P0442 (Small Evap Leak) Mystery

[Gildan]

P0442 Small Leak Detected code that cannot be cured. Think of all the possible causes and it is not any of them. Anyone have any ideas what is being missed?
I have a perennial P0442 code that has been nagging me and the dealership (and a whole parade of mechanics for years now). Nothing that has been done gets rid of it and no diagnosis or procedure known to anyone can find what is causing this issue. Everything has been replaced (and I mean everything), and at each fix, you get about two weeks before the CEL pops on with a P0442. The EVAP I/M system has been smoked tested more times that a Cheech and Chong movie.

Every part, component, etc., has been replaced, tested, replaced again and tested again to no avail. Every single thing checks out correct, and the freeze frame data shows absolutely nothing that is out of specs at all. We even closed all the assorted valves and applied a vacuum to the system at the proper amount and let is sit all night without so much as thousandth of a millimeter loss of vacuum. Nothing.

Here are two ways the problem can be reproduced every single time to get the code to set:

1.) Reset the code; wait for the EVAP I/M to go through its drive cycle. Drive around to your hearts content and no code gets set, that is, until you get down to exactly 1/4 tank of gas at which point, the next time you start the engine, and the P0442 code sets.

2.) After the EVAP monitor drive cycle completes, part the Yukon on an unlevel surface where the passenger side of the vehicle is even the slightest bit lower than the driver's side.


And, to make it all the more bizarre:

1.) If you keep the fuel level about 1/4 tank *and* you don't park on an unlevel surface sloping down to the passenger side, you can go forever without the P0442 code lighting up the CEL.

The only thing that I haven't tried is to call an Exorcist, and I may even try that. Ya' never know.

 

[Fless]

I missed the year, what are you driving?

Fuel tank pressure sensor been verified as good, or replaced? Fuel pump gasket?

EDIT: if the FTPS is not reporting the tank pressure correctly, the evap code can be triggered. Leak or no leak.

 

[Gildan]

I missed the year, what are you driving?

Fuel tank pressure sensor been verified as good, or replaced? Fuel pump gasket?

EDIT: if the FTPS is not reporting the tank pressure correctly, the evap code can be triggered. Leak or no leak.

All of that is verified as good and replaced while we were in there checking things out.

 

[Fless]

What are you driving?

Would you list "everything" that's been replaced? Are the replacement parts OE, or aftermarket?

 

[Gildan]

What are you driving?

Would you list "everything" that's been replaced? Are the replacement parts OE, or aftermarket?

2008 Yukon Denali. All OEM.

 

[OR VietVet]

Thanks for sharing the year and model. Put that info in your signature so it is always there when you start a thread or respond. Or not.

When you say everything has been replaced, are you saying the tank was dropped and all was replaced, including the fuel pump or all was tested and the pump is still the same?

Your first #1 makes it sound like you get to 1/4 tank and the light will come on even if level or am I missing that at 1/4 tank you always have to park at the lean to get the light on?

After reading all the threads here about evap problems, especially when replacing one part at a time, I had no problems but decided to replace the fuel pump and the entire evap system before the inevitable happened.

 

[Fless]

One of the reasons the code is set in the morning, and not during the day drives, is that a cold start is needed to initiate the drive cycle. There are several temperature requirements for the IAT (intake air) and the ECT (engine coolant), as well as the fuel tank level being between 1/4 and 3/4 full (technically I think it's 1/8 to 7/8th full).

Partial clip (more on the site) from https://www.smogtips.com/smog-question/3715/How-To-Perform-a-Chevy-Suburban-Drive-Cycle:

A. Cold Start: Begin from Cold Start, and ignition in OFF position for at least 1 hour. Ensure your Chevy Suburban's fuel tank is between 1/4 and 3/4 full. Engine coolant temperature must be below 122°F and within 11°F of the ambient air temperature. Do not leave the key on prior to the cold start or the heated oxygen sensor (O2) diagnostic may not run.

So since it's been indicated that everything's been replaced, does this include:
Purge valve on engine replaced
Blowing air through the purge valve line to the charcoal canister
Canister replaced (were loose charcoal bits found coming from the canister?)
Vent valve replaced (including the filter and relocating the filter)
Evap lines from canister to tank blown clear
Fuel tank pressure sensor replaced
Fuel pump replaced (if not, what is the condition of the top of the fuel pump?)
Fuel pump gasket replaced
Gas cap replaced
Gas filler tube replaced

Anything else found and/or replaced during the diagnostics?

 

[Gildan]

Everything has been replaced. New tank, filler neck, pump, pressure sensor, vent valve, charcoal canister, various solenoids, gas cap, vacuum lines, wiring harnesses, etc., and a whole list of things I haven't listed. All the same exact results. A perpetual P0442 code. You can fix it for about two weeks and it always comes back like clockwork.

There is one component that no one can check because no one, including the service technicians (and its location is not given in the service manuals but appears in the schematics), knows where it is: A vacuum pump that applies vacuum to the whole evap system for a period of 10 seconds at every start up before or as the engine starts to detect a vacuum leak of any size. And no one knows exactly where it is and how to get to it, and the literature is decisively lacking that information.

 

[mrhp]

Have you replaced your purge valve, not the vent valve? Can both valves be bi-directionally function tested with the use of a scan tool? An intermittent sticking valve will set this code. The purge valve is often overlooked.

 

[Gildan]

One of the reasons the code is set in the morning, and not during the day drives, is that a cold start is needed to initiate the drive cycle. There are several temperature requirements for the IAT (intake air) and the ECT (engine coolant), as well as the fuel tank level being between 1/4 and 3/4 full (technically I think it's 1/8 to 7/8th full).

Partial clip (more on the site) from https://www.smogtips.com/smog-question/3715/How-To-Perform-a-Chevy-Suburban-Drive-Cycle:

A. Cold Start: Begin from Cold Start, and ignition in OFF position for at least 1 hour. Ensure your Chevy Suburban's fuel tank is between 1/4 and 3/4 full. Engine coolant temperature must be below 122°F and within 11°F of the ambient air temperature. Do not leave the key on prior to the cold start or the heated oxygen sensor (O2) diagnostic may not run.

So since it's been indicated that everything's been replaced, does this include:
Purge valve on engine replaced
Blowing air through the purge valve line to the charcoal canister
Canister replaced (were loose charcoal bits found coming from the canister?)
Vent valve replaced (including the filter and relocating the filter)
Evap lines from canister to tank blown clear
Fuel tank pressure sensor replaced
Fuel pump replaced (if not, what is the condition of the top of the fuel pump?)
Fuel pump gasket replaced
Gas cap replaced
Gas filler tube replaced

Anything else found and/or replaced during the diagnostics?

All that was done. Everything was checked or replaced and it still throws the code. Like clockwork. I did talk to a couple of GM techs about this and they said that there is some kind of vacuum pump on the EVAP system that generates a slight 1 inch water's worth of vacuum and holds the system sealed for about 10 seconds to see if there is any drop in the vacuum. The problem is that the techs have no idea where that pump is located nor does their service literature tell you where the vacuum pump is located.

I've even done a service bay reset of the EVAP I/M (GM locks and hides this service feature, so you can't do it unless you have the means to hack your own scanner, but that ability is there) so that you don't have to go through hoops of fire to get the drive cycle to do its magic. The only thing that cannot be diagnosed, because you can't find it, is the vacuum pump that engages to apply a vacuum for the test. If you do a service bay reset on the EVAP system, you get exactly 21 warmup cycles and the error code gets thrown (and that's if you don't deliberately trigger it by parking on a slope or have your tank less than 1/4 full).
Either way, during the test, there appears to be a leak down of vacuum somewhere. I suspect it's at the elusive EVAP vacuum pump that no one can locate or they just won't tell you where it is. The P0442 code problem is a known issue on a lot of GMCs and no one, not even the dealership, knows how to fix it once it starts. That is, unless you can find that vacuum pump which location is apparently more secret than the contents of Fort Knox.