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Hello, new member so I look forward to learning a lot on this forum. I am looking into purchasing a low mileage and well cared for 1988 Iroc. I do have one question that I would like to have answered.
The car ran for about 30-45 minutes, idling and some low speed moving around the garage, with no issues that could be heard. Once the car was shut off it would sound like it would vent or hiss from near the rear of the car. Sounded like it was from the gas tank/cap area. I was wondering if this is normal and if it is not then what could be causing this? Possible fixes or issues to look out for?
I search the web for the past hour or so and I couldn't find anything that sounded like what I am hearing.
There's a charcoal canister under the hood (probably behind the driver side headlights). gas tank fumes collect in there, and then under certain conditions, the ECM will open a valve letting those fumes vent into the throttle body, and then into the intake for combustion. If the canister is clogged and fumes can't get in, pressure can build in the tank. If fumes can get IN the canister, but the valve is damaged, stuck etc. etc., then fumes can't get out. Same thing, pressure in the tank. When it builds to a certain point, it's gonna vent with psssshhhhhh.
On my car I had the same thing. It would do this everytime I ran it for more than a few minutes. To technically remain emissions compliant you need to fix it. In NC where I live, we only have safety inspections for OBD1 cars ...anyhow, I built a charcoal canister/filter out of PVC pipe, filled it with fish tank charcoal, drilled some vent holes, and connected it to the metal line coming from the gas tank. No gas smell underhood, no driveability issues, no extra pressure build up in the tank.
I built a charcoal canister/filter out of PVC pipe, filled it with fish tank charcoal, drilled some vent holes, and connected it to the metal line coming from the gas tank. No gas smell underhood, no driveability issues, no extra pressure build up in the tank.
That's brilliant. I have got to see your DIY canister!
There's a charcoal canister under the hood (probably behind the driver side headlights). gas tank fumes collect in there, and then under certain conditions, the ECM will open a valve letting those fumes vent into the throttle body, and then into the intake for combustion. If the canister is clogged and fumes can't get in, pressure can build in the tank. If fumes can get IN the canister, but the valve is damaged, stuck etc. etc., then fumes can't get out. Same thing, pressure in the tank. When it builds to a certain point, it's gonna vent with psssshhhhhh.
On my car I had the same thing. It would do this everytime I ran it for more than a few minutes. To technically remain emissions compliant you need to fix it. In NC where I live, we only have safety inspections for OBD1 cars ...anyhow, I built a charcoal canister/filter out of PVC pipe, filled it with fish tank charcoal, drilled some vent holes, and connected it to the metal line coming from the gas tank. No gas smell underhood, no driveability issues, no extra pressure build up in the tank.
Thank you! What would be the fix or things to look at that are mostly likely the culprit? The vehicle is all original and I would like to keep it that way. Is there a way to unclog that charcoal canister?
EDIT: If unable to unclog it would a whole new canister and canister valve help this?
Last edited by midwestiroc; 05-11-2017 at 01:43 PM.
First thing I'd do is search for canister, charcoal, emissions etc. etc, and read up on exactly how it's supposed to work. I KNOW I've read the operation of the ECM, and the purge valve etc. etc, but can't recall exactly the how and when of it's operation. Find a schematic, grab your multi-meter and test the wires, make sure it's function properly, and make sure the vacuum line isn't broken, etc. etc. If all that's working properly, it could be the canister. I don't "think" it's designed to be serviced, but anything is possible. Just search. I know I've seen people dig into them and refresh them. ....I was going to go down this road myself, but with the safety only inspection, and then the final blow being the new Holley EFI not having controls for the emissions, I just went the home-made filter route.
" vent or hiss from near the rear of the car. Sounded like it was from the gas tank/cap area"
This guy is back buy the axle, drivers side. It was probably relieving pressure. Its a 2 way valve. Remove it and take it apart carefully and clean it up. Sometimes it will even GROAN and MOAN.
some good info. Search for "Charcoal Canister" ...not ALL of it is useful, but it's worth looking through. Here are a few that looked promising. Literally 4 pages of search results. Lot's of em are for sale, and some are the same ole same ole, but look through em and read em. ...you'll learn a ton. Search is your friend.
Bob, I was under the impression that THAT valve let air IN....under vacuum conditions due to fuel consumption. ***DISCLAIMER...that may be false, but that's what I thought. ...regardless, I disassembled and rebuilt mine. There's a great thread about doing just that.
Abubaca, that's what I thought I had read.....both ways. I did mine not too long ago and it did indeed go both ways. I don't know,...mine really groans on occasions.
It vents both ways. It vents out once the tank reaches 7 psi. If its venting out after only a little while then the vent line up from the tank before the carcoal canister is plugged or has failed
05-09-2017, 01:28 PM
