Fuel vapor purge problem in 1991 system.
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,386
Likes: 534
Fuel vapor purge problem in 1991 system.
Just curious, anyone ever had a compromised evap system audibly communicate with you? 1991 car, ZR-1. I have an aftermarket stereo and speakers. I thought this was built up current discharging out of the passenger rear speaker after the car was turned off.
Audible humming noise. I pulled every fuse and the humming continued. Finally crawled under the car to smell gas and found the check valve / line assembly making the humming noise. I tapped the evap lines at the check valve. That made the hum change pitch. Vented the tank by gently loosening the cap, big whoosh of pressure, humming immediately stopped. So something’s not happy. What I have done.
Wondering if anyone else has had a humming evap system and what it ended up being.
Audible humming noise. I pulled every fuse and the humming continued. Finally crawled under the car to smell gas and found the check valve / line assembly making the humming noise. I tapped the evap lines at the check valve. That made the hum change pitch. Vented the tank by gently loosening the cap, big whoosh of pressure, humming immediately stopped. So something’s not happy. What I have done.
- New charcoal canister, can’t remember if existing was the original. Could not push air though the old canister, it only appeared to vent out of the top and not out the “evap” port. New canister was also the same way however.
- Purge line disconnected at canister and plenum, then 50 psi of air applied to the line. No debris in the line. Air passes freely.
- Check valve removed and vacuum checked. 5 psi keeps it open to allow air to pass through. Maintains vacuum. Snaps shut as intended when vacuum released.
- Purge solenoid replaced a few years ago when I first had this issue. Bench test 12 volts did nothing to old solenoid. Replacement opened and closed as designed with 12 volts applied. I vacuum tested the solenoid at the plenum line on 5/11/26. Vacuum can be applied, but does not hold. 20 seconds to leak from 10 psi back to 0. When diagnostic terminals A and B are grounded, cannot create vacuum, which tells me the solenoid functions properly with voltage applied during diagnostic test.
Wondering if anyone else has had a humming evap system and what it ended up being.
Melting Slicks
Joined: Nov 2016
Posts: 2,256
Likes: 3,084
From: So Cal
2023 C4 of the Year Finalist - Unmodified
There is a Tank Pressure Control Valve in the vent plumbing. It is located in the RR of the car. From the FSM 6E3-C3-2: "When [tank] vapor pressure exceeds approximately .7 psi the diaphragm valve opens, allowing vapors to vent to the canister. A 1.14 mm (0.045 inch) orifice in the passage leading to the cannister tube causes pressure to drop slowly, preventing the valve from oscillating (buzzing)."
Sounds like the orifice is plugged. Based on the description "in the passage leading to the cannister tube" it may be possible to find the orifice and clean it out. Happy hunting.
Cannister purge, the solenoid and ECM control and strategy have nothing to do with the vent system prior to the cannister.
Sounds like the orifice is plugged. Based on the description "in the passage leading to the cannister tube" it may be possible to find the orifice and clean it out. Happy hunting.
Cannister purge, the solenoid and ECM control and strategy have nothing to do with the vent system prior to the cannister.
Last edited by IHBD; May 12, 2026 at 02:40 PM.
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,386
Likes: 534
There is a Tank Pressure Control Valve in the vent plumbing. It is located in the RR of the car. From the FSM 6E3-C3-2: "When [tank] vapor pressure exceeds approximately .7 psi the diaphragm valve opens, allowing vapors to vent to the canister. A 1.14 mm (0.045 inch) orifice in the passage leading to the cannister tube causes pressure to drop slowly, preventing the valve from oscillating (buzzing)."
Sounds like the orifice is plugged. Based on the description "in the passage leading to the cannister tube" it may be possible to find the orifice and clean it out. Happy hunting.
Cannister purge, the solenoid and ECM control and strategy have nothing to do with the vent system prior to the cannister.
Sounds like the orifice is plugged. Based on the description "in the passage leading to the cannister tube" it may be possible to find the orifice and clean it out. Happy hunting.
Cannister purge, the solenoid and ECM control and strategy have nothing to do with the vent system prior to the cannister.
You’re the guy that got me through that cheap keyless entry security system wired in as planned, so we’ll see how this advice lands. Will report back!
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,386
Likes: 534
Just a follow up, the culprit was an iffy check valve. It allowed pressure past when applied, but something in it after all these years allowed the buzzing/humming. Replacement check valve, and an hour of driving, and no more audible humming after a spirited drive.
Last edited by 1985 Corvette; May 17, 2026 at 10:26 PM.
Melting Slicks
Joined: May 2019
Posts: 3,245
Likes: 1,404
From: Mid-Atlantic
2025 C4 of the Year Finalist - Unmodified
2023 C4 of the Year Finalist - Unmodified
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I've heard that same humming noise .
Which check valve specifically?
I have a 90, so all of that stuff is up in the front, LH side of the bumper.
Which check valve specifically?
I have a 90, so all of that stuff is up in the front, LH side of the bumper.
Thread Starter
Le Mans Master
Joined: May 2004
Posts: 5,386
Likes: 534
A link to the part in question:
https://www.zip-corvette.com/91-96-v...hoC_AwQAvD_BwE
Link says it’s for 91-96 only, but I believe this check valve is compatible with 85-96 purge systems.
Melting Slicks
Joined: May 2019
Posts: 3,245
Likes: 1,404
From: Mid-Atlantic
2025 C4 of the Year Finalist - Unmodified
2023 C4 of the Year Finalist - Unmodified
2022 C4 of the Year Finalist - Unmodified
2021 C4 of the Year - Unmodified Finalist
A link to the part in question:
https://www.zip-corvette.com/91-96-v...hoC_AwQAvD_BwE
Link says it’s for 91-96 only, but I believe this check valve is compatible with 85-96 purge systems.
https://www.zip-corvette.com/91-96-v...hoC_AwQAvD_BwE
Link says it’s for 91-96 only, but I believe this check valve is compatible with 85-96 purge systems.
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