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how to unblock fuel tank breather


Well-Known Member
Mar 1, 2010
Every time i went to fill up with diesel i got this hissing sound as i unscrewed the fuel cap as air rushed into the filler pipe. I have been told by Julian at Overland Cruisers that my fuel tank breather is blocked and that it is in the fuel cap. As i have fuel starvation issues i decided to get it sorted and write a guide as i could not find one anywhere.

First remove the inner threaded part from the handle by holding the handle firmly in one hand and gently pulling them apart at an angle

This is a picture of the filter element, i took all these photos after i cleaned it as i thought it was solid plastic there due to all the dirt on it :roll:
you can remove it with a tiny flat head screwdriver or with a sharp knife

also pry out this bit of plastic on the other side, just be careful as there is a loaded spring underneath, take note which way round the valve is under the spring.

Now clean all the parts. i have laid out all the parts below in the correct order of assembly

to assemble follow the steps in reverse :lol:

when assembling check if the valve is not stuck by inserting a small screwdriver as below
nice one and thanks. just been out and cleaned mine up - easy 15 minute job.
Did it sort the hissing and fuel starvation issue for you, out of interest? I assume 'yes'?

haven't had it running for a while, still waiting for some time during the day to install the fuel pick up pipe filter in the tank. Will report back this weekend
Right took it for a drive and hissing seems to have gone.

Success :D :D
Cheers for that Rob

My filter was like yours, you could'nt tell it was one until it was clean.
Dead easy 2 minute job with your help.
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I have found on some 80's there is an additional breather, it is fed from the tank filler neck and leads down under the tank and is literaly poked into a hole in one of the chassis cross beams in front of the spare wheel! Of course it was full of crap and whatever and yet the car ran ok, I assume the filler cap vent was letting in enough air but it may be worth checking?


Going to check the breather today as the Toy sucks in air when I fill her up.

Just to make sure I understand the principles at work here.

In order to have an odorless tank, Mr Toy has to regulate the tank pressure.
When a tank and its contents heats up, you releave some pressure through the breather stuffed into the chassis. Like the Mr Toy diff breathers???

When a tank cools down you need to get some air in it through the filler cap breather. Looking at the pics I understand its a one way valve.

Now to ensure you've got an odourless tank Mr Toy puts a spring into both breathers and designs the system so there is always a small vacuum inside the tank to allow air moleculles to dissipate into the tank in stead of outwards from the tank.
Air and odour travel through rubber I've learned and probably read somewhere.

Correct me if I'm wrong? Because I've messed up filler caps in the past and ended up with a smelly garage and unhappy missus!!
Excellent, thanks for this Rob - just had a go myself. For clarification, one is attempting to pull all the white part from the grey cap in the first step and not just the threaded bit - ie the whole clicky mechanism.
Thanks for this very informative piece of information. My fuel cap works differently and looks different to the one in your photographs, but I stripped it nonetheless and the whole thing popped apart as I was busy prying the inner part out. In my cap everything fits from the rear end of the cap and not from the front like yours. The reason I was searching about this is because I've lost many litres of fuel from the breather at the top of the main (original) fuel tank. The fuel was actually streaming from it and the fuel cap was sealing as tight as hell.
Since having followed your example, when blowing into the fuel cap from the front (the part that goes inside towards the tank) I can actual blow it, but it would obviously mean that one would smell fumes coming from the tank, passing through the fuel cap not so? This being the case, surely it must be a fire hazard should someone light a cigarette standing next to the vehicle.
The other question is; Is it proper for the breather that should lead into the chassis to spurt petrol if the fuel cap is sealed due to dirt or whatever reason?