Y-pipe).

[verse]

What Gearhead read of to you was a TSB for your vehicle under that code. That's what GM has said they've had happen and that was the fix for it.

 

That was the only TSB other than fuel contamination and to check the ground on the backside of the right head I found for that code in GM dealerworld.

[AJ]

spray carb cleaner on the intake, almost sounds like an intake leaking. there is a redesigned gasket to correct this issue. have seen many do this, we have also had a problem with a few injectors but more offten than its the intake.

[Guest Removed]

Plug looked clean and like the others.

 

Now I'm much more familiar with OBD-I vs. OBD-II, as I've done tuning with OBD-I, but at WOT, isn't the O2 sensor just giving a reference voltage, and not actually used in changing a/f ratio at all. Further, even if the O2 sensor can change the A/F ratio at WOT, why would it only impact cylinder #2, and not 4, 6, and 8? I'm asking, not saying the O2 isn't the cause, and can easily swap the two and re-test.

 

Now the exhaust backpressure, that is something that could be an issue. The truck was throwing a P0430, Catalyst Efficiency low-bank 2.

 

I cleared that code, and it hasn't came back in the several minutes I've driven the truck. It still may, but I haven't had the chance to drive it long enough yet. When I banged on the Y-pipe, it get a rattling inside of it, but I can't tell which side is rattling (almost sounds like both sides, as if something is in the Y-pipe).

i know we have been having issue with o2's causing miss fires like crazy lately.

 

and i had assumed you have check all the basic's. compression, spark, fuel pressure/volume,vacuum leaks etc.

[Dave1647545494]

I had a cracked intake gasket do that a couple months ago in a van it would idle fine and break up under a load

[Guest Hal]

I would think the intake gasket would be likely. My first thought jumped to injector but, that seems unlikely.

[Nitrousbird]

Well, the problem got 20x worse, but I found the cause.

 

I had to tow my boat over to my other house, so I could winterize/store it. The Avalanche was not fond of that across Columbus trip, and had to do half of it on surface streets as freeway towing wasn't cutting it.

 

Sprayed carb cleaner around the intake, with no results. Truck was pulling a P0300 now, random misfires. Pulled the Passenger pre-cat O2, started it up, and presto...truck runs perfect. Why? Clogged cat; with that hole there, it had room to release the exhaust fumes.

 

Pulled the Y-pipe (and other 3 O2's, and the crossmember...thank god for the impact gun), and you can hear a piece clunking in the cat.

 

Now it is gutting vs. welding in a new cat; I can't see paying $400-500 for a new catted Y-pipe, which has to be ordered anyway.

[AJ]

how many miles are on it? cats are 8 years 80,000 miles. last one i did we had in stock. they had issues with them and some trucks got extended coverage. when i read your orginal post i was hopeing it wasn't a cat since you only had the mis on one cylinder. don't drive it if you can help it as i have seen two engine failures because of this problem.

[Nitrousbird]

how many miles are on it? cats are 8 years 80,000 miles. last one i did we had in stock. they had issues with them and some trucks got extended coverage. when i read your orginal post i was hopeing it wasn't a cat since you only had the mis on one cylinder. don't drive it if you can help it as i have seen two engine failures because of this problem.

Truck has almost 117k on it; I knew about the 80k warranty, and knew I was way out of that.

 

I am waiting for Mario to bring the welder over; already cut out the old cats, and new...ummm...very very high flow "cats" are going into its place. Yes, I know a reprogram or sims will be necessary.

 

The offender had the front honeycomb come completely loose, and looks like it turned itself sideways. The other one was starting to move around a bit but wasn't loose yet.

[Guest Hal]

Rob was right on with the backpressure. I know that issue is common on some aftermarket cats but, I hadn't seen it on these.

[AJ]

I know that issue is common on some aftermarket cats but, I hadn't seen it on these.

 

90% of the time everything is fine and then boom misfire on a whole bank, 6 months later you have herpies.

[Guest Removed]

Rob was right on with the backpressure. I know that issue is common on some aftermarket cats but, I hadn't seen it on these.

kind of what i was thinking...#2 being up front was getting the most back pressure. i didnt think to post compression and vacuum leak thinking that stuff was checked before, coils and injectors went in.