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Fuel tank builds pressure

'83 Turbo Coupe, the fuel tank builds pressure and fuel vapor eventually gets into the cabin. I've had the tank off and don't see any issues with the lines. When there is pressure in the tank fuel vapor cannot be pulled by vacuum to the charcoal canister, relieve the pressure and vapor flows. This has me really stumped and I can't safely drive the car. Thanks for your help.

Fuel tank builds pressure

Reply #1
the 90deg fitting on the drivers side of the tank is a one way check valve.

the valve ties to a rubber & metal transition path up home run to the charcoal canister.

the charcoal canister is a simple pass through container.

the purge solenoid located close to the charcoal canister gets triggered to turn "on" by the EEC.

when the purge solenoid is on, the engine is able to suck vapor from the tank, through the line, through the canister and back up to the intake.


-unplug vac line from tank to purge solenoid
-unplug vac line from the rubber to metal transition back near the tank.
-you have now isolated the metal line path.
-verify you can blow through the line.
-plug off one end and suck on the other or by vac pump,, verify it holds pressure to confirm no breaks.

-you currently have a rubber line from the fuel tank check valve.
- loosen gas fill neck cap and go back under car
-suck on the check valve rubber hose to insure vapor can be sucked from the tank.
-blow through the line and verify you *can not* push air back into the tank.

-free up the purge solenoid tee fitting.
-blow through the TEE and verify you can *not* do so.
-unplug electrical connector to purge solenoid
-with gator clips manually 12dc to the solenoid
-listen or feel for a click as power is applied.
-blow through tee fitting to verify you *can* pass air through.

-blow air through charcoal canister.

-blow air through the vac line that leads to your engines vac port fitting.


=========================
-inside trunk familiarize yourself with the fuel fill neck and the plastic surround that has an integrated drain hose leading to the rear bumper cover.
-verify inside the trunk below the fill neck  down deep into the deep rear qtr void that there is no evidence of fuel spilling inside the trunk.



********improper tank removal step by step process will cause the fill neck surround to be broken.  this will allow everything from rain, car washes or gas overspill to build up in the trunk area and then into the car.


=========== if you look at the exterior and interior skin of the lower rear qtr panels where they meet, there is a drain hole on the bottom side closest to the tire.
-lay a shop light under the rear qtr with bulb facing up.
-inside the trunk pass side, look down into the rear qtr area to locate the light
-shop vac out the valley down in there.
-study the shape and design of the drain hole.
-insert a punch into the opening and wobble the drain hole out more than what the factory did so that bigger and larger items can drain out.

Fuel tank builds pressure

Reply #2
jcassity.........awesome troubleshooting write-up! Sticky that one!

If it was me, Gman I'd start at testing the solenoid valve. As Jcassity says, it needs to be OPEN when power is on ignition, so it is powered open and shut when power is removed. If it fails, then gases will not vent from the tank.

If the valve is good, go after the hoses and check valve tests next.

Just a suggestion.

Fuel tank builds pressure

Reply #3
It's TC so no solenoid. Supposed to have a large hose from canister to airbox. If you ditched the airbox you are only able vent as pressure builds. Check the vent line from the tank. They have been known to gunk up and clog.

John

Fuel tank builds pressure

Reply #4
Yeah, it looks like the Turbo is purging any time the engine is running. Unless there is some kind of valving inside that air box.
The 3.8L doesn't have a solenoid either. It has a vacuum operated purge control valve. The same vacuum that goes to the EGR valve also goes the purge control valve.

Maybe somebody knows better than me, but here is how I think the carbon canister works:
If there is pressure in the gas tank with the engine not running, or it is not being purged, it goes through the carbon granules in the canister and out the air vent.
On its way through the canister the carbon granules absorb the fuel vapors.

When the engine is running and the canister is being purged, air is drawn in through the air vent and through the carbon granules. This removes the fuel vapors from the carbon granules and sends them on to the intake manifold.

If you remove the gas tank vent hose from the canister and this releases the pressure, this means the canister is plugged up.

The mushroom shaped thing on the canister in the pic is the air vent.

Fuel tank builds pressure

Reply #5
Quote from: softtouch;466239
If you remove the gas tank vent hose from the canister and this releases the pressure, this means the canister is plugged up.

I've removed the line from the canister, no vapor comes through it when the fuel tank has pressure in it. Unscrew the gas cap and vapor flows through the line.

I appreciate all of the responses.

In regards to a purge solenoid,  NAPA doesn't list one for an 83, but they do for an 84. With that being said, I have yet to locate this solenoid on my 83 and I don't recall one being on my 84.(wouldn't be the first time NAPAs has told me wrong. :): )

Fuel tank builds pressure

Reply #6
Quote from: G-MAN;466242
I've removed the line from the canister, no vapor comes through it when the fuel tank has pressure in it. Unscrew the gas cap and vapor flows through the line.
Sounds like you have a vacuum in the tank not pressure. Do you have the stock gas cap? 
How do you tell vapor is flowing? Smell it?

I appreciate all of the responses.

Quote
In regards to a purge solenoid,  NAPA doesn't list one for an 83, but they do for an 84. With that being said, I have yet to locate this solenoid on my 83 and I don't recall one being on my 84.(wouldn't be the first time NAPAs has told me wrong. :): )

Like I stated above, you don't purge active for vapor to flow. It should vent through the canister under it's own pressure.

The 84 EVTM doesn't show an purge solenoid for any of the three engines.
Does your car match the pic above?

Fuel tank builds pressure

Reply #7
Quote from: softtouch;466243
Sounds like you have a vacuum in the tank not pressure. Do you have the stock gas cap? 
How do you tell vapor is flowing? Smell it?

I appreciate all of the responses.

 
Like I stated above, you don't purge active for vapor to flow. It should vent through the canister under it's own pressure.

The 84 EVTM doesn't show an purge solenoid for any of the three engines.
Does your car match the pic above?

The gas cap is not stock but has been on for years, long before this problem started. Slowly unscrew the gas cap and there's enough pressure I can feel it moving through my fingers, out from the cap. On the line from the tank to the canister, I put my vacuum pump on the line and pump half a dozen times. No fuel smell, release the pressure by unscrewing the gas cap, pump a few more times and I can smell gas coming from the vacuum pump. My car matches the diagram, it is stock with no cold air intake or other mods.

Fuel tank builds pressure

Reply #8
Wow, I have no idea what is going on but it has nothing to do with purging the carbon canister.

If you have a piece of hose, put it on the tank vent line nipple of the canister and see if you can blow through canister.
If you can't blow through the canister, leave the tank vent line disconnected and see if the tank still pressurizes.
What do you have to do to get the tank pressurized?

Fuel tank builds pressure

Reply #9
It takes a short trip, about 20 minutes, to build up a good amount of pressure.

Fuel tank builds pressure

Reply #10
Well you have taken me into parts of the 84 shop manual I've never been to before.
Turns out there is a test procedure for the fuel vapor tube.
Basically it says you should be able to blow through the tank (with gas cap removed) and blow through the carbon canister.
Of course they use a hand operated pressure pump with pressure gauge. The description in the A1 & A2 flow test chart is a little screwed up. The removing the gas cap blurb should be in the A1 section.

At the tank they have a vapor separator and roll over valve assembly. To keep liquid fuel out of the vapor line and I guess to stop fuel leaks if your car is shiny side down.

There is also a blurb about the gas cap being a back-up pressure relief incase the normal venting system fails.

Fuel tank builds pressure

Reply #11
Have you tried taking the tank vent line off the canister and letting it vent to atmosphere to see that stops the pressure build up?
Have you tried blowing through the tank vent nipple on the canister to see if the canister is clogged up?

If you have one of those vapor valve/ roll over valve assemblies on the tank, maybe pressure in the tank is somehow causing that valve to close. when you take the gas cap off to release the pressure it opens.

Fuel tank builds pressure

Reply #12
Quote from: softtouch;466294
Have you tried taking the tank vent line off the canister and letting it vent to atmosphere to see that stops the pressure build up?
Have you tried blowing through the tank vent nipple on the canister to see if the canister is clogged up?

If you have one of those vapor valve/ roll over valve assemblies on the tank, maybe pressure in the tank is somehow causing that valve to close. when you take the gas cap off to release the pressure it opens.


This is my best guesstimate. I'll do some troubleshooting when I get the car out of storage, in about a month.

Fuel tank builds pressure

Reply #13
Quote from: G-MAN;466301
This is my best guesstimate. I'll do some troubleshooting when I get the car out of storage, in about a month.
10-4

Fuel tank builds pressure

Reply #14
Had time to do any troubleshooting?