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Hi all , after taking my 75 for a drive today I noticed hissing and a gas smell coming from my gas cap ..When I unscrewed the cap it was under a lot of pressure… Is this anything to be concerned about as always thank you
Hi all , after taking my 75 for a drive today I noticed hissing and a gas smell coming from my gas cap ..When I unscrewed the cap it was under a lot of pressure… Is this anything to be concerned about as always thank you
...this used to happen on my '74 coupe many years ago and I found that the rubber vent line going to the steel vent line to the vapor canister was kinked and wouldn't let the fuel tank "breathe". This line is on the left side of the fuel tank going to the steel line at the LR frame.
Do you still have the fuel vapor canister in your 75, or was it removed by the prior owner?
Have you checked to make sure the filter isn’t clogged?
The vapor canister is how your fuel tank vents, if the lines are pinched, filled with fuel or the filter is clogged, your tank will not vent and becomes a sealed system with a vacuum.
The hissing is the result of something wrong with the system.
Make sure the canister is hooked up to engine vacuum properly, and check all the lines from the tank forward.
In the mean time, you can always put on a vented fuel cap and still drive it without any issues, but it’s best not to drive it with the sealed non-vented fuel cap until you find and repair the issue.
You can purchase a vented cap for a pre 72 from most auto parts stores.
I purchased one from Napa Auto Parts for about $35.00.
The only draw back with running a vented cap is you MAY smell gas fumes in the garage when it’s parked.
I’ve been running with a vented cap for several years and have never smelled gas in my garage, but I never park it after a drive with a full tank of gas either.
...this used to happen on my '74 coupe many years ago and I found that the rubber vent line going to the steel vent line to the vapor canister was kinked and wouldn't let the fuel tank "breathe". This line is on the left side of the fuel tank going to the steel line at the LR frame.
Do you still have the fuel vapor canister in your 75, or was it removed by the prior owner?
Have you checked to make sure the filter isn’t clogged?
The vapor canister is how your fuel tank vents, if the lines are pinched, filled with fuel or the filter is clogged, your tank will not vent and becomes a sealed system with a vacuum.
The hissing is the result of something wrong with the system.
Make sure the canister is hooked up to engine vacuum properly, and check all the lines from the tank forward.
In the mean time, you can always put on a vented fuel cap and still drive it without any issues, but it’s best not to drive it with the sealed non-vented fuel cap until you find and repair the issue.
You can purchase a vented cap for a pre 72 from most auto parts stores.
I purchased one from Napa Auto Parts for about $35.00.
The only draw back with running a vented cap is you MAY smell gas fumes in the garage when it’s parked.
I’ve been running with a vented cap for several years and have never smelled gas in my garage, but I never park it after a drive with a full tank of gas either.
thank you my 75 is all original so I will check the canister and lines
The fuel cap on the 75-77 has a Higher Relief Pressure pressure then just a standard fuel cap.
When my 1977 still required Emission testing about every 2 years the fuel cap pressure test would fail, so off to the dealer to get a new correct cap and retest.
I have a CAP history, lol....
The correct new Stant Brand Fuel Cap is Part #10817
I would also replace the fiberglass filter on your Charcoal Canister, it is not expensive but the drivers side rocker molding and splash pan will need to be removed for easy access.
When the car is running the charcoal canister via vacuum it is pulling vapors from fuel tank directly to the carburetor.
When the engine is OFF vapors from the fuel tank become neutralized when they pass though the charcoal and vent through the fiberglass filter, it doesn't sound like you are getting a gas smell when engine is off so maybe the filter is just dirty or clogged?
Once the rocker molding is removed, there are 3 bolts that have to come out to remove splash pan, 2 behind the rocker molding and one to frame rail.
2 behind rocker molding photo
There should be a cap with a drain spout that is on the bottom of the canister, that cap simple pops off and you will see the filter.
A fiberglass replacement filter is around $3.00
O'Reillys auto parts:
Microgard #42998
Standard Motor Products #CF1
Wix #42998
See the cutout on the canister where you insert a flat screwdriver to pop cap off.
The 1975-1977 fuel sending unit has an internal liquid/vapor separator unlike the 1970-1974 that had a external liquid/vapor separator.
In the photo below you will see a red arrow, that is the check of the separator.
You will also notice a photo from the service manual, the fuel vapor pipe and the short rubber hose connected to the tanks equalizer pipe are restricted by a .035 orifice. I don't think this is your problem but if either of these are blocked, especially the main discharge vapor pipe your tank pressure could exceed the maximum relief pressure of your fuel cap and the cap itself would do as it's designed and vent.
Hissing gas cap
