Petrol vapour leak - how to find?

Submitted: Thursday, Oct 30, 2008 at 18:11

ThreadID: 63053 Views:2679 Replies:4 FollowUps:3

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Member - John R (QLD)

Hi all,
I've noticed recently that there is no release of pressure when the fuel cap is opened on our Hilux, and there is a petrol smell noticeable around the fuel tank area when you pull up. No obvious sign of liquid petrol leaking though (it's a cab chassis, so you can see the filler pipes and tank).
Has any forum reader had experience in efficiently finding what is probably a small crack in a filler pipe somewhere, or a faulty petrol cap? Have thought of using very soapy water to try to find where the vapour is leaking, but not sure if this is the best idea.

Thanks, John

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Reply By: Robnicko - Thursday, Oct 30, 2008 at 19:03

Thursday, Oct 30, 2008 at 19:03

John,
Most likely the seal on the fuel cap. Being a petrol engine the seal on the fuel cap is part of the fuel system pressure.
On a sunny day if there is a fuel vapour leak you may see a shimmering effect like at the servo when you fill up.
I very much doubt it would be a crack in the filler pipe as they usually have a rubber section between it and the tank to allow for body flex. Check the vent hose, if there is one, along the filler section.

Rob

PS> apart from the obvious check with a match!....had to get that in before someone else

AnswerID: 332714

Follow Up By: Member - John R (QLD) - Thursday, Oct 30, 2008 at 19:42

Thursday, Oct 30, 2008 at 19:42

Match test might be a bit too permanent, Rob :-)

The reason I suggested a crack is that I have seen this where the small bypass(?) pipe joins the main filler pipe just below the cap (I assume when you fill, liquid petrol goes down big pipe, displaced air/vapour back up and out bypass pipe).

Thanks, John

FollowupID: 600586

Follow Up By: Robnicko - Thursday, Oct 30, 2008 at 22:04

Thursday, Oct 30, 2008 at 22:04

The bypass pipe is to allow for quicker filling (air dissipation), bit like the spout nozzles on jerry cans. If cracked it would most likely be the source of the leak as it is directly connected to the tank.

Good luck
Rob

FollowupID: 600627

Reply By: simple - Thursday, Oct 30, 2008 at 19:04

Thursday, Oct 30, 2008 at 19:04

i always thought the breathers were meant to prevent that, but i may be wrong

AnswerID: 332715

Reply By: Grizzle - Friday, Oct 31, 2008 at 10:15

Friday, Oct 31, 2008 at 10:15

I had the same issue on a Mitsubishi Triton. It turned out to be the filler cap. It has and internal valve which was playing up on mine. $14 and it was fixed. I had had it to the dealer on numerous occasions but they couldn't find anything. The silly thing was that I still got the release of pressure when taking the cap off to fill up.

Also it's probably the easiest and cheapest thing to check.

Cheers

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AnswerID: 332822

Follow Up By: Member - John R (QLD) - Friday, Oct 31, 2008 at 19:11

Friday, Oct 31, 2008 at 19:11

Thanks, Grizzle. I'll see if the Hilux has a similar valve that can be replaced. Hoping not to have to replace the lot, thereby needing another key for the lock.

Cheers, John

FollowupID: 600756

Reply By: Member - John R (QLD) - Tuesday, Nov 18, 2008 at 19:51

Tuesday, Nov 18, 2008 at 19:51

Followup: from above advice I tested with some soapy water, and found vapour bubbles coming out around centre of fuel cap (where key turns). Took cap apart, and found quite a bit of red dust, presumably from desert trips in 2007. After cleaning all parts of dust with soapy water and reassembling cap, problem has been fixed.

Cheers, John

AnswerID: 335949