Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Faint hissing coming from fuel cap

Had a good drive out in the roadster on Tuesday making the most of the nice weather. Today, I hear a faint noise hissing noise coming from the fuel cap. Thus stopped after I opened the fuel cap and the familiar whooshing sound. I've never heard thus hissing before, but it was really faint. There also seemed to be a whiff of fuel before I opened the cap, although not particularly strong. I understand that the fuel system might be pressurised, but don't know how this works. I'm guessing u need a new seal, but is this an urgent job, and are there any problems that this can cause? It had been approximately 24 hours between the drive and discovery of the hiss.
Thanks

I had the same problem a few years ago, mine was fixed at the factory in the end as it wasn't a simple fix. The cause was a kink in the fuel line and a problem with the roll over valve.

Seems to be lots going on with the modern Morgan fuel tanks, roll over valves, charcoal canisters, computers that tell the valves when to purge the canister of vapour etc etc. Not like older cars with self vented fuel filler caps.

The first thing to check (IMO) is the roll over valve (No 18 in the image) but this wasn’t easy to access on my 4/4 Sport with no spare wheel, but might be easier on your Roadster ?

My car has the set up Craig has pictured, on my car above the tank is the plastic storage insert/tray, the roll over valve (18 on the diagram) didn’t have enough room to be mounted vertically, Morgan just left it flapping around in the hope it would jiggle around enough to allow the pressure in the tank to equalise, it didn’t , the tank would constantly vacuum.
I extended the fuel pipe and relocated the valve and it’s been fine ever since.

As an aside, looking at Craig’s diagram, there are an awful lot of flexible pipes there that may ( or may not) need replacing when the dreaded E10 petrol arrives! £££££. Will MMC ever tell what needs doing or will they fob it off to dealers to sort out and yet more £££?

Thanks guys, and jezz for that above picture. It looks very familiar to what I saw yesterday when I had the spare wheel out. Where did you come by this picture, as it would be good to see what all the component parts are described as by the numbers? Thanks

For me it was the split fuel tank that came first, and once the new tank was fitted the hissing of air when opening the fuel cap developed worse. Some occasions I was even sprayed with petrol.





This is where the roll over valve was repositioned.

While you are there, do check the tubing from the filler cap to the tank (33 on the diagram, although your car might have a little different configuration). Over years the rubber can get hard as rock, crack up and start leaking. Mine did (same car and year as yours), took me a while to find out where that strange smell came from as it was intermittent - only when the tank was nearly full and mostly in left hand corners. Well known weak point in Morgans. Look for cracks, especially round the jubilee clips.

Thanks Robbie. The tube from the fuel cap has already been replaced thankfully, and looks in good nick. I haven't pulled the plate that the spare wheel sits on again but removed the wheel and give the fuel tubes a good wobble, and they all seem free and kink free. I'm wondering if its just normal evaporation if the fuel back up the filling tube direct from the tank. I'm not sure if there's a one way valve, but that would seem to make sense to me. Its never spat fuel at me before.

Pictures are far better than words.
Have check all the tubes and no kinks. Everything seems in order. Would a faulty roll over cause the pressure/vacuum? I'm not sure which it is, as I've not been spat in the face with fuel, although I could smell fuel the other day, so I'm guessing it's positive pressure trying to escape.


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