Fuel tank breather / rollover valve issue - ’92 4.0 pre-cat

[jauntyviper]

Hi everyone, I'm quite new to the TVR world and hoping for your help, please.

I’ve run into what I think is a tank venting problem on my pre-cat Griff. The first sign was an odd bong sound as the tank changed shape under pressure. When I opened the filler cap (unvented type), I could feel the pressure equalise very quickly.

For now I’m running with the cap cracked open slightly so the tank can breathe, but I’m very aware that’s not a safe long-term solution.

Here’s what I’ve checked so far:

• Followed the breather hose from the nipple on the filler neck.

• Disconnected it and tried pressure testing with ~30 psi of air from the compressor. Totally airtight.

• Went underneath the car, found what I think is the other end of the pipe tied near the rear left suspension. Untied that, tried a pressure test again – still airtight.

• Pulled some of the boot carpet back and traced the hose down behind the tank. My hand found a component with a part number on it, which identifies it as a rollover breather valve.

My working assumption is that this valve has failed closed. Does that sound likely? The way it’s mounted looks like it passes through the boot wall with two rivet-style fittings.

Questions for the hive mind:

• Is there anything else I should be checking before blaming the valve?

• If it is the valve, can it be replaced in situ, or am I realistically looking at removing/moving the fuel tank to get at it?

Any advice is very much appreciated – I'd love to avoid pulling the tank completely if there’s a simpler way through this.

Thank you all,
Daniel

[Belle427]

Im no expert on rollover valves but it should be open to allow the tank to vent and then seal off in the event of the car overturning.
I would check the rubber hose has not been squashed somewhere, sadly will probably mean tank moving but this isnt a huge issue if its fairly empty.
It can be a good opportunity to change the rest of the fuel hoses as if they are an unknown age they can be a real safety issue.
Check your fuel cap too to see if it has a breather hole in it.

[Loubaruch]

The roll over valve is usually just a ball bearing that shuts off the tank vent if the car turns over. The plastic one on my 1996 car was a rather flimsy affair that broke. Aluminium replacements available from racing parts supplier are much stronger and more compact.

[jauntyviper]

Thank you for your responses, helpful stuff

I had some time today to get into it, so I elevated the back of the car, disconnected the battery, removed the boot carpets and interior carpet on the rear bulkhead, released the two bolts either side for the top and bottom of the tank straps, and removed the filler and flexible pipe from the neck.

I didn't drain or fully remove the fuel tank: by removing the cable ties from the main fuel supply line underneath the car, and dismounting but not disconnecting the fuel pump, I was able to provide enough free play in the pipes through the bulkhead to scoot the tank far enough rearwards to be able to work behind it.

I removed both halves of the breather line from the roll over valve, both of them allowed air to pass easily, so I took the rivets out that held the valve to the bulkhead, and took the valve body to the workshop. It wouldn't allow air to pass either way, so I cut it open to satisfy my curiosity. I think I can safely say I found the problem:







I'm just now searching for a replacement valve, I'll fit it and I think I'll be good to go. Fortunately all of the fuel pipes have been replaced recently and are in good condition, so I think I can keep it simple.

Thanks, guys.
Daniel

[Belle427]

There are lots of this type around if you can find a way of mounting it.
https://www.ebay.co.uk/itm/362928785828?chn=ps&amp...