Bypassing the EVAP system? Any thoughts or experience?
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5th Gear
Joined: Jun 2025
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Bypassing the EVAP system? Any thoughts or experience?
So I’m very close to solving an issue I had with my 1993 stalling. I realized the line going to my purge solenoid is completely clogged and the purge solenoid isn’t functional. When taking off the gas cap I get a 10 second “whoooosh”.
Normally I’ll stall after 10 minutes of driving, but decided to try driving around with no cap just to see what happened. No stalling and no longer any bogging or shaking like before. I’ve never heard the car run so well.
So obviously driving with no gas cap on a regular basis isn’t a good idea, but how about getting a cap that lets air vent out instead of only in? Or drilling holes in my cap? The long term plan is to replace the EVAP system, but what could I do for the time being? There is no emissions testing where I am, I’m more concerned about safety.
Normally I’ll stall after 10 minutes of driving, but decided to try driving around with no cap just to see what happened. No stalling and no longer any bogging or shaking like before. I’ve never heard the car run so well.
So obviously driving with no gas cap on a regular basis isn’t a good idea, but how about getting a cap that lets air vent out instead of only in? Or drilling holes in my cap? The long term plan is to replace the EVAP system, but what could I do for the time being? There is no emissions testing where I am, I’m more concerned about safety.
Race Director
Joined: Jun 2004
Posts: 13,961
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From: WI
The cap should vent, turn it over and look to see if it's not smashed or stuck etc. You could use a lawn mower or snowmobile or whatever cap that has a hole in it. I made a thread about evap removal on a track car long back: https://www.corvetteforum.com/forums...terectomy.html
Race Director
Joined: Jun 2004
Posts: 13,961
Likes: 707
From: WI
Every cap should be vented since gas tank invented. The ones on modern cars are pressure tested at emissions testing station if you go to one ask about it. Canister is also vented and lines are known to clog search threads might turn up more.
Melting Slicks
Joined: Nov 2016
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From: So Cal
2023 C4 of the Year Finalist - Unmodified
I'm not sure of the exact C4 year that applies (It may be when the charcoal can went to the rear of the car) , but my 1992 has a Tank Pressure Control Valve between the tank and the charcoal can. The valve maintains appx .7 psi. So some woosh is normal. My 92 occasionally is much higher than .7 psi, usually after sustained highway running in high ambient temperatures. I haven't investigated it. There is a control diaphragm on the TPCV. If the diaphragm fails, or something else goes wrong with the TPCV the tank won't purge properly. There is a test procedure for the TPCV in section 6E3-C3 in Book 2 of the FSM.
An electrical failure of the purge solenoid should set a Code 26. (Code 26 does not turn on the Service Engine Soon light.) Use the onboard diagnostics to look for H24 in Module 4. If H24 is present, check the solenoid coil resistance and connections.
If the purge line is clogged (It may not be. The TPCV can "block" the line until minimum pressure is present), and the solenoid is non-operational, I don't see how tank pressurization or removing the cap to relieve the pressure can influence "stalling". If excessive vapor is reaching the engine, Code 45 and/or Code 65 (rich exhaust indicated) should result.
Cheers.
An electrical failure of the purge solenoid should set a Code 26. (Code 26 does not turn on the Service Engine Soon light.) Use the onboard diagnostics to look for H24 in Module 4. If H24 is present, check the solenoid coil resistance and connections.
If the purge line is clogged (It may not be. The TPCV can "block" the line until minimum pressure is present), and the solenoid is non-operational, I don't see how tank pressurization or removing the cap to relieve the pressure can influence "stalling". If excessive vapor is reaching the engine, Code 45 and/or Code 65 (rich exhaust indicated) should result.
Cheers.
Last edited by IHBD; Jun 19, 2025 at 08:17 PM.
Race Director
Joined: Jun 2004
Posts: 13,961
Likes: 707
From: WI
