Fuel - Gas - Petrol Cap Vacuum Vented

[Col_Callan]

UK spec car. Numerous attempts to cure an ongoing misfire that comes in after 20 mins running. Appears under load or under a good welly full and gradually creeps down the rev range.
No heavy load and it will slide up to the ton quite easily. When it does throw the toys it wont start unless left for 30 mins when all is once again hunky dory...for 20 mins or so.
New dizzy coil, etc etc K&Ns with corrected needles - straw coloured plugs when its behaving itself. No fuel filter..

but.. take the fuel cap off and it sounds like a bottle of coke opening. So is this a vacuum building up that stops my wee car? Should it (the fuel cap) do this? Cap has AS18.8 stamped on it and "Vented" printed on it. Ive tracked the fuel line back for pinches etc none. Help!... or I have to drive a Mondeo to Le Mans!

[mb4tim]

The MGB I had in high school did the same thing. A vacuum would build up so strong, that I had a hard time getting the cap off. I drilled a tiny hole in the inner portion, and all was well. But, I'm sure there was something else at fault. You should have a vent somewhere in the tank.

My TR7 is fuel injected, so I guess my fuel return line is the vent.

-Tim
http://www.morSpeedPerformance.com

[FI Spyder]

UK cars have a "vented" setup, there should be no build up of vacuum in the tank. N/A cars are "sealed" to prevent gas fumes escaping into the air a causing all that smog in the seventies. They should have a mild vacuum in the tank. As your UK car has a vented cap, the vent is plugged. Not actually seen a vented cap so other than that I can't help. A friend had bought a "vented" locking cap and could smell gas fume until he replaced it with the correct N/A sealed cap.



- - - - TR7 Spider - - - - - - - - 1978 Spitfire - - - - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - - Yellow TCT

[Hasbeen]

I reckon it's probably an in line fuel filter approaching totally blocked, but allowing enough fuel to run gently. Try a new one, they are cheap.

Try a standing start, revving hard though all the gears. This was my test, & it would start to starve about 4500 in 3Rd gear.

Hasbeen

[Col_Callan]

Thanks Guys - I've been back under the thing and cant see a single filter anywhere. I traced the fuel line inch by inch (with the exception of the few inches of rubber pipe that runs through the rear body work to join the steel pipe onto the tank - there's a small blind / untouchable part of probably only 3-4 inches - surely not a filter there? - if so that's just plain malicious!

As for breathers My Haynes says they come down to the left rear chassis rail but no sign of them. Are they really well tucked away? If so I suspect these are going to need a pit or mechanics lift to see properly.

[mb4tim]

maybe disconnect either end and blow some air through it - into a jar, can, or big bottle, of course.

-Tim
http://www.morSpeedPerformance.com

[Col_Callan]

Thanks for the info guys. - Removed the fuel cap and took it for a run. No lower / creeping death issues now, except a persistent miss when flooring it at top end - more on this in a mo.
Opened the panel in the boot to the filler and am now planing to cut the overflow return pipe and tee off a short length with a one way valve in it to provide a one way breather.So that should sort the breathing out.

As for the miss at the top I think its down to the needle. Spoke to the nice guys at Burlems (sucarb.co.uk) who concur. Rather than trawl charts and graphs they recommended I ask here. Its profile sounds fine until the very end - ie needs to be richer at the end. Any anecdotal recommendations I can research guys?

[Hasbeen]

With a good exhaust & a very easy flow air filter, mine could not get past 3500RPM on full throttle, with the stock needle.

I went through a few before getting to a BAL needle, & all is now well. Others have come to the same result.

Hasbeen

[Col_Callan]

Many thanks Hasbeen. Ill get then ordered. Much appreciated !

[Beans]

As for the vented fuel filler cap there are two pretty small holes which might well have blocked up with dirt over the years.
But that is pretty easy to sort as the cap can be taken apart ...

In the pictures the parts on top/ on the left are vented, and the others are of the non vented variety.
Spot the difference ...

<center>

[Col_Callan]

I feel a session with my ultrasonic cleaner coming on....
Thanks Beans.

[john lewis]

hi noticed lately when removing gas cap there is a vacuum lock and it is hard to lift it up, and when u do u hear a loud sucking sound, where would vent be blocked, thanking u in advance john

[FI Spyder]

It is not blocked, that is the normal condition. North American cars have a closed loop gas tank system to prevent gas fumes from entering the atmosphere. When these gas fumes react with sun light you get smog. The vacuum is supplied by the intake manifold through the charcoal cannisters which have a line that goes back to the gas tank. These charcoal cannister soak up the fumes when the car is stopped and when it is running the intake vacuum sucks them into engine to burn. The gas cap is sealed. The vacuum is not enough to over power the fuel pump.

In contrast Europe has an open system. The gas cap is not sealed, they don't have the same emissions stuff. As the tank level goes down the air is drawn into the tank by the gas cap. If the gas cap is plugged the vacuum can develop to a high enough level that it will over power the pump and stall the engine through fuel starvation.

When I stop my car in the garage I can hear the air being sucked into the gas tank from a leak in the sealed gas cap. I don't know if this normal (the leak). I live in a very quiet neighbourhood. Of course I can only hear it if the birds aren't chirping, the ravens aren't flapping their wings or the deer aren't chewing their cuds too loud. Fortunately the bald eagles fly too high and are usually soaring so don't make any noise (we keep them safe here for America, it's their national bird). Egads, I'm starting to sound like Mildred, I think I've been reading too many of her posts.[:p]

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[HDRider]

I don't think that is normal. I have several NA TR7's with and without FI and none of them do that.

Edward Hamer
Petaluma CA

[FI Spyder]

Doesn't make any difference if it's FI or not. If you don't have a vacuum your system is blocked or it has been modified. All my cars have the slight vacuum.



- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT