Fuel Filler Cap

I discovered an anomoly today while trying to fix a fuel leak at the filter. Spent the better part of last year trying to get a gas gauge that would show when the tank was really empty. I think I may of found the problem. THE FUEL FILLER CAP

While I was under the car, engine running, I had brother Gord remove the filler cap. The damn tank expanded almost 2 1/2 inches !!!! Turns out the evap solinoid is stuck open (engine vacuum applied to tank) and the check valve in the cap is stuck closed sucking the tank flat.

Explains why last summer when the car crapped out at 1/4 tank, I could loosen off the cap and get another 20 miles. Actually thought the system was suppposed to be under a bit of vacuum. By the way, when I released the vacuum, the fuel gauge showed I was actually empty. Guess I was over thinking the problem.

Either way, before you blame the sender unit, check to see if your evap solinoid is working as it should and that the cap is working. That last 2 1/4 inches makes a big difference in what the gauge says. At least now I have a new pump and sender and will have a new solinoid and cap.

Bill

Two & a half inches? Wow. I assume that the newer tanks with more baffling inside would collapse less?

That would explain the thump I heard yesterday when I was filling up the OCC.On my Stant
locking gas cap you just push the key inward to release tank pressure before turning to remove.
The low fuel chime had been chiming for about 15 miles before I stopped.ABout 2 or 3 seconds after pushing in the key I heard a very low thump from under the car and now I know it was the tank expanding.

jimbeau wrote:

Two & a half inches? Wow. I assume that the newer tanks with more baffling inside would collapse less?

Now that I think about it, the baffle may be what is keeping it from collapsing further. If memory serves, the baffle in the Cadillac (LT1) was about 2 1/2 inches from the top of the tank. Going to have to check the old sender to see if the pump and sender has enough travel. Just saw the tank from the corner of my eye so can't say if the area under the sender was flexing that much. I'm assuming the Caddy is the new style but that would answer some questions. For now I'm running with a rag stuffed in the pipe.

According to All Data, the cap should not hold vacuum, just pressure (3 PSI I think).

Bill

wouldn't the rag cause a check engine light? (open cap)

" Check to see if your evap solenoid is working as it should and that the cap is working."

How does one go about checking it? Any one have info to share on that?

Dave

Krzdimond wrote:

wouldn't the rag cause a check engine light? (open cap)

Black electric tape fixes that

Bill

lakeffect wrote:

" Check to see if your evap solenoid is working as it should and that the cap is working."

How does one go about checking it? Any one have info to share on that?

Dave

You can follow the procedue in the FSM or:

1. remove connector and vacuum lines from solinoid
2. I use a hand vacuum pump (brake bleeding pump) and try to apply vacuum to the engine side vacuum port. If it will hold a vacuum, than the valve is not stuck open.
3. Reconnect the electrical connector and the engine vacuum line. Leave the canister line off and connect vacuum gauge (or thmb) to that port. Ground out the green/white wire.
4. Turn ignition on. There should be vacuum at the canister port.
5. Start engine. The valve is a simple on or off valve so you should only get vacuum under the following conditions:
•Coolant temperature above 60°C (140°F)
•Vehicle speed above 2 mph
•Throttle off idle, above 1.6%

Spent some time last night tracing the system and have come to the following conclusion. In order for the engine to apply vacuum to the tank than:

1. The purge solinoid is on
2. The canister filter is plugged solid making it into a vacuum resevoir.
3. The pressure valve is open allowing the engine vacuum to suck down the tank. This valve is also referenced to the canister so if the canister is under vacuum than this valve will stay open.

Anyways at the very least it looks like I'm in for at least a canister and cap and maybe the solinoid and pressure control valve (think maybe the pressure valve is included with a new canister). Will know more over the weekend when I get the time to do the above.

Bill

I have noticed in some pictures of our wagons gas tank straps that there is sometimes a small
gap between the straps and the bottom of the tanks.Would this be a sign that your cap isnt venting?Am thinking that just looking at your tank as someone starts your car and revs the
engine would tell you a lot.

Problem solved. Wasn't the cap/canister or solinoid. It was probably the hose routing at the solinoid, though the pressure control valve was a little wonky until I used some PB Blaster to free it up.

Turns out the upper port on the solinoid is "vented". This allows the system to equalize when ever the solinoid is in the off position. The hoses were reversed, with the manifold vacuum working through the vented port and the canister/tank hose on the dead end port. Even with the solinoid off, the system held its vacuum, thus a constant vacuum being applied to the tank when the engine was running, regardless if the solinoid was on or off.

Theroetically, the cap should have released the vacuum so am still going to have to change that and the canister vent hose was also plugged. The whole system was being sucked down. Drove it for 1/2 hour today after cleaning everything up and NO vacuum in the tank

Bill

Thanks for the update. Good stuff to know know.