Bad Misfire and Dieseling Upon Shutdown after Driving in the Rain

[Wasted Income]

Guys, looking for advice....

I was touching up the idle tune on my buddy's 383 LS6 CTS-V last night. Ran great all day, until I got caught in the rain a few miles from home.

It started missing / shaking really bad, both at idle, and under load. I also noticed that it is now dieseling at shutdown. (runs for a couple seconds after the key is turned off).

I searched around a bit, and it seems that O2 sensors can cause the rain misfire, but what about the dieseling? Idling so rich, that it continues to run on after key-off?

The car has longtubes and extensions with new(er) sensors.

I took a datalog with EFILive last night, and it appeared that the O2 voltage was hanging "high" around 700-800 mv, with very little, if any rich/lean switching.

Please help, my bud is supposed to sell his car on Saturday, and I want to square this away before giving it back...

I can take a datalog tonight if anyone is interested...just let me know what PIDs you'd like to see?

[DrX]

Does reverting to a previous known good tune make any difference? If yes, do a tune compare and review all differences.

Did any of the connectors under the hood get wet, including the PCM connectors? Are all connections secure?

[slows10]

Is it currently running with a maf? Does it have some kind of a aftermarket cold air intake? Does it look like it got wet? Try running a log with looking at all the basic engine sensors and see what they are doing. See if any of the values look strange.

[Wasted Income]

Does reverting to a previous known good tune make any difference? If yes, do a tune compare and review all differences.

Did any of the connectors under the hood get wet, including the PCM connectors? Are all connections secure?

I haven't tried that yet, but it ran great for about 30 miles, and about 1-2 miles after it began raining, it started missing real bad, and shuddering, etc. I believe that everything is secure. I'm going ot go over everything tonight and check it all out.

Is it currently running with a maf? Does it have some kind of a aftermarket cold air intake? Does it look like it got wet? Try running a log with looking at all the basic engine sensors and see what they are doing. See if any of the values look strange.

Yes, it is running MAF right now. It has an aftermarket intake tube, but I believe the airbox is stock. I will log some "normal" engine values tonight to see if anything looks weird.

[DrX]

Even though the connectors are "sealed" direct water spray or water dripping onto them can cause a short. If the issue is still present, try blowing out the MAF connector with compressed air.

[Wasted Income]

Thanks I will give that a shot.

[Wasted Income]

So apparently the rain had nothing to do with it.....and the "exhaust leak" that the owner heard was really a broken valve spring. Of course it broke worse when I was driving it. Started clattering quite a bit while I was troubleshooting last night. Off comes the valve cover aaaaaaaaaand......



Inner was in 3 parts, outer was in 4 parts. Awesome.

Runs great now though

[joecar]

Was that the only spring that broke...?

What is the cam spec, and what springs are they...?

Good thing you found it right away