Dreaded P0442
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Dreaded P0442
I'm stumped... Short story is dreaded P0442. I've smoked the system and found no 'leaks'. Here is the kicker... I can command the system closed for testing purge/vent sealing but when I open the fuel cap I see no change in the fuel tank pressure signal while monitoring... doesn't seem logical to me. I have smoked the filler neck after making an adaptor from an old cap and have no leaks or had no previous or current fuel filling issues. Is it possible to have a bad roll over valve in the tank? I see there is a small hose as well as a larger hose connection on these...
2001 GMC Jimmy 4.3 4X4 106,000 miles.
2001 GMC Jimmy 4.3 4X4 106,000 miles.
BF Veteran
Joined: Jan 2007
Posts: 4,629
From: Florida
Are you commanding the vent valve, the purge valve or both closed?
What is the fuel tank pressure reading before and after this command with the fuel cap in place?
Have you tried just replacing the fuel cap?
George
What is the fuel tank pressure reading before and after this command with the fuel cap in place?
Have you tried just replacing the fuel cap?
George
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Beginning Member
Joined: Dec 2017
Posts: 19
Using the scan tool, I closed the vent solenoid and observed the fuel tank pressure sensor value (I believe it was in H2) change then opened the vent valve to watch the value return to it's starting point. In theory that should show a good system to start.
I then ran the same test and took the gas cap off and No change in value as it did when I turned the vent valve back to venting.
Then I ran the service bay test and it failed the EVAP small leak vacuum decay rate test.
[img alt="Service bay test result
"]https://cimg4.ibsrv.net/gimg/blazerforum.com-vbulletin/1280x800/evap_test1_259aba02cf27776f12096675adf59e3cc624e4d 0.png[/img]
Service bay test result
Again the at rest value of the fuel pressure sensor responds to sealed and unsealed commands and no smoke is observed at the lines or fuel pump area.
When I do inject smoke at the test port near the engine, I do not see any smoke to enter the fuel filler neck and if you remove the large line off from the EVAP Canister there is a woosh of stored smoke in the canister. I did remove the canister and it shows no deterioration or signs of being damaged or plugged.
I did replace the gas cap as it is always suspect at over 100,000 miles with a NAPA cap that has the same OE style O-ring. Same test results using old and new cap.
Quick note: I did have my truck in the shop in March for a Lean code bank 1&2 and the faulty Purge solenoid was replaced.
I then ran the same test and took the gas cap off and No change in value as it did when I turned the vent valve back to venting.
Then I ran the service bay test and it failed the EVAP small leak vacuum decay rate test.
[img alt="Service bay test result
"]https://cimg4.ibsrv.net/gimg/blazerforum.com-vbulletin/1280x800/evap_test1_259aba02cf27776f12096675adf59e3cc624e4d 0.png[/img]
Service bay test result
Again the at rest value of the fuel pressure sensor responds to sealed and unsealed commands and no smoke is observed at the lines or fuel pump area.
When I do inject smoke at the test port near the engine, I do not see any smoke to enter the fuel filler neck and if you remove the large line off from the EVAP Canister there is a woosh of stored smoke in the canister. I did remove the canister and it shows no deterioration or signs of being damaged or plugged.
I did replace the gas cap as it is always suspect at over 100,000 miles with a NAPA cap that has the same OE style O-ring. Same test results using old and new cap.
Quick note: I did have my truck in the shop in March for a Lean code bank 1&2 and the faulty Purge solenoid was replaced.
Last edited by KevinZ; Jul 5, 2025 at 08:56 AM.
BF Veteran
Joined: Jan 2007
Posts: 4,629
From: Florida
Just to make sure that we are on the same page:
The purge valve is at the engine, the vent valve is at the “intake” side of the circuit at the back of the truck. The canister, fuel tank, pressure sensor and gas cap are in between.
Under normal operation with the truck running, the gas cap is sealed, the vent solenoid is open and the purge solenoid is partially open at around 20%. So while the engine is running and not under test, the evap system is using engine vacuum to constantly pull fresh air through the canister and into the engine. If you open the gas cap under this condition not much will change. The fuel tank pressure is higher.
If you close the purge valve then no more engine vacuum is applied to the circuit. Opening the gas cap will have little effect. Same is true if you now close the vent solenoid. Opening the gas cap will not do much.
If the engine is running and you leave the purge solenoid partially open and close the vent solenoid the evap system pressure will start to drop as a vacuum develops. Under this scenario then opening the gas cap will cause a major system leak and the fuel tank pressure will rise. Opening the vent solenoid will do the same thing.
You can watch the system being perturbed by watching the fuel tank pressure or short term fuel trims if there is a pressure sensor problem
So have the truck running and not under test. Confirm that the purge valve is partially open. Look at the fuel tank pressure. Close the vent valve and watch the fuel tank pressure drop. Open the gas cap and watch the fuel tank pressure rise.
George
The purge valve is at the engine, the vent valve is at the “intake” side of the circuit at the back of the truck. The canister, fuel tank, pressure sensor and gas cap are in between.
Under normal operation with the truck running, the gas cap is sealed, the vent solenoid is open and the purge solenoid is partially open at around 20%. So while the engine is running and not under test, the evap system is using engine vacuum to constantly pull fresh air through the canister and into the engine. If you open the gas cap under this condition not much will change. The fuel tank pressure is higher.
If you close the purge valve then no more engine vacuum is applied to the circuit. Opening the gas cap will have little effect. Same is true if you now close the vent solenoid. Opening the gas cap will not do much.
If the engine is running and you leave the purge solenoid partially open and close the vent solenoid the evap system pressure will start to drop as a vacuum develops. Under this scenario then opening the gas cap will cause a major system leak and the fuel tank pressure will rise. Opening the vent solenoid will do the same thing.
You can watch the system being perturbed by watching the fuel tank pressure or short term fuel trims if there is a pressure sensor problem
So have the truck running and not under test. Confirm that the purge valve is partially open. Look at the fuel tank pressure. Close the vent valve and watch the fuel tank pressure drop. Open the gas cap and watch the fuel tank pressure rise.
George
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Beginning Member
Joined: Dec 2017
Posts: 19
Just to make sure that we are on the same page:
The purge valve is at the engine, the vent valve is at the “intake” side of the circuit at the back of the truck. The canister, fuel tank, pressure sensor and gas cap are in between.
Under normal operation with the truck running, the gas cap is sealed, the vent solenoid is open and the purge solenoid is partially open at around 20%. So while the engine is running and not under test, the evap system is using engine vacuum to constantly pull fresh air through the canister and into the engine. If you open the gas cap under this condition not much will change. The fuel tank pressure is higher.
If you close the purge valve then no more engine vacuum is applied to the circuit. Opening the gas cap will have little effect. Same is true if you now close the vent solenoid. Opening the gas cap will not do much.
If the engine is running and you leave the purge solenoid partially open and close the vent solenoid the evap system pressure will start to drop as a vacuum develops. Under this scenario then opening the gas cap will cause a major system leak and the fuel tank pressure will rise. Opening the vent solenoid will do the same thing.
You can watch the system being perturbed by watching the fuel tank pressure or short term fuel trims if there is a pressure sensor problem
So have the truck running and not under test. Confirm that the purge valve is partially open. Look at the fuel tank pressure. Close the vent valve and watch the fuel tank pressure drop. Open the gas cap and watch the fuel tank pressure rise.
George
The purge valve is at the engine, the vent valve is at the “intake” side of the circuit at the back of the truck. The canister, fuel tank, pressure sensor and gas cap are in between.
Under normal operation with the truck running, the gas cap is sealed, the vent solenoid is open and the purge solenoid is partially open at around 20%. So while the engine is running and not under test, the evap system is using engine vacuum to constantly pull fresh air through the canister and into the engine. If you open the gas cap under this condition not much will change. The fuel tank pressure is higher.
If you close the purge valve then no more engine vacuum is applied to the circuit. Opening the gas cap will have little effect. Same is true if you now close the vent solenoid. Opening the gas cap will not do much.
If the engine is running and you leave the purge solenoid partially open and close the vent solenoid the evap system pressure will start to drop as a vacuum develops. Under this scenario then opening the gas cap will cause a major system leak and the fuel tank pressure will rise. Opening the vent solenoid will do the same thing.
You can watch the system being perturbed by watching the fuel tank pressure or short term fuel trims if there is a pressure sensor problem
So have the truck running and not under test. Confirm that the purge valve is partially open. Look at the fuel tank pressure. Close the vent valve and watch the fuel tank pressure drop. Open the gas cap and watch the fuel tank pressure rise.
George
I did check with the engine running the same test for vent solenoid actuation on/off and commanded the vent closed at idle, watched the pressure value change, took off the gas cap to watch the pressure value change back to vent ,heard the air rush in, and value went back to original reading.
Sorry to not have the readings as I am still learning the retrieval in my new scanner.
Here is what I have so far:
EVAP Values
EVAP readings at idle.
EVAP service bay test.
Thread Starter
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Beginning Member
Joined: Dec 2017
Posts: 19
I did gaze in disgust at my scan tool, as everything seemed 'normal' when I just kept looking at the vent on/off data as well as pressure sensor readings, when all the once I saw the pressure glitch... I wish I could have graphed and recorded this, It seemed to drop off and then back to normal. (this was just sitting there at idle)
Now I am recalling the mechanic said he lost communication with all of his scanners for a while, when road testing my truck.
Could this be a bad ground??? Bad sensor???
I would like to clean these grounds, as I found the one located above the vent valve, but haven't located ground #100 with California emissions, (CKT 1550) located at the air harness on the lower right radiator support. The G100 is the main ground for the computer control of many things including EVAP.
Any suggestions on G100 location as my ALDATA does not seem to give only a description rather than a sketch...?
I think we are making headway, as it may not be mechanical example physical leak, But may be electrical.???
Truck runs great with only the P0442 code and will not pass the EVAP service bay test.
Thoughts???
By the way, Thank You for the Excellent description of the EVAP system George
, now I can understand how it really works.
Now I am recalling the mechanic said he lost communication with all of his scanners for a while, when road testing my truck.
Could this be a bad ground??? Bad sensor???
I would like to clean these grounds, as I found the one located above the vent valve, but haven't located ground #100 with California emissions, (CKT 1550) located at the air harness on the lower right radiator support. The G100 is the main ground for the computer control of many things including EVAP.
Any suggestions on G100 location as my ALDATA does not seem to give only a description rather than a sketch...?
I think we are making headway, as it may not be mechanical example physical leak, But may be electrical.???
Truck runs great with only the P0442 code and will not pass the EVAP service bay test.
Thoughts???
By the way, Thank You for the Excellent description of the EVAP system George
, now I can understand how it really works.