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Jeremy airjer W

A fun little challenge

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So I was the third guy to look at a 2004 Chevy Suburban with a 5.3 liter. Customers complaint was runs ruff, low idle, doggy on initial acceleration. The intake gasket, spark plugs, fuel filter had been replaced already. The check engine light was on and the only code was a P0300 Multiple cylinder missfire. A quick look at the miss fire counters did show a lot of misfires on all cylinders with no particular cylinder that stood out except for 1 and 3 which had a lower history.

Other scan tool data showed the coolant temp was fine, MAF readings normal and nothing else out of the ordinary except for Long Term Fuel Trim at -25%, Short Term Fuel Trim at -32%, and the Upstream O2's lingered at around .975 volts.

Fuel pressure was fine at 55-60 PSI at idle. Fuel pressure held steady with the key off. Manifold vacuum at idle was at 10-12 which was odd. Back pressure on both cats less than 1/2 psi and likely much closer to 1/4 psi. The spark plugs where new and the correct plug but black and sooty. Black smoke would poor out of the except when revving the engine.

Armed with this info I did one more thing to confirm my suspicion. Any Ideas? I'll post the results and fix in a couple of days!!!

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Well this old shade tree mechanic would have checked the choke. If that was open next suspect would be the float level adjustment. However, comma pause for effect, it is not near that simple anymore! Maybe the guy put a couple gallons of diesel into the thing by accident. Buddy of mine did that to his car one time while trying to get some free gas from his dad's farm tank. Oppps! Will be interesting to find out what it was. Take care and N Joy the Hunt././Jimbo

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Fuel is fine, not contaminated. This has 8 individual coils with eight individual wires. Since there are misfires across the board so it would be unlikely that all eight coils/wires/injectors would fail.

Duckslayer, your on the right track but your heading the wrong way! (that should throw some confusion into the mix grin)

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Ok this is what I see of this. Random missfire, p0300. Missfire counts prove it is random. Fuel pressure good and holds pressure(rule out drippy injectors, leaky fuel pressure regulator). Blows black smoke, O2s reading rich, pcm trying to correct rich condition (cant). Engine has low engine vacuum, possibly low compression, or caused by excessive fuel in the cylinders, or by a weak spark. Or plugged exhaust, but back pressure tests ruled that out.

I would check to make sure the spark is good check air filter. Try to dry cylinders and perform a compression test.

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If its reading rich and can't correct, with lots of black smoke and a misfire there could be a massive vaccum leak. I thought there was a problem with the intake manifold cracking by the throttle body? Run a smoke test and see where it leaking. Maybe a dirty/bad mass airflow sensor? Really, really, dirty air filter (like a squirrel made a home in the box).

Should be interesting to see what it is...

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I didn't take the time to do an actual compression check but i did do a running compression on several cylinders all where about 100 psi which ruled out week compression. It was smoked and thats why the plenum gasket where replaced. No other vacuum leaks where found by myself. the air filter is like new, with no restriction. The MAF is not the issue. There is plenty of spark out of the coils sampled.

I'll be honest this one had me stumped and took a little over an hour for me to figure out! Heres a clue, there are several scenerios that can cause a missfire. grin

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My best guess would be the high speed bumper bolt, or the muffler belt grin

Seriously, this is one of the best forums on the site. The professionals here willing to share their knowledge is incredible!!

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No distributor and all the other sensors are reading normal except the ones that I gave specific info on.

The key word here is "lingered". The front O2's held pretty steady at over .9 volts. Nothing I could do would get them to show lean. After I figured that out my next step was to simply unplug the upstream O2's and see what happened. The PCM defaulted a value of just under .5 volts and surprise surprise, the idle returned to normal and the missfires slowly started to clear. A quick test drive with the O2's unplugged and the black exhaust cleared out. Since the upstream O2's where essentially stuck at a rich value it was giving bad info to the PCM. The PCM doesn't know any better and starts backing off fuel, a lot! Enough to start causing a lean misfire, which explains the random missfires.

So this one was running to lean not to rich as the scan tool data suggested. Two upstream O2's and it was good as new.

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Was he running 92 or higher octane fuel? There is still a reason that BOTH upstream O2 sensors were bad. Or was the intake leaking anti-freeze into the combustion chamber via a intake port? I think there is a underlying problem that has been missed. Have you taken time to run a boro-scope down a intake runner to check the condition of the intake runners or through a spark plug hole to look for carbon fouling in the combustion chamber? Something caused both to go bad at the same time. What was it?

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