ThreePointFive Posted December 5, 2009 Share Posted December 5, 2009 Hi all, since replacing my old fuel filler cap I've been getting a vacuum in the filler pipe, after only a short run it creates enough suction to make it quite strongly resist being opened. It also sounds like it's drawing fuel up the pipe as when opened a gushing sound is very audible. Is this a normal thing? My cap never worked properly and I only replaced it because off road fuel was dripping from it so I don't know how it should be, but I'm guessing it really shouldn't be like that. Just wondering what I can or should do about it. The breather for the tank (I assume that's what the smaller pipe above the filler main pipe is) exits behind the cap anyway and so does nothing to get air in to relieve the vacuum. Quote Link to comment Share on other sites More sharing options...
Aragorn Posted December 5, 2009 Share Posted December 5, 2009 The vent pipe must be kinked or otherwise blocked if your drawing a vacuum like that... There are two "vent" pipes, one is perhaps an inch in diameter and connects to the filler neck as you describe, the purpose of that is to stop you airlocking the tank when filling up. The other is small, about the same size as a fuel line, and will either terminate at a high point like a axle breather, or it will be piped along with the main fuel lines and into a carbon canister then into the inlet, depending on the design/age/etc Its the smaller one you need to be looking at, as thats what allows air into the tank to replace the fuel the engine has consumed. Quote Link to comment Share on other sites More sharing options...
mickeyw Posted December 5, 2009 Share Posted December 5, 2009 I think the early type large diameter ones with no lock are vented caps. I'm not sure if there's a ball valve inside that is meant to stop fuel leaking when on an incline. I believe the later type with the lock are not vented and rely on a separate vent hose. If you have replaced a vented with a non-vented type then you will get a vacuum build up. Quote Link to comment Share on other sites More sharing options...
ThreePointFive Posted December 5, 2009 Author Share Posted December 5, 2009 Thanks for replies. I've had a look and I can't see any breather type pipes coming from the tank - I'm thinking that it must be the vented type cap required. It's an 1988 90. If it was to have a breather, where would this be? Quote Link to comment Share on other sites More sharing options...
Aragorn Posted December 5, 2009 Share Posted December 5, 2009 Usually comes off the top of the tank. TD5 tank certainly has one, but thinking about it, i dont recall seeing one on the old diesel tank, so the vented cap could be a good call. Quote Link to comment Share on other sites More sharing options...
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