fuel tank pressurizing

[krisp]

Hi...thought someone maybe able to give me the heads up.
My fuel tank in my 96 V8 in pressurizing badly.
I spoke to the mehanic and he informs me they do tend to pressurize but this appears to be more extreme than slightly.I believe there is some form of an electrical switch release on the tank???
I can hear a high pitched buzzing (like a fuel pump or switch) coming from the tank when I stop the vehicle and get out... this disappears when I release the pressure by undoing the fuel cap. Any ideas whats causing it??? Thanks

[Adam in NYC USA]

When the pressure builds up, it is suppose to release the pressure. If you want, change the cap with a new one and see if it still does that.

Now if the old cap does that in your hand afterwards, with the new cap in place,then you DO have a problem.

[TerryS]

Originally Posted by Adam in NYC USA

When the pressure builds up, it is suppose to release the pressure. If you want, change the cap with a new one and see if it still does that.

Now if the old cap does that in your hand afterwards, with the new cap in place,then you DO have a problem.

I don't think that's right. The Purge Control valve, mounted on top of the charcoal evaporitive canister is operated, in pulses, by the ecu, and allows engine vacuum to bleed off pressure in the tank, through the charcoal canister, and back into the right side of the ram housing base plate. If the valve is not operating, pressure, created by the returning excess fuel, will increase in the tank. The cap on American cars is not supposed to release any pressure to the atmosphere.
One of two things may be happening. Either your charcoal cannister is plugged up (don't buy a new one from Land Rover, it's made right here and available at less than a quater of Land Rover's price at NAPA) or the solinoid valve has failed. With the engine running, check for the presense of voltage on the whit with purple tracer wire at the purge valve. The ground path (slate and yellow wire) is controlled by the ECU, and should be checked with a scope. You can connect a test light accross these two wires and the light should be blinking at kind of a random rate. If it is, try applying 12 volts directly to the valve and listen for it clicking. If it is clicking, see if you can blow through the valve. You should check it out soon. I have seen severly distorted tanks from excess pressure. Leaving your cap slightly loose will probably trip a code and CEL (Chk Engine Light) but it won't make the engine run poorly, but you won't burst your tank (plastic ones can) and you won't be flooding your engine if your injectors are old and weepy.

[Disco Mike]

Terry,
I think you are right on the money.
Mike

[Bene]

crap, i thought that was normal

looks like i have something to do tomorrow.

[downunderdisco]

Terry
Could this build up of pressure and fuel being pushed through worn injectors lead to a motor running rich? I have a 96' 3.9 no o2 sensors in the cat and have been chasing a problem with the motor running rich. It had its ecu reset by a LR dealer but I still seem to have the prob. I also get bad pressure build up when it's hot.

[TerryS]

Excessive pressure WITH worn injectors can certainly contribute to rich running, whereas, If the injectors are good, The pressure alone isn't likely to cause the motor to run rich.
I would suggest you investigate the condition of your fuel temp sensor in the rail, look to see that your O2 sensor outputs are ok, and that the water temp sender to the ECU ( The one mounted vertically, with plug/socket similar to tose on the injector)Not the temp sender, mouted horizontally, that's for your guage) hasn't failed.
Those are the three that most effect injector pulse duration.