Excessive fuel tank pressure/Build-up.

[Dustin89]

I have a 89 SHO and lately I have noticed when i remove the gas cap for refueling there is alot of pressure in there. To much in my opinion. Usually it will just release pressure for a split second but not this. I open it real slow and i can feel fumes blowing and hissing out around the cap and my hand. I takes atleast 5-10 seconds or more for it to all release!

I replaced the gas cap with a new one and that did not help. Also replaced the fuel filter about 1 month ago. I checked fuel pressure at rail and it was 34-35 at idle. Pump seems fine but it should not build pressure like that!

Any suggestions?

Thanks
Dustin

 

[projectSHO89]

Service your evaporative emissions system.

You probably have a clogged vent line or a clogged charcoal canister.

Steve

 

[pjtoledo]

A gas cap is supposed to let air in, not out of a tank. So the cap is doing it's job. Most likely the vapor vent line going from the tank to the Evap canister is clogged. There is a restrictor section in the tubing near the canister/purge selonoid that can get clogged up. Clean it out,,,but DO NOT enlarge the hole in the middle.

Perry

 

[Dustin89]

Thanks for the help guys.

But where is this located? What should i clean it with?
This is all under the hood correct?

Dustin

 

[Off Road SHO]

That's correct. On 2nd gen it's below the antilock brake computer near the left front wheel. There is a solenoid that is controlled by the computer that is suppossed to vent that cannister into the engine every once in a while I believe.

Tom

 

[DHMag]

A gas cap is supposed to let air in, not out of a tank. So the cap is doing it's job. Most likely the vapor vent line going from the tank to the Evap canister is clogged. There is a restrictor section in the tubing near the canister/purge selonoid that can get clogged up. Clean it out,,,but DO NOT enlarge the hole in the middle.

Perry

i thought the fuel cap was similar in operation to the radiator cap. itll allow X amount of pressure to build in the tank before opening up.

regardless, id say the charcoal cantister needs replacing.

 

[shojuan]

i thought the fuel cap was similar in operation to the radiator cap. itll allow X amount of pressure to build in the tank before opening up.

regardless, id say the charcoal cantister needs replacing.

No. The whole idea is to contain the fumes and not release them into the environment. The charcoal canister is supposed to be able to absorb all reasonable excess. And it IS quite reasonable. Have you ever seen how much liquid charcoal can absorb? A large volume of fumes is really only a fairly small amount of liquid. Anyhow, that's getting beyond the question. The pressure builds and the pressure drives the fumes into the charcoal canister. When the fuel tank cools down, overnight for instance, it needs to draw in air through that cap valve to prevent the tank from the possibility of imploding. Flatbed a car with a partially full tank from California to Manitoba in winter and that would be a very real concern if it weren't for a proper fuel cap in good shape.

I think charcoal canisters can be salvaged somewhat when they misbehave by forcing compressed air through them. Worth a shot when one is suspect.

 

[Rockledge]

EVAP Vacuum

A little more on the EVAP system and air pressure in the system, per Ford:

Fuel Vapor Emission Control System

As a part of the fuel system, vehicles are equipped with a fuel vapor evaporative emission control system designed to meet Federal and state requirements in effect at the time of production.

The fuel tank evaporative emission system allows for controlled release of fuel tank vapor to the fuel vapor canister, Under normal operating conditions, this system will allow sufficient venting to prevent a build-up of internal fuel tank pressure.

Some operating conditions may cause temporary internal fuel tank pressure. In a normally functioning system, pressure will be relieved through vapor venting. Some of these conditions are:

- On warm or hot days, parking the vehicle after filling the fuel tank the fuel is cool from underground storage and vaporizes rapidly when warmed.

- Parking after driving over rough roads, washboard, etc., after filling the fuel tank. Agitation of fuel increases vaporization.

- Parking after driving long distances in high-temperature conditions with low fuel level.

- Climbing long grades, especially while towing a trailer or while fully loaded.

No service is required if these conditions caused the customer concern.

A normally-functioning fuel tank evaporative emission system will relieve the pressure build-up.

A blocked fuel tank evaporative emission system can cause abnormal fuel tank pressure and must be serviced