Gas Tank Pressure
 

When I drove this last weekend, I had something I had never experienced before happen. I heard a noise under the hood and when I lifted it to find what it was, I found that the valve that goes to the black canister was making the sound. I slowly opened the gas cap and it had a huge amount of pressure. It took over a minute to let it escape. What would cause this back pressure? Thanks in advance. :confused:

Re: Gas Tank Pressure
 

Gid'ay Earl. Was it positive or negative pressure. The carbon canister can put a negative pressure in the tank as it evacuates the fuel fumes, It is controlled by,,,,,,,,yes another solenoid.

If the pressure is positive then there is a problem with the for mention solenoid.

Harvey. ;)

The gas vomited out was pretty impressive. :eek:

Hope I can help Earl.

N.B. In order for there to be a negative pressure in the fuel tank the spring loaded integral relief valve in the fuel cap would have to be faulty and this is unlikely.

It would appear from your remarks that you have positive pressure, in which case you should investigate the Purge control solenoid valve. This is in the circuit of a tube which runs to the throttle body. If removing this tube from the canister releases the pressure you are on the right track. In the event that pressure is released leave the tube disconnected until you are able to fix the problem. The only effect will be to allow fuel tank fumes to be released into the atmosphere

I can assist you with suggestions re checking the solenoid valve if you shout.

Going on what you have reported it appears unlikely that the roll over valve is causing the problem.

It was positive pressure (which was quite negative!) After I messed with the check valve and a couple of other things that I am sure really didn't help, I didn't have any more problems. Untill I can find a problem that happens all of the time, it will be hard to figure it out. I will keep you posted. Thanks all for your suggestions. :)

Earl had a good think about it, if it is positive pressure as you say, the problem would have to be either the line from the tank to the carbon canister is blocked or the hose that runs out of the bottom of the carbon canister and plugs into the frame rail under the air box is blocked.
To check them you can remove the pipe from the center of the canister, remove the tank cap and blow in the pipe, it should be clear. (my 95 model has a small one way valve fitted to the pipe at the canister, I don't if all models have this valve, if it has you will have to remove it to blow). Then blow into the pipe in the center of the canister. This should be clear right through to the frame rail.

The way the system works, which may help you or somebody searching the subject, is.
The system is to prevent hydra carbons from escaping to the atmosphere. This is done by fitting a canister filled with carbon to the tank breather pipe. As the tank contents expand and contract, the fumes pass through the carbon which removes the hydra carbon from the air, so that the fumes don't escape, while the car is standing.

When you start the car, the ECU waits till the engine is up to temp and is cruising, to open the purge valve solenoid to apply manifold vacuum to the canister. This pulls air through the tank cap, through the tank to carry the fumes up to the inlet manifold to be burnt. Also at the same time the manifold vacuum pulls air up the hose to the bottom of the canister, through the carbon, to remove the suspended hydra carbons,which are burnt in the engine. At this time it consumes the fumes from the tank and cleans the carbon of the hydra carbons held there.

It is normal to find a small negative pressure when removing the petrol cap, the volume of air rushing in depends on the amount of fuel in the tank at the time. It should never be a positive pressure.

Harvey.;)

Thanks for the informaton. I have a one way check valve on one of the hoses that go to the canister, and I took it off of the hose and messed with it, and then put it back. I could blow both directions with it. I thought that this was a one way valve.

I have not got the positive pressure again. All seems to be ok again, but will watch it closely. :)

One way valve
 

Harvey, quotes from your post : -

Ò (my 95 model has a small one way valve fitted to the pipe at the canister, I don't if all models have this valve, if it has you will have to remove it to blow)Ó.

This is NOT a one way valve. It is a spring loaded restrictor valve which opens with a level of positive pressure. This valve maintains a constant positive pressure in the fuel tank resulting from return delivery from the pressure regulator.

I do not agree with your statement, quote : -

Ò It is normal to find a small negative pressure when removing the petrol cap, the volume of air rushing in depends on the amount of fuel in the tank at the time. It should never be a positive pressure. Ò

N.B. The petrol filler cap is fitted with a valve to prevent significant negative pressure. There should be no Ò volume of air rushing inÒ and noticeable when removing the cap. If there is pressure it WILL be POSITIVE pressure but again not at a level which will be noticed when removing the cap.

You also state : -

Ò This pulls air through the tank cap, through the tank to carry the fumes up to the inlet manifold to be burnt. Also at the same time the manifold vacuum pulls air up the hose to the bottom of the canister, through the carbon, to remove the suspended hydra carbons, which are burnt in the engine. Ò

I believe the above should read : -

This first pulls air through the vent in the bottom the canister and when pressure in the fuel tank rises above the set pressure of the restrictor valve fumes will also flow from the fuel tank. If the return from the fuel regulator is not sufficient to equalise tank pressure this will occur via the fuel cap which has a pressure loaded inlet valve.

I see that you have been extracting information from the manual as there is no mention of the restrictor valve there, but possibly this is an oversight by the authors. My 92 ex Japan has it.

You are however correct in drawing attention to the fact that the problem raised in this thread is not so likely to be the Purge Control Solenoid Valve as the canister is provided with a bottom vent, something I did not pick up on.

Trevor.

:confused: :)

Hrrrmmm mine just did the same thing on the N Cascade pass (w/o expelling gas though), didnt have a problem with pressure before. :confused:

I hate to hijack, but it is on the same topic kindof. I broke the fitting on the vacuum solenoid for the line that goes to the charcoal canister when I put in my engine. It is the drivers side line on the charcoal canister. The fitting for that line is broken off on the vacuum solenoid.

I've been driving that way ever since and haven't noticed anything strange.

I managed to get a used solenoid on ebay but haven't bothered to pull the intake to install it yet.

Am I going to die?
Am I losing vacuum or just venting fumes?

Earl, was the fuel level low? When my fuel gets low and it is rather warm I get positive pressure. I think it might just be fumes building up in the tank from the heat. Other cars I've had did the same thing.

Mine was nearly full, I opened the cap slowly because I remember that Earl's sprayed gas everywhere. :eek:

Re: Gas Tank Pressure
 

Going back nearly 10 years on this one!!!

Since I have owned my car I have had two observations that I wouldn't call issues, per se. The first is variable gas tank pressure. Sometimes I get a little burst of air when unscrewing the gas cap and sometimes I get a hurricane. Today, I got a hurricane and gas sprayed everywhere. So that makes it a bit of an issue.

The second is that I have always had a high long term fuel trim reading (>10) on my OBD2 reader but only for one bank. The gas mileage has always seemed decent and the car has always run well so I eventually ignored it.

Based on what I read above, I wonder if they might not be related. I don't know how the purge solenoid, the restrictor valve, the canister, the intake manifold and the tank are plumbed up but it sounds like it could be related.

The first question is, what is the most likely issue causing the massive fuel tank pressure?

Second, could this be related to the high long term fuel trim on one bank?

Third, could this be related to the starting issue I had a couple weeks back?

Re: Gas Tank Pressure
 

As far as the 10+ LTFT here is a pasted comment.



Running too lean – High positive fuel corrections can be traced to MAF and O2 sensor faults, vacuum leaks from intake gaskets/hoses, unmetered air (intake snorkel leak), clogged or dirty fuel injectors, fuel delivery issues, and exhaust restrictions such as a clogged catalytic converter.

Diagnostic Tip:

For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak. After the repair, reset the KAM and start the vehicle. Monitor the fuel trims to make sure they are within the normal ranges. It could take up to 10 miles of driving for an accurate LTFT reading.

Re: Gas Tank Pressure
 

Going back to close this one out. The check valve to the canister was backwards. I have no idea how or when it happened.

And thanks to fasteract for the diagnostic suggestion. I still need to put that mystery to bed.