Physh1

2003 Tahoe, 5.3, 225k, blackbear tune around 75k ago. Zero issues until a few weeks ago. Started fine, went to lock up at work, came back out & truck was missing quite badly. Turned it off & left it for our mechanic. He's been really busy but he has lent me some of his diagnostic tools so this is where I'm at.

Codes were running lean bank 1 with misfires on 3 & 7.

I replaced the o2 sensors with delcos (under warranty to cover my bases), new injectors, swaps coils around (no change), swapped plugs around (no change), swapped wires around (no change).

- Used a noid light on all injectors...all are good.
- Oil pressure good, no coolant in oil, no shavings in oil.
- Compression test...180 (plus or minus 5) on all 8
- Vacuum is 19-20 at idle constantly...
- Head gaskets are good...no hydrocarbons in the coolant
- Best I can tell I DON'T have castech heads
- I recently (15k ago) did intake gaskets & did them again last week just in case...no change
- fuel pressure is 62psi & the FPR is good as well
- sprayed carb cleaner EVERYWHERE & there was zero change

Weird things...plugs on cylinders 1&2 are reading rich, 3&4 better but still rich, 5&6 are normal, 7&8 are lean. Also, there is a definate change in how the truck runs when cold (open loop) than warm (closed loop).

Ses goes from solid to blinking randomly with no real change in engine sound or misfire counter.

Mechanic I work with is not sure what's going on. He hasn't really had time to put a few minutes at a time in on it & answering my dumb questions.

What am I missing & what should I look at next?

Cameron

Rekeever

Clean the mass air flow sensor and the throttle body.

Atomic

iTrader: (42)

Any exhaust leaks? Do a leakdown test (not just a compression test).

So it drives fine in open loop (when its cold)?

trxmxzx

iTrader: (7)

Seems like you covered the basics geeze I hate finding the ghost- I may have missed this if you tried it but

How about unplugging your maf? See if it gets better before it dumps more fuel- my 99 did this once went lean as hell- unplugged it-it went rich drove it home like that and put a new maf on and it was good to go- still ran rough but it wasn't 17-19 afr with it plugged in- it went to 10.5 to rich (220k)

Rekeever

I'm leaning toward the MAF because you said it runs alright in open loop. but I also wouldn't rule out an intake gasket leak. I've usually found the intake gaskets fail on the bottom where it can be hard to detect using a richening spray. But obvious with a smoke machine.

You can fake an impromptu smoke machine by getting a big fat *** cigar lighting him up and wave it around your suspected leaks

Physh1

Maf is clean, it's cleaned religiously. I forgot about the smoke test but I did that. No leaks. Tb is clean. Mechanic scanned the info off the maf & said it's good. I don't believe it's the maf...hard to have that be bad & make the front cylinders rich with the rear running super lean.

Sometimes it runs ok in open loop sometimes it doesnt. Same with closed loop. Every time though you can feel & hear when it switches from open to closed.

Cameron

Anything else?

Rekeever

If the fronts are rich and the rears are lean they must be getting unmetered air in there.

Or some freak of nature event has happened. You did say it's pretty high mileage, perhaps the intake is full of pvc puke. This is a wild *** theory but let's say the front of the intake runners are clogged up. Or carbon build up on the valves near the front. So the engine is metering the correct amount of air but it is skipping over the forward cylinders making them rich and going into the rear and causing them to lean out.

Physh1

I use Mopar combustion chamber cleaner & I also used out parts washer to do an extra cleaning on my intake. It really was un needed as I have a oil catch can. That along with the MCCC keep things nice & clean.

I'm going to do a leak down test in the next day or two & see what it says.

Cameron

Rekeever

Well you have a counter for every theory I have. I'm starting to think it's sadly internal failures. Lifter flat perhaps

gonzo 6.2

iTrader: (8)

borrow a good maf