[C6Dave]

I have noticed that the last couple of times I went to add fuel to the car it seemed that the tank had a 'vacum' in it as once the cap was loosend air rushed in.

Yesterday I struggled to turn it as it was so tight so should the cap be vented or not?

Here's a pic of what is on the car atm.
.

[Mr Teddy Bear]

First question has your car been equipped with a cat'?

Prior to 93 [?] they were optional I believe.

Certainly when cats were fitted as standard the fuel system has a charcoal canister fitted under bonnet that absorbs fuel vapour and thus I assume the tank isn't vented to atmosphere.

My 89 216GSi also wasn't vented to atmosphere and didn't have a charcoal canister either for what's that is worth.

[crepello]

Mine does the same (1991 non-cat). I used a smear of rubber-friendly grease on the sealing ring to make opening easier.

[C6Dave]

First question has your car been equipped with a cat'?

Prior to 93 [?] they were optional I believe.

I have no idea if it has a Cat and is a single OHC Honda engined car, but I will take a look, thanks for the heads-up

Mine does the same (1991 non-cat). I used a smear of rubber-friendly grease on the sealing ring to make opening easier.

Thanks, I have some 'Red' rubber grease so will add some.

[g259fsg]

My car is a 1990 214, and it has a breather system on the tank with a pressure relief valve. When it's working there is a slight vacuum in the tank produced as fuel is pumped out. I think the valve releases the vacuum when it gets to a certain level and can produce a fog-horn sound. If you are not getting a vacuum it could mean there is a leak on the tank or the filler pipe and airlock tube which are sealed when the petrol cap is on. Don't know if the 216 is the same. There may be some archived technical articles on the Viking about the petrol tank. I did one a few years ago.

[C6Dave]

My car is a 1990 214, and it has a breather system on the tank with a pressure relief valve. When it's working there is a slight vacuum in the tank produced as fuel is pumped out. I think the valve releases the vacuum when it gets to a certain level and can produce a fog-horn sound.

I am getting Vacuum, making the cap hard to get off at times. When you look at the cap from below (tank side) in that hole in the middle there is a spring inside, so wonder if that is the vacuum release. Gave it a good spray of cleaner and let it soak to see if it makes a difference.

Speaking to another couple of owners today they get vacuum as well to varying degrees, the car runs fine (120 miles today) so it's probably not an issue.

Thanks for all the feedback everyone who posted.

[crepello]

When you look at the cap from below (tank side) in that hole in the middle there is a spring inside, so wonder if that is the vacuum release.

I'm pretty sure the spring acts on the floating 'ears' to set the clamping force of the cap.

[C6Dave]

I'm pretty sure the spring acts on the floating 'ears' to set the clamping force of the cap.

That makes more sense, thanks for the feedback

[alviseven]

Our two early 90 216 GSi with SOHC honda are both the same - really hard sometimes to get the cap off and always have been - owned the one for nearly 20 years and sometimes takes two hands to remove the cap. Think it is why the car talks to you when parked up as the pressure is released but it has never affected performance or caused me an issue in over 100K miles. Have tried at least three different caps too over the years and couple of fuel tanks too! All the same - mine are non cat early cars of course.

[Johnny 216GSi]

My 216GSi the same too. Sometimes quite hard to turn, and then you get the sound of air as the vacuum is released. A bit like in Silence of the Lambs when they take the bug cocoon out of the victims mouth with a pair of tweezers