DTC-P0442

vhato · September 21, 2005

It is driving me nuts. My local dealer says it is common on 2005 trucks, but they don't know what it is. I don't really believe it is common and GM can't figure it out.

Anyway it is 2005 Chevrolet 1500 Crew Cab w/Vortec 5300 and 4L60-E. About every 2 weeks I plug in AutoTAP and clear the code. We (me and dealer) have verified the tightness of the gas cap after fill ups and I can't see any hoses that are cut or removed.

Any ideas??? How exactly does the PCM detect a small Evap leak anyway? If I knew that I could start doing better tracing.

Thanks in advance. Glad you are here.

V

GM-Tech · September 21, 2005

Down below is how the system runs the test. If they've just verified the cap is tight and visually inspected lines, that won't tell 'em squat. The test can detect a leak as small as .020" and that's not something you're going to see.

Have they done a smoke test? That helps, but isn't always successful at finding 'em either. Is it the original cap?

Here's how the test is performed by the vehicle...

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SMALL LEAK TEST

The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the evaporative emission (EVAP) system. While previous leak detection methods were performed with the engine running, the EONV diagnostic monitors the EVAP system pressure or vacuum with the ignition OFF. Because of this, it may be normal for the control module to remain active for up to 40 minutes after the ignition is turned OFF. This is important to remember when performing a parasitic draw test on vehicles equipped with EONV.

The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, then starts to drop. The EONV diagnostic relies on this temperature change and the corresponding pressure change in a sealed system, to determine if an EVAP system leak is present.

The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 in). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed, a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 in) leak is present, often little or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.

J45p3r · October 24, 2005

My fathers 2004 GMC Sierra 4.8L Z71 is doing a similar thing. First, while driving, the DIC will come up with "Tighten Fuel Cap". So far this has only happened with the tank at 1/2 or below. After a few driving cycles when he starts the truck, the CEL will be on with a code P0442. We replaced the gas cap with a new GM replacement and just today he got the Tighten Gas Cap message.

Any additional thoughts on the matter?

Thanks!

vhato · October 25, 2005

The dealer ran the smoke test and found a fuel tube or vent hose or something dosconnected from the top of the fuel tank.

GM-Tech · October 26, 2005

So far this has only happened with the tank at 1/2 or below.

The P0442 test will only run when the fuel is at a certain level (I think 15%-85%), and if certain other conditions are met, so this fact doesn't mean much.

I have no real good suggestions because you just have to find the leak. If I had to guess, I'd guess vent valve, but there's only a 50/50 chance it could be that. It needs to be tested with smoke as previously mentioned in this thread and the leak found and a 0.020" pinhole isn't always so easy to find.