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Finding a fuel leak
Slimy38 - 25/8/25 at 06:58 PM

I have my car almost finished, but the smell of gas is still with me. It's an MX5 1.6 donor, with two copper fuel lines running front to back. At each end are fuel approved rubber lines going to the fuel rail on the front, and to the fuel pump (out) and swirl pot (return) on the tank.

The tank itself is welded up steel that I have pressure tested. I then have a filler with 2 inch hose going to a vented Mini cap. When I run the car and then retire it to the garage I get strong fumes for ages. Gut feel is that it's longer than you might expect for just exhaust fumes, so that's led me to believe I have a fuel leak somewhere.

None of the joints and connections are damp, or show any sign of a leak. I've even laid paper towels underneath them and not seen anything, but I'm aware that to have fuel dripping is quite significant. I've refurbed the injectors so they've all got new seals, and again on the engine there's no sign of leaking fuel.

What's next to find and fix a leak? I'm thinking of some smoke but given it's full of fuel I'm not sure what would be safest (I used plumbing matches initially to generate smoke but that was before the first fill up). Is there some tint or colouring I can add to the fuel that would show up? Is there any other way of seeing where vapours are escaping (short of using a naked flame and listening for the explosion!).

jps - 25/8/25 at 09:20 PM

I’ve no experience and simply googled after your suggestion of a dye, but it seems that is very much a thing, with UV being common, which makes a lot of sense in terms of being able to spot it! From the amount of hits I got back, I think you’ve got a reliable way forward.

[Edited on 25/8/25 by jps]

Benzine - 25/8/25 at 10:28 PM

How long was it pressure tested for? How was it welded? I always thought MIG was inherently porous - fine for a quick pressure test but may leak petrol on a microscopic level.

Also, 'gas'? We can do better than that. The less association we have with the currently insane USA the better.

gremlin1234 - 26/8/25 at 06:34 AM

you say
"fuel approved rubber lines "

does that include modern fuel with Ethanol?

also I seem to remember there were some Fake batches of hoses

Slimy38 - 26/8/25 at 07:21 AM

quote:
Originally posted by gremlin1234
you say
"fuel approved rubber lines "

does that include modern fuel with Ethanol?

also I seem to remember there were some Fake batches of hoses


Oh I really hope it's not fake stuff!! While it was from Ebay, it was quoted as 'Textile Reinforced Rubber Fuel Hose - BSAU108/2 / SAEJ30R6 - Suitable To Use With E10 Fuel' and came from a place in Stoke.

Slimy38 - 26/8/25 at 07:25 AM

quote:
Originally posted by Benzine
How long was it pressure tested for? How was it welded? I always thought MIG was inherently porous - fine for a quick pressure test but may leak petrol on a microscopic level.

Also, 'gas'? We can do better than that. The less association we have with the currently insane USA the better.


I spent about an hour on it, there were places where I'd not quite got it right and had to grind and redo the welds (MIG). The final test was at least five minutes, I just left it pumped up a while to see if anything would come out over time. After that it got a layer of thick paint.

And yeah, sorry about that. That was just laziness on my part, half the number of letters....

[Edited on 26/8/25 by Slimy38]

Slimy38 - 26/8/25 at 07:29 AM

quote:
Originally posted by jps
I’ve no experience and simply googled after your suggestion of a dye, but it seems that is very much a thing, with UV being common, which makes a lot of sense in terms of being able to spot it! From the amount of hits I got back, I think you’ve got a reliable way forward.

[Edited on 25/8/25 by jps]


I was doing the same, and found UV dyes for AC leaks (which is where my first thought came from), but going a bit deeper I saw that they were actually fuel and oil safe too. I've ordered a bottle, lets see if it finds anything. If it gets picked up by the vapours then fingers crossed something will show up.

gremlin1234 - 26/8/25 at 07:41 AM

also, what hose clips are you using? there are whey better clips than jubilee clips.

Slimy38 - 26/8/25 at 12:18 PM

quote:
Originally posted by gremlin1234
also, what hose clips are you using? there are whey better clips than jubilee clips.


Yeah I don't seem to have much luck with jubilee clips on any application!! The ones I have on my air intake have already failed.

I use this type for rubber to copper;



I don't have the tools to flare the copper, so at each end there's about ten centimetres of overlap (copper inside rubber), with two of those clips per pipe.

[Edited on 26/8/25 by Slimy38]

Slimy38 - 26/8/25 at 12:25 PM

This is what I've done;


Slimy38 - 2/9/25 at 03:01 PM

Quick update, I got the dye, added it, ran the engine, and.... nothing. No sign of any leaks at all. Either the dye doesn't work with vapours or it's my imagination.

Having said that, another discussion on here regarding catch cans has got me thinking. The MX5 has a breather connected to the air intake, but as I'm not using any part of the intake I've swapped out the breather for a small air filter. I've got no sign of oil or fuel coming out of the filter, but I am wondering whether it's responsible for the smell.

I'll have to figure out what's going on at some point, for now it's got a first layer of paint so my garage is now full of paint fumes rather than petrol fumes!

cliftyhanger - 3/9/25 at 07:56 AM

quote:
Originally posted by Slimy38
quote:
Originally posted by gremlin1234
you say
"fuel approved rubber lines "

does that include modern fuel with Ethanol?

also I seem to remember there were some Fake batches of hoses


Oh I really hope it's not fake stuff!! While it was from Ebay, it was quoted as 'Textile Reinforced Rubber Fuel Hose - BSAU108/2 / SAEJ30R6 - Suitable To Use With E10 Fuel' and came from a place in Stoke.


R6 is not rated for ethanol containing fuels as it is Nitrile. Viton is needed for ethanol.
You want R9 rated hose. And branded from a proper source. Not AFS on ebay etc, I know several people who bought from them and teh hose fell apart within months.

Merlin motorsport, burtons, etc will see proper stuff. Gates Barricade, Cohline or Codan. I won't use anything else.

Slimy38 - 3/9/25 at 10:08 AM

quote:
Originally posted by cliftyhanger
R6 is not rated for ethanol containing fuels as it is Nitrile. Viton is needed for ethanol.
You want R9 rated hose. And branded from a proper source. Not AFS on ebay etc, I know several people who bought from them and teh hose fell apart within months.

Merlin motorsport, burtons, etc will see proper stuff. Gates Barricade, Cohline or Codan. I won't use anything else.


Bu**er... Yep, I've messed up. I went through all this very early on. I knew that R9 was the good stuff. So why on earth did I buy R6... Even the working pressure is wrong, R9 is needed for fuel injected cars.

Thankfully as I've gone for hard lines for the tunnel distance I only have short lengths to replace either end.

I'd like to ask about the significance of 'Viton' though. A quick Google for R9 hose gave me this as an example;

Nitrile rubber hose

I appreciate what you're saying about just going for proper stuff, but is Viton part of the R9 spec?

Edit: A bit more Googling suggests a Viton lined rubber hose, is that an option?

[Edited on 3/9/25 by Slimy38]

cliftyhanger - 3/9/25 at 01:12 PM

quote:
Originally posted by Slimy38
quote:
Originally posted by cliftyhanger
R6 is not rated for ethanol containing fuels as it is Nitrile. Viton is needed for ethanol.
You want R9 rated hose. And branded from a proper source. Not AFS on ebay etc, I know several people who bought from them and teh hose fell apart within months.

Merlin motorsport, burtons, etc will see proper stuff. Gates Barricade, Cohline or Codan. I won't use anything else.


Bu**er... Yep, I've messed up. I went through all this very early on. I knew that R9 was the good stuff. So why on earth did I buy R6... Even the working pressure is wrong, R9 is needed for fuel injected cars.

Thankfully as I've gone for hard lines for the tunnel distance I only have short lengths to replace either end.

I'd like to ask about the significance of 'Viton' though. A quick Google for R9 hose gave me this as an example;

Nitrile rubber hose

I appreciate what you're saying about just going for proper stuff, but is Viton part of the R9 spec?

Edit: A bit more Googling suggests a Viton lined rubber hose, is that an option?

[Edited on 3/9/25 by Slimy38]


Nitrile has poor resistance to ethanol. Viton has good resistannce to ethanol. So a proper bit of fuel hore should have a viton liner.
That hose yopu linked to sounds like it has a viton liner (fluro is the clue) but it is unbranded and I wouldn't be buying it.

[Edited on 3/9/25 by cliftyhanger]

Slimy38 - 3/9/25 at 07:32 PM

quote:
Originally posted by cliftyhanger

Nitrile has poor resistance to ethanol. Viton has good resistannce to ethanol. So a proper bit of fuel hore should have a viton liner.
That hose yopu linked to sounds like it has a viton liner (fluro is the clue) but it is unbranded and I wouldn't be buying it.

[Edited on 3/9/25 by cliftyhanger]


Perfect, yeah it was the hose having a Viton liner that just confused me a bit. Thanks for the guidance, I know where I'm looking now.

PorkChop - 3/9/25 at 08:27 PM

I wouldn't be surprised if you replaced the fuel lines with R9 and still have the same issue, injection systems usually have non vented fuel caps.

Slimy38 - 4/9/25 at 07:37 AM

quote:
Originally posted by PorkChop
I wouldn't be surprised if you replaced the fuel lines with R9 and still have the same issue, injection systems usually have non vented fuel caps.


Interesting, I'm using a vented cap for IVA purposes but what would be a better option?

40inches - 4/9/25 at 09:04 AM

quote:
Originally posted by Slimy38
quote:
Originally posted by cliftyhanger

Nitrile has poor resistance to ethanol. Viton has good resistannce to ethanol. So a proper bit of fuel hore should have a viton liner.
That hose yopu linked to sounds like it has a viton liner (fluro is the clue) but it is unbranded and I wouldn't be buying it.

[Edited on 3/9/25 by cliftyhanger]


Perfect, yeah it was the hose having a Viton liner that just confused me a bit. Thanks for the guidance, I know where I'm looking now.

I would only buy fuel hose from Car Builders Solutions, safe to say they sell quality fuel hose.
Ebay fuel hose after only 500 miles.
Description
Description

adithorp - 4/9/25 at 08:25 PM

quote:
Originally posted by Slimy38
quote:
Originally posted by PorkChop
I wouldn't be surprised if you replaced the fuel lines with R9 and still have the same issue, injection systems usually have non vented fuel caps.


Interesting, I'm using a vented cap for IVA purposes but what would be a better option?


In tin-tops perhaps. They also have carbon canisters/evap system to absorb and deal with the fumes.

Not something you'll find in most kit cars. If they don't have a vented cap then they'll have some other form of tank venting. Usually with a 2-way valve that alows gas intout but not fuel. Without that they'd inflate the tank. With that I very rarely get fuel smell from mine.