I had filled the tank and drove about 45 miles climbing in altitude from sea level to about 10k'.
The last 10 miles at less than highway speed and a much steeper grade.

When I stopped at about 10k', I heard the fuel fumes venting.
I loosened the cap reducing pressure, but it kept producing fumes because the tank was very warm.
I kept loosening the cap and bleeding the pressure off.
I was completely paranoid about fire from the fumes.
I finally got it stabilized, then turned around and coasted back down the mountain to the highway.
It did not over pressurize again.
I drove it back to sea level and it still did not over pressurize again.
The entire return to sea level was basically a down hill coast with engine running and in gear.

The entire drive has an altitude gain of 13 to 14k'.
I still had 3 to 4k' to go.
I've done it before with no problems.

What would cause the fuel tank to heat up and over pressurize?
Filter problems?
Return valve?
Fuel Pump?
???

 

Do you bring oxygen with you to 13K plus feet? Air becomes basically unbreathable above about 12K.......

As you climb, atmospheric pressure drops considerably. (that's why your ears pop.) The fuel system, being completely closed, is 'supposed' to vent thru the charcoal canister, and then be burned off by the engine. (canister purge valve opens, and engine can draw air from the charcoal canister.) As you come DOWN, atmospheric pressure increases, so, it simply isn't an issue.

 

His profile says Hawaii so I believe it, driving around the volcanoes.

Yes it's supposed to purge through the charcoal can instead of the cap (the intake manifold vacuum is actually supposed to suck the fumes back from the fuel tank) but sounds like that didn't happen.

 

@ HeyYou
You have a very misinformed idea of what happens due to increases in altitude.
But, I'm not here to give a lecture on the gas laws and how they affect respiration.
Hundreds of people climb to the summit of Denali at 20k' every year.
Many people in the Andes live every day of their lives at 15k' or higher.

Something went wrong with the fuel system on my Dak.
The fume venting was caused by, or made worse by the gasoline in the tank being heated up.
I'm thinking that a fuel return line may have been bringing hot fuel back to the tank.
The increase in altitude could have made the problem worse by lowering the boiling point of the fuel.

@magnethead
Yes, the road to the top of Mauna Kea reaches almost 14k'.
There are a number of observatories, and the people that man them, up there all the time.
I have driven up there several time before.
The only problem I had found before, was a huge loss in power from the engine, as well as symptoms of O2 starvation in myself.
Becoming acclimatized to the altitude could fix that.
Something I found out later was that I should have stopped a few times along the way, and turned the vehicle off to allow the computer to reset itself so that it could adapt to the extreme changes in altitude.

I'm wondering if that may have been part of the problem with the fuel vapors pressurizing the tank.
When I loosened the cap to vent the pressure, then tighten it again, it would build up pressure again and need to be vented again because of the hot fuel in the tank.
It seems that when the tank cooled down and I began the descent, then the problem went away.

To restate the problem:
The gasoline sold on the islands for vehicle use is 10% ethanol.
The fuel in the tank was not hot before I started out on the trip.
The fuel in the tank was getting hot, creating a buildup of pressure in the fuel tank.
Apparently, the fuel evaporation control system was not handling it.
I assume that the fuel feed for the injectors has an excess fuel return line back to the tank.
That could account for the fuel in the tank heating up.

I thought that evaporation control was part of the fuel system.
It isn't.
It's part of the emission control system.
Computer malfunction could be at the bottom of this.

How do I check the function of the fuel evaporation control system?
Is there an excess fuel return line to the tank?
Any other ideas as to what might be the cause of this happening?

 

Yep, lots of people do it....

I am not even going to bother to argue the point though.

 

gas is volatile...increases in altitude decreased boiling point temperature. water will boil at room temperature if in a vacuum. The fuel isn't boiling but more vapours were created (i speculate) due to the pressure drop than the evap system could release (possible due to a evap venting problem). Fuel caps to my knowledge has a vac and pressure release as a safety measure when the evap is blocked.

the map sensor is used to compensate for altitude changes. Its updates at wot or key on engine off.

 

Well, I'm going to try going up again today.
There was a big dump of snow on top of the mountain through the last week.
I'm going to try getting some skiing in for the first time in years.

Still don't know why the fuel in the tank heated up.
That is what seems to be the cause of the problem.
I'll try stopping every few thousand feet and shut down for a short while.
That'll let me monitor it and maybe solve the problem for me.
If I don't blow up, I'll up date this when I get back.

 

The fuel pump creates heat. As oxygen does down, injector pulse goes down to hold stoich. As flow does down, rail pressure can go up (the pump has in internal regulator to hold at ~49psi), if the regulator is working well the pump should always see the same load. If not, it may be getting loaded up, causing it to work harder and make more heat.

But my first guess is that the EVAP system is not charging. Were you above 3/4 tank or below 1/4 tank? EVAP does not cycle with a full or empty tank.

 

what year, engine dakota?

 

sig says 99 5.2