Idle misfire - cant figure it out
 

have an annoying misfire at idle. I did plugs and wires and it helped nothing. I tried doing a case learn and for some reason the VCM scanner would not allow me to do it. I followed the directions but the case learn button would not highlight to allow me the option to try the feature out.

What else can I try to get rid of this annoying misfire at idle? Here is my tune and a log at idle. I scanned each cylinder for misfires and misfire history.

02 silverado 4x4 5.3

mods: Comp Cam XR265HR 212/218, .522/.529, 114LSA, Pacesetter LT's & ORY, Intake, Gibson catback, 35" tires, 4.10 gears, stock 1800 stall


thanks in advance.

What rpm do you idle at? I can't view your log, but do the misfires show up within? If so, can you correlate them to your air/fuel ratio?

idle at 700rpm

the log shows the misfire.

air fuel should be fine. i have leaned it out and richened it up to see if anything helps and nothing does.

What's your idle timing like? Are the misfires localized to a specific cylinder, or do they occur randomly?

Thanks,
Justin

Idk if this will help...

idle timing is showing between 15-16 (hit 20 on one cell). I revved the truck up really quick just to show the misfires.

http://i50.photobucket.com/albums/f3...SS/scanpic.jpg

Your long term trims are rather high. If they're being caused by a leak of some sort, then that could be causing an intermittent lean misfire, especially with a cam. Can you step through a log and look at your O2 sensor voltages at exactly the point that a misfire occurs and check them out? Even in closed loop, the switching action of the O2 sensors can trigger a lean misfire here and there.

I raised my idle timing to 35* and the truck has a noticeably better idle.

I was also told that my idle fuel trims need to be tweaked. I dont know which tables to tweak though :bang:

Fuel trims are a learned value based on the "error" between the PCMs calculation of required fuel for a given operating condition and actual. If your trims are positive (which they are), then the PCM is undercalculating the amount of fuel needed. This calculation is based on MAF, VE, IAT, ECT and IFR primarily. Airflow/airmass calculations made from MAF, VE, IAT, ECT are then used to calculate needed fuel flow based on commanded AFR and the injector flow rate.
If you're seeing a lot of reversion due to that cam, could be causing some of your idle fueling issues.

Best bet is to get your VE table nailed down via some method (wideband is preferred), then nail down your MAF calibration as needed.

also checked for leaks last night. spent about an hour looking. did all my old carb tricks to find one and nothing.