Fuel tank under pressure

[marcthelegend]

Hi All,

This weekend was driving on the sand up near Two Rocks in WA, having a blast and came across someone waving at the top of a hill, upon investigation it turns out there were 2 vehicles that had driven down to the beach but the sand was too soft for them to get back up and the tide was coming in.

We linked 3 snatch straps together and managed to pull them both out of there pretty quickly which was good, they were mighty embarassed that their heavily lifted vehicles with 35's needed to be recovered by a Pajero.

Anyway, whilst recovering the vehicles I looked in my mirror and noticed what looked like vapours coming out of the fuel filler pipe (you can see it diffusing your view). Once we recovered them I headed to a clearing to let the car cool down as the coolant temp had crept up to 92deg and removed the fuel cap to allow it to vent.

About 10 seconds into venting the car started to struggle as if it were losing fuel and came so close to dying, venting of the tank took a whopping 5 minutes and you could hear bubbling from within the tank.

I'm trying to work out what has gone wrong here, looking at the service manual for the tank there are 2 ways to vent the tank, the first being a vent on the uppermost rear section of the tank, this goes up to a small check valve near the filler neck, google says the valve should allow air in but not out, others say it should allow air out but not in, others say it should do both, does anyone know what the check valve actually is meant to do?

Other way to vent is through the rollover valve / charcoal cannister / purge solenoid. I believe this is working as the purge solenoid will go from ticking loudly to not ticking at all and cycle like that, in addition, if I plug the "vent" side of the charcoal cannister there is a slight vacuum created though if there is a way to test the cannister I'd be keen to try that as well.

Any help is greatly appreciated

Cheers,

Marc

[vladguan]

Not sure which way but the name itself, checkvalve, means flow in one direction only.

[marcthelegend]

Thanks Vlad,

That was my thought as well but I remember finding a post on here some time back and I swear I recall the OP saying a brand new valve flowed both directions, easy to blow air through it one way but with some resistance the other way. Then again I believe the cap seals when shut so it wouldn't really matter what the valve was doing.

[vladguan]

https://www2.pajeroclub.com.au/forum/showthread.php?p=298097&postcount=9

[erad]

Firstly, I have no direct knowledge of the arrangement of the fuel systems in your car, but this may help to diagnose the cause...

I presume your Pajero was working very hard, and not moving fast, if at all. Therefore the underside areas of the car, especially around the exhaust pipe would have become very warm, if not hot. The fuel returning to the tank from the pressure regulating valve has to run somehow back to the tank. The return line may even run beside the exhaust pipe.

Years ago, I had a Triumph 2000, and being of English heritage, it liked to leak oil. The underside of the car was caked in oil over the years. Add a little mud, and it sets like a thick coating of underseal. The fuel line ran from the tank to the engine up the transmission tunnel, right beside the exhaust pipe. I used to make regular runs uphill through the Snowy Mountains, and very hot days, the car would make it to a certain point and die. This was in a long box cutting and luckily there was one area where I could pull off and fix the problem. First time was scary, the other times, I knew what was wrong and easily fixed it and got going again. The problem was that the petrol was boiling inside the glass bowl of the fuel pump. I carefully poured some water over it and it cooled down fairly quickly and then I got under way again. Only a short distance to the very top of the climb and after that, no problems. Eventually, I was looking at the car and noted all this caked oil/mud everywhere. I wiped the crud from the fuel line (and the brake line as well), and I never had any more problems with fuel boiling after that.

If your fuel return line to the tank runs close to the exhaust pipe, the return fuel will get warm or hot. If the lines are caked with mud etc, they will get hot and stay hot. When you released the fuel tank cap, this dropped the pressure inside the tank back to atmospheric pressure, and if the whole tank was warm enough the fuel may have been boiling inside the tank. This would go on for 5 minutes easily until the temperature of the remaining fuel had dropped to a level where the vapour pressure was higher than atmospheric pressure and the bubbling then stopped. Note that to change from liquid state to gaseous state requires a certain amount of heat. In generating this change (boiling the fuel), this would drop the overall temperature in the tank and the boiling would automatically stop. Alternatively, if you had put the cap back on and pressurised the tank again, the boiling would have stopped.

Having said all this, there should be some venting of the tank so that excessive pressure cannot build up. From memory, there is a purge valve at the cannister in the engine bay. Maybe this was not operating? I would say that the hard work you trusty old Pajero did heated up the petrol in the tank and this caused even more pressure buildup in the system.

[marcthelegend]

https://www2.pajeroclub.com.au/forum...97&postcount=9

That's the one! I'll pull the valve out and see the condition of it and which was air flows and the level or resistance!

[marcthelegend]

Firstly, I have no direct knowledge of the arrangement of the fuel systems in your car, but this may help to diagnose the cause...

I presume your Pajero was working very hard, and not moving fast, if at all. Therefore the underside areas of the car, especially around the exhaust pipe would have become very warm, if not hot. The fuel returning to the tank from the pressure regulating valve has to run somehow back to the tank. The return line may even run beside the exhaust pipe.

Years ago, I had a Triumph 2000, and being of English heritage, it liked to leak oil. The underside of the car was caked in oil over the years. Add a little mud, and it sets like a thick coating of underseal. The fuel line ran from the tank to the engine up the transmission tunnel, right beside the exhaust pipe. I used to make regular runs uphill through the Snowy Mountains, and very hot days, the car would make it to a certain point and die. This was in a long box cutting and luckily there was one area where I could pull off and fix the problem. First time was scary, the other times, I knew what was wrong and easily fixed it and got going again. The problem was that the petrol was boiling inside the glass bowl of the fuel pump. I carefully poured some water over it and it cooled down fairly quickly and then I got under way again. Only a short distance to the very top of the climb and after that, no problems. Eventually, I was looking at the car and noted all this caked oil/mud everywhere. I wiped the crud from the fuel line (and the brake line as well), and I never had any more problems with fuel boiling after that.

If your fuel return line to the tank runs close to the exhaust pipe, the return fuel will get warm or hot. If the lines are caked with mud etc, they will get hot and stay hot. When you released the fuel tank cap, this dropped the pressure inside the tank back to atmospheric pressure, and if the whole tank was warm enough the fuel may have been boiling inside the tank. This would go on for 5 minutes easily until the temperature of the remaining fuel had dropped to a level where the vapour pressure was higher than atmospheric pressure and the bubbling then stopped. Note that to change from liquid state to gaseous state requires a certain amount of heat. In generating this change (boiling the fuel), this would drop the overall temperature in the tank and the boiling would automatically stop. Alternatively, if you had put the cap back on and pressurised the tank again, the boiling would have stopped.

Having said all this, there should be some venting of the tank so that excessive pressure cannot build up. From memory, there is a purge valve at the cannister in the engine bay. Maybe this was not operating? I would say that the hard work you trusty old Pajero did heated up the petrol in the tank and this caused even more pressure buildup in the system.

Thanks Erad, That has given me a few diagnostic related ideas!
I've extensively gone over the fuel system many times, dropped the tank more times than I care to admit (bad fuel causing recurring problems and a massive dent from a miscalculated rock climb) so know it well!

The closest the fuel lines get to the exhaust would be a distance of around 150mm and thats on the firewall, nearest the drivers side exhaust headers, from there back its exhaust down the passenger side and fuel related stuff down the drivers side.

The heat from the recovery combined with the sand (which tends to trap the heat i find) definitely caused the temps to jump a considerable amount and no doubt boiled the fuel for a little while. I was anticipating the pressure to be higher as the purge solenoid can only shift so much air at any given time and having seen many a jerry can expand in just the slightest amount of heat and at such an incredible rate I wouldn't have expected it to keep up too well.

That being said it was spitting fuel out around the cap and under an abnormally high amount of pressure so something is amiss somwhere, when I get home I'll remove the charcoal cannister and see if I can blow air through it, I know there is a clear path between purge and air but haven't yet tested the path between tank and purge.

I know the purge valve works as it creates a decent amount of suction against my finger.

If all that checks out it'll either be the line running to the rollover valve in in the tank .. or the valve itself which I am really hoping it isn't as they're a pain to get to!!

Cheers,

Marc