Fuel tank pressure.

Submitted: Friday, Jan 30, 2009 at 09:01
ThreadID: 65538 Views:4962 Replies:9 FollowUps:5
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On the last two trips that I have taken, one into the Vic High Country and the other into Bendethera Valley, I have had reports from the drivers that there is a strong smell of petrol coming from there vehicles.

What is happening is the fuel tank is just about full, vehicles are using low range with a lot of revs, climbing steep rough tracks on a hot day. The fuel in the tank starts to vapourise and and build up pressure. The pressure escapes through the carbon canister which is usually found under the bonnet.

The carbon canister can handle petrol vapour on most occasions but when the vehicle is working hard on a hot day the carbon canister fills up and petrol fumes can be smelt.

What we have done when this happens is to slowly release the pressure build up in the fuel tank by unscrewing the filler cap very slowly. You can hear the pressure escape, but if the filler cap is removed too quick fuel can spray out. Make sure that there are no naked flames about because the fumes will travel.

Also on the last trip we had a late model diesel Pathfinder that had power steering fluid leak. The leak was traced back to the lock nut on the high pressure pipe the goes into the steering rack.
The vehicle had less that 10,000km on the clock and we can only guess that it might not have been tight when the vehicle left the production line.

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Reply By: ingo57 - Friday, Jan 30, 2009 at 09:27

Friday, Jan 30, 2009 at 09:27
Gday Wayne

I had that happen to me in my old Pathy at Bendethra, we were stopped for lunch and I could hear a hissing sound as If I had a tyre leak. Followed the hiss to the petrol cap, I got the shock of my life when I started to unscrew the cap, I recall the built up pressure escaping around the cap for around a minute.

AnswerID: 346705

Reply By: Member - John and Val W (ACT) - Friday, Jan 30, 2009 at 12:21

Friday, Jan 30, 2009 at 12:21

Can be really interesting with a full tank and parked on a slope with the filler on the downhill side and the temperature going through the 40's.

My Troopy has two petrol tanks, and uses 2 valves to change between them - one to select which tank to draw from and the second to send excess fuel back to the tank. When one of these fails you can draw from one tank and return to the other. Now that's a mighty effective way of pressurising a fuel tank. Happened to me a long time ago and took a while to figure out what was happening.

BTW - Very impressed with your signature photo, though it's hardly a glamour shot!

J and V
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AnswerID: 346723

Follow Up By: Wayne (NSW) - Saturday, Jan 31, 2009 at 07:04

Saturday, Jan 31, 2009 at 07:04

A dual tank plumbing system is very simple if you can work out what is happening when you switch tanks.

If there is no power going to the change over relay the fuel will be drawn from the main tank. With diesels vehicles fuel can be drawn from either tank as opposed to a petrol vehicle where fuel is pumped from the sub tank into the main tank and a high pressure pump pushes the fuel to the injectors.

When I had my 80 Series the return fuel line snapped after the return port. When the motor was running fuel was leaking out of the return line. When the motor was turned off the fuel stopped leaking. Switched tanks and there was no fuel leaking at all.

Took at while to work out what was happening, but I was able to run on the other tank until I got home to have a good look.

Signature photo, It would have been more glamorous if those two female were not in the photo. :-))


FollowupID: 614882

Follow Up By: Member - John and Val W (ACT) - Saturday, Jan 31, 2009 at 08:38

Saturday, Jan 31, 2009 at 08:38
G'day Wayne,

Wouldn't argue about diesel setup, but my old petrol Troopy uses 2 solenoid operated changeover valves - one selects the supply tank, the other the return tank. The second tank doesn't get pumped to the main.


J and V
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FollowupID: 614895

Reply By: Rangiephil - Friday, Jan 30, 2009 at 16:14

Friday, Jan 30, 2009 at 16:14
It's not really just the petrol in the tank getting hot that causes this.
Most injections except the very latest continually recirculate petrol through the fuel rail and back to the tank via the fuel pressure regulator.

The petrol is returned to a sump where the fuel pump pickup is , some it then once again goes to the front and some is bled back to the tank.
you can see the potential for the fuel to heat up as it traverses the engine bay . Nearly all injections have a fuel temp sensor to compensate for the hot fuel so there is little effect on performance.

So the most heat is probably generated in slow going with small throttle openings, as the bypass back to the tank is greatest then.
I have a really high build up in my 140 litre LR tank and have been thinking of a way to overcome it. Maybe a fuel cooler on the return line as some diesels have.
Regards Philip A
AnswerID: 346755

Reply By: Splits - Friday, Jan 30, 2009 at 20:16

Friday, Jan 30, 2009 at 20:16

Are you sure these cars don't have something wrong with their tank vapour systems?

I just had a look at a the operation description for the evaporative emission control system on a VP Commodore in a genuine factory manual that I have. The whole system looks the same as any other car and I would imagine they all work the same way.

It says the tank is not vented to the atmosphere but the filler cap is fitted with a pressure and vacuum relief valve. This would be to prevent these problems.

It goes on to say the fuel vapour is absorbed by the charcoal. When the engine is running above idle speed, air is drawn into the canister through the atmospheric port at the bottom. The air mixes with the vapour and the mixture is drawn into the intake manifold via the canister purge line.

The purge valve is controlled by a ported signal from the throttle body.

When you study the whole operation, you can see a few places where things could wrong. If everything is working properly though, the symptoms you describe should not happen.

AnswerID: 346780

Follow Up By: Wayne (NSW) - Saturday, Jan 31, 2009 at 07:14

Saturday, Jan 31, 2009 at 07:14

What you are saying is right on modern motors. We only noticed the problem after we had stopped and the engine was turned off.
The petrol was still expanding/ vaporising and this must of filled the canister causing the petrol smell. Once the filler cap was opened and the pressure released the petrol smell stopped.

FollowupID: 614883

Reply By: mechpete - Friday, Jan 30, 2009 at 20:37

Friday, Jan 30, 2009 at 20:37
hi there ,
I had the same problem with my old GQ on lpg ,if we went up into the high country with a full tank of fuel and used lpg but kept the fuel for emergency the same thing would happen because the fuel level would expand and push it up the canister and then run on both .
to fix it run it for about 1/2 hr on fuel to lower the fuel level so its to low for the canister purge hose to pick it up .
AnswerID: 346787

Reply By: Rangiephil - Friday, Jan 30, 2009 at 20:55

Friday, Jan 30, 2009 at 20:55
"Are you sure these cars don't have something wrong with their tank vapour systems?"

You have a point. The petrol should never vent to atmosphere, but many cars do not seem to have relief valves. My Range Rover Classic does not seem to.
The ECU switches the purge valve at light throttle , while the car is moving , usually at revs over injector cut off, which is usually 1500RPM.
If one of these conditions is not met it doesn't purge.

While my tank is well pressurised it never leaks to atmosphere and does not seem to affect the running at all.
Regards Philip A
AnswerID: 346790

Follow Up By: Splits - Friday, Jan 30, 2009 at 21:51

Friday, Jan 30, 2009 at 21:51

I have just read the trouble shooting section of the book and it lists overfilling the fuel tank as a reason for fuel getting into the canister.

Pressure in the tank is said to be caused by a faulty pressure relief valve but it goes on to say some pressure in the tank is normal in high temperature conditions.

It would appear that if the system is working properly and you don't try and fill the tank right up to the cap, you should not get fuel or vapour seeping out anywhere. This would make sense because I could not imagine any manufacturer allowing fuel leaks or smells to occur and saying it was normal.

The whole purpose of the charcoal canister system was to stop all fuel vapours escaping to the atmosphere.

FollowupID: 614851

Reply By: Member - Roachie (SA) - Friday, Jan 30, 2009 at 21:49

Friday, Jan 30, 2009 at 21:49
Okay, this has made me think of another question......

I have a diesel and a year or so ago I managed to forget to re-fit both my fuel caps when I filled up at Pt Augusta. I realised my error when we got to Pt Pirie and went to an auto shop. They only had petrol type caps, so I bought 2. I still have them fitted, but they do stipulate they are for PETROL only.

Now, is it only petrol vehicles that have this charcoal canister venting set-up? I often hear my fuel tanks making gurgling noises in the carport, sometimes several days after I last drove it. I haven't noticed any drop off in performance of the rig (as if it was starving of fuel), but have experienced the rush of air as I remove the filler cap/s.

AnswerID: 346800

Follow Up By: Wayne (NSW) - Saturday, Jan 31, 2009 at 07:32

Saturday, Jan 31, 2009 at 07:32

Only you could forget both filler caps.

What I do is to leave the filler cap/caps on the side step near the drivers door. That is why Toyota have the fillers on the drivers side on the Troopies. I started doing this after I forgot to replace a cap once.

With a diesel tank there is no fancy charcoal canister only a short fuel line with a one way value and the end of the fuel line is feed into the chassis rail.

That gurgling noise that you can hear coming from the fuel tanks is made when the vehicle is not used enough. If you hear that noise again, you should pack the vehicle straight away and go camping. Well that is what I do to prevent the dreaded gurgling.

FollowupID: 614884

Reply By: Member - Bucky, the "Mexican"- Saturday, Jan 31, 2009 at 04:16

Saturday, Jan 31, 2009 at 04:16
Back in the days when we owned a Commorore, I decided that I would fill the tank right up to the very brim. You know the situation !
"I'm gunna drive it till it stops" syndrome, and just see how far the thing gets.

It was a 40c day and I got 6 km down the road when I thought we were doomed.
The Commodore sputtered, big time, and then Lyn picked up on the smell !

What have I done, "she has blown up". I immediately stopped, and lifted the bonnet and to my surprise, there was lots of petrol pouring out of the canister, dashboard lights on !

Panic Panic Panic.

I released the fuel cap, and she hissed like a mad cat.

In my wisdom, I decided to get the hell out of there, heat and petrol do not mix anyway.

5 km down the road, all was good, and I never filled that vehicle to the brim again..Lesson learnt.


AnswerID: 346819

Reply By: Richard W (NSW) - Saturday, Jan 31, 2009 at 06:58

Saturday, Jan 31, 2009 at 06:58

Recently did a trip through the High Country (Australian Alps ;) ) with a 100 series petrol. After the driver filled the tank and the engine had done some hard work, when he stopped, he heard a boiling sound from the fuel tank. He opened the filler cap and the gas vented for about 5 minutes. Sounds like this might be a fairly common problem.
AnswerID: 346824

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