L.t.f.t. Maxed out at idle only

[smslyguy]

This is on a completely stock 99 chevy silverado 5.3. The L.T.F.T become maxed out while idling for a while then i notice a multi cylinder missfire, which is probally do to the fuel trims being maxed out. While driving down the road the L.T.F.T ARE within reason.

I understand that a vacum leak would throw off the fuel trims, but wouldn't it even under load? I replace the o2 senser, which was bad anyways, fixed an exhaust leak, cleaned the injector's ( profesionally cleaned and fuel filter.

The fuel pressure at idle is normal and under load as well. No vacum leaks were ovious or able to find. Here is a copy of the scan i ran. any help would be appreciated. I did own a 2000 gmc with a 5.3 with a similiar problem and i realize these are comon for a intake manifold gasket, but it didn't do a thing on my 2000 after replacing it.8748

[joecar]

At idle, an airleak is a significant portion of airflow, and becomes less significant as throttle opens.

LTFT's pegging positively can be due to:
- airleak at intake or exhaust,
- insufficient injector/fuel flow,
- EGR malfunction causing excess dilution,
- EVAP malfunction causing excess un-metered airflow.

If the LTFT's peg, I would expect DTC's reflecting this. Do you see any other DTC's...?

You may also have two concurrent problems, one causing LTFT's to peg and the other causing misfires.

Check the coil connections and grounds, check ignition wires/plugs.

Might want to clean the MAF... do you use an oiled air filter...?

I haven't had a chance to look at your file yet... later tonite.

:)

[joecar]

Ok, I looked at your log file...

Some comments:

The misfire counters show almost no activity, this low amount of activity is considered normal (and there is no DTC's present for this);

DTC P0449 (EVAP vent solenoid) is present, see P0449 (http://www.ls2.com/boggs/dtcs/DTC%20P0449.htm);
disconnect the EVAP tube from the intake manifold, and plug the intake manifold, see if this has any effect on your LTFT's, if it does then you found the source of the problem;

The O2 sensors are switching quite slowly,

Remove TPS[V], RAMFAIL, ROMFAIL, CYLAIR_DMA, MFCYCLxC and add TP[%], MAF, FUELSYS, VSS, DYNCYLAIR, DYNAIR and log again (keep pid channel count to 24 or less, we want highest sample rate to see HO2S voltages).

[smslyguy]

Thanks joe, i will try to plug the evap tube tonight. I did replace one of the 02 sensor's , which was bad. I will get back to you later on the results. Thanks for the reply.

[smslyguy]

No luck with evap tube, however, the initial pump shot is below specs and it doesn't hold pressure, but while the vehicle is running the fuel pressure is normal. Don't know why it looses pressure. Unless it is related to the evap system??? It turns out i had a bad fuel pressure regulator. It wouldn't hold pressure until the vehicle was running.