Wierd idle problems

[chrleb2]

I have a customers truck in the shop right now. It is an 03 chevy 1500 and the stock tune is in it. The truck has headers and an intake right now. It came in because the CAT was dead on the driver side and getting even more plugged. I cut the cats for right now to test the motor. The plugs were fouled in it.

Right now I am having a misfire problem. It consistantly misfires on #2 and #7. If you give it throttle, it clears up and doesn't miss. I've changed the o2s, plugs, moved spark plug wires, injectors, coils, and checked the MAF. After all of this, it still misfires. Now for the wierd part, if i add global timing into it from the scan program, the missing goes away and fuel trims equal out close to 0. When missing the ltft's are 16-18 for the driver bank and 0 - .8 for the pass bank. This is driving me up the wall. The part that has got me wondering is the effect of added timing (10-12*) at idle.

[beaukz28]

I'm new here and a beginner but I think I can help. I work at a Chevy dealer and have seen a lot of these trucks with misfires. A very common problem is bad intake gaskets causing a vaccum leak from the valley area. GM has come out with a revised gasket set and they are green in color. Possible ways to check are to look at the maf values with a scan tool. They will be low with the leak. Also sometimes you can take some brake line and feed it under the valley to spray in something flammable and watch the fuel trims to see if they settle back to 0. I think the timing you are adding is just covering up the lean misfire by giving it more time to burn. This is probably what caused the cat to burn up. Hope I helped. Let me know.

[chrleb2]

I checked for leaks around the manifold. I sprayed starting fluid around while it was running.

[beaukz28]

Make sure you spray into the valley. Thats where they always leak. You have to force a piece of brake tubing under the intake through the foam insulator and then spray into the valley.

[ChipsByAl]

Another possibility is an injector clogged. #2 and #7 injectors are the most common to clog. Pull the #7 first and inspect. You will likely find it semi clogged. Injector balance tests do not always reveal it.
Al