L31 Vortec questions

[travis]

So...the LT fuel trims are still maxed out...

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The ST fuel trims start out at 40-48% on a cold start. Where they are in the above pics is after it is fully warmed up.

Idle vacuum is a rock solid 20”

From the scan tool, idle timing varies between about 20* to 23.5*. I also put a mark on the balancer and watched it with a timing light...it moves all over about a 1” range. So most likely the timing chain is shot.

I cannot find a vacuum leak. I’ve sprayed brake cleaner around every gasket surface and vacuum operated accessory that I can reach. All upper end gaskets have been replaced twice. There is no oil on the plugs to indicate an internal intake gasket leak. I’ve plugged off the very few vacuum ports to rule out anything beyond the engine as well.

[turbo camino]

Spark timing is determined by the 4x crank signal, nothing to do with the distributor. CMP is only used for discrimination. See the "Enhanced Ignition System Description" section in the service manual.

[travis]

So...I did end up finding a vacuum leak after all. There is a “can” built into the plastic upper intake manifold that has 2 vacuum connections coming off of it. 1 to the hot water control valve, and 1 to the PCV valve. It has a removable lid on it...that wasn’t on correctly. Fixed that...the fuel trims came down a bunch but still too high (13-18% on the LT’s). Still surging and bucking at light throttle.

After getting nowhere troubleshooting further, on a whim I disconnected the MAF sensor. The engine bogged heavily for a split second, then resumed idling. LT trims started dropping, and the ST’s dropped to +.08% to -3.1%. Took it for a test drive with the MAF still unhooked, and it runs and drives perfect now. LT’s still show high when driving but the ST’s are perfect now.

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[GARY C]

I know who I am calling if my 09 starts acting up...

[dannobee]

We did have problems with people over oiling K&N style air filters. The oil would stick to the little sensor in the MAF and let junk deposit on it. You can carefully clean it off with a Q-tip and some brake cleaner. Look inside the MAF and you'll see what looks like a resistor perpendicular to the air flow.

[travis]

We did have problems with people over oiling K&N style air filters. The oil would stick to the little sensor in the MAF and let junk deposit on it. You can carefully clean it off with a Q-tip and some brake cleaner. Look inside the MAF and you'll see what looks like a resistor perpendicular to the air flow.

This has a stock replacement paper filter. I’ve always heard that the K&N’s oil will mess up the MAF’s on EFI engines

[travis]

I know who I am calling if my 09 starts acting up...

That’s fine as long as you don’t expect your truck back for the next year or 2