Fuel tank pressurization

[taylormj4]

Hi all,
started up the Chimaera today and a few seconds later heard a load clang from the boot which actually shook the wing mirror.
After an inspection under the car etc and finding nothing, drove off only for it to happen again. It seems to be the fuel tank expanding / contracting.
This has happened on a second journey today and have noticed pressure change when opening the fuel cap.

Is this a blocked breather? If so, does anyone know where the fuel tank breather pipe runs? I really wouldn't like it to go pop on me!

Cheers,
Matthew

[shpub]

Yes. Probably. Could be the small kids that have the requisite small hands to put these cars together that stowed away and escaped but I doubt it....

Breather pipe runs behind the tank - can get kinked and then to the front wing and the carbon cannister. Getting to that is a swine.

[rev-erend]

It's probably worth reading the TVR owners manual.

I'm sure it says that the fuel car should not
be fully tightened but I agree with Steve the
breather is probably blocked.

[SGee]

I have had the same problem with my 450 and the solution for me is that suggested by rev-erend. Simply screw the petrol cap on tight and then unscrew until it clicks. The cap will then be raised slightly and will allow movement up and down, but rest assured it is still locked on, and petrol does not come out.

This I guess allows the air pressure/vacuum to be released/equalised so I no longer get the banging problem.

I have also noticed that the car runs better , presumably because the vacuum was reducing the flow of fuel.

[M@H]

is this the "someones locked in the boot" type clanking noise ?? I think I have the same thing..

Cheers
Matt.

[Ford Prefect]

My 2001 Chim 450 has been doing this from new as far as I can recall. When I turn on the ignition the fuel pump starts running and particularly if the tank is low on fuel I get a couple of clanking/clunking noises coming from the tank.

I had always assumed that this was because the tank was being pressurised by the pump - are you saying that the pump is in the fuel line to the engine and the clanking/clunking noise is caused by a vacuum being created in the tank because the tank can't breathe? - The pump seems to run for a hell of a long time sometimes just to pressurise the line to the engine.

[tweety]

both mine and my brothers Chimaeras had a central hole drilled in the cap, I assume to stop this problem.

[Ford Prefect]

Bloody hell, all this time I had thought that the hiss when I took off the fuel filler cap was positive pressure and now it looks like it was negative because the tank isn't breathing properly.

Now for my next problem, the filler cap on mine is either tight on or free to remove there is no in between - no second safety position so what next....do I try and drill a smaall hole in the cap or what would you suggest please?

[MikeyT]

The missus bought me a Leven one for me birthday and when done up normally I just can't get the bugger off so I'm doing it up either a tenth of a turn or not at all. Where is it going to go anyway? Not far. When the bootlid goes down, there's a shape in the lid that it fits into – the cap can't move.

[shpub]

I have a lot of deja vu about this. This has come up before this week....

Anyway. Don't drill holes as there is a breather vent system that does this job. 1/2 inch pipe and all that. It is routed through a carbon cannister that filters the fumes as part of the emission control. It can and does get blocked. If this is blocked or the valve is not working, then you get the vacuum problem. All that drilling a hole does is solve the symptom and not the cause.

[Hut49]

Faced with the two choices: drill a hole or track the entire route of the half inch hose to the carbon cannister looking for a blockage, I probably take the former and use the time I saved to redecorate the house.

Having said that, is there a problem with just treating the symptom and drilling the hole? Can't see how anything gets worse unless you happen to be lying under the car when it's upside down and fancy a fag.

Hutch

[shpub]

Yes. Safety regs dictate that if the car tips then fuel should be prevented from leaking via the breathing vent. This is done by a roll over valve that seals if the car is tipped. Drilling a hole allows fuel or vapours to escape and increases the risk of an explosion or fire in an accident.

There is also the problem of fuel escaping as it is forced to one side in the tank when cornering. Seen that come out of the cap vent on many an occasion particularly on M series car.

If I see a hole drilled through a cap then I immediately ask what other shortcuts have been taken on this car? What else couldn't the owner be bothered to sort out?


>> Edited by shpub on Friday 11th October 12:24

[Hut49]

Good points, Steve - especially the last one.