Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Vacuum from fuel tank

I have been experiencing what I feel is an excessive equalisation of pressure when I remove my filler cap to refill the tank. I recently had a leaking tank replaced under warranty together with modification to a new roll-over valve in a new location. However there is still an alarming amount of suction when I take the cap off (when the tank is low) I know there will be some pressure differential, but my question is, how much is too much?

As I have said before Craig I don't have any pressure build up in my tank in either direction but being older it will not be subject to the same environmental issues as yours.

This year however a couple of weeks ago when in for her MOT one of the checks was removal of the filler cap - so I wonder is it something Morgan specific or just a change in the rules.

Others in the trade on here will know.

Thanks Richard

I just filled it to the top with fuel, then opened the fuel cap when I arrived home after 10 miles, and now all seems fine.

But when I open the cap on an empty tank, there is a huge rush of air 💨 that lasts a few seconds

As long the tank doesn’t split / fail again (on the weak weld joints as some say) I’m happy. At least it doesn’t spray me with petrol anymore, the new roll over valve in a new location solved that.

My car does the same, I’ve never routinely removed the cap just to see, only when I need petrol and I generally brim full the tank and refill when near empty.

I think I’ll start opening the tank before and after journeys to find out what makes a difference, it can’t be good for it.

Re. MOT, cap security has been a standard part of the test for quite a while, it has to fasten on and seal.

Thanks Ewn

That’s good to know

Craig - for what its worth my 4/4 does not have this issue.

I agree with Ewn - it cannot be right for the tank to create this vacuum - surely it will cause distortion of the tank - or at least undue pressure on the welds/seals.

Might be worthwhile calling MMC warranty team?

Thanks Neil

I’m waiting for the dealer to contact me this afternoon. This is weird as some do this & some don’t.....

Like I said, I’m not overly worried as long it doesn’t contribute to splitting the tank.

I will wait & see what they say this afternoon. If I’m honest I can’t see how much more they can do, they have pretty much changed everything.

At least I know my tank isn’t cracked, it wouldn’t suck if it was

Do you notice a change in engine performance as the tank empties?

If there is a significant vacuum in the tank either your fuel pressure would be dropping or your fuel pump would have to work a lot harder to maintain the pressure at the regulator.

No loss of performance Bob, not that I can notice anyway.

However, I friendly man at MMC has arranged for my car to be collected and checked over at the factory, next month.

It will feel very odd having an empty garage 😢