I noticed the fuel gauge acting funny recently. It was showing a bit over 1/4 tank then a few miles later it went to empty and the fuel light went on. It only had 200 miles since the last fill and I went ahead and filled it up. It only took 12 gal. When I removed the gas cap, there was a noise like air escaping from the tank, however there was no odor of gas so I'm guessing it was vacuum in the tank rather than pressure. This is a 1997 Thunderbird with the V-8 engine. What's the most likely problem?

Thanks,
Jim
1997 Ford Thunderbird with 140,000 miles.

 

When fuel is consumed , some air has to be able to replace the fuel withdrawn.
There is a vent valve that allows filtered air to enter the tank.If the filter on the valve is plugged, air cannot get in to replace fuel withdrawn and sometimes than can affect performance due to insufficient fuel.that can't overcome the vacuum condition caused by a plugged filter on the vent valve. If suspect this to be a problem , just replace the vent valve assy.

 

Air is allowed to enter the tank to replace the volume of used fuel through a one way check valve in the "vented" fuel cap.The cap also allows a controlled release of pressure as the cap is turned to prevent fuel spillage. There is also a one way vent valve located on top of the tank to allow fuel vapors to enter the evaporative system and also prevent fuel loss in the event of a rollover. The vapors are stored in a charcoal storage cannister somewhere in the engine compartment to be burned. The cannister-purge valve solenoid is what is controlled electrically to allow the vapors to pass into the engine for burning.

I don't believe your gauge issue is related.A sticking float arm assy or a heavy float is a possibility. I can tell you from experience with my Birds and Cougars is difficulty actually getting the tank filled.I always had to go to a pump with an incline.Then around 1989-1990-97 Ford designed a strangely shaped tank making it more difficult to fill where the drive shaft actually is above the middle of the tank making it a bigger job just to change u-joints.

 

Before emissions were considered a problem, fuel tank caps were allowed to vent, but not with today's vehicles
If the cap is vented or on loosely the CE light is illuminated to notify the operator the cap is loose or a leak is present in the system. Some vehicles will even provide a message to check fuel cap to be sure it is on tight enough.

 

This link is for the un-informed on the operation of a modern gas cap. It is a calibrated device just like a cap on a radiator but with a different function. A vented which is the most common will have a hole or a series of holes acting as an orfice for the valve.Yes, a malfunctioning or loose cap will set a CEL, no denying it.Old vehicles before emissions were closely monitored had an open vent line above the tank.
Second link shows the exact type of vented cap the OP would have on his 97 Thunderbird, that is what we are talking about, right? I hope my information helps all and hope we can get back on course with this to help the OP.

https://itstillruns.com/vented-gas-c...k-5057554.html

https://www.rockauto.com/en/catalog/...+tank+cap,5900

 

Thank You for reminding me it was a 97 year vehicle.

 

Thank you for the links. I have ordered the cap and I hope that it fixes the problem I didn't get a check engine light so I'm guessing I didn't drive far enough to cause an error to be sensed. I did have a light show up a few weeks ago and was scheduled to have service so I asked the mechanic to see what the light said. When I got it back it was off but there wasn't anything on the bill about it, so I figured it was trivial. I'm guessing now that might have been the same issue and they just cleared the error. I failed to ask them since I didn't inspect the bill until I got home. I'll let you all know if that was the fix, but it's logical. I had my suspicions about venting but I didn't really know how the system worked. I think I had read about systems that returned fuel to the tank and wondered if that might be what I had. However the only systems like that i know about were diesel and I think it was to cool the injectors. I've not had much experience with modern engines. I was a gearhead in my youth, but that was 50+ years ago.
Thanks, Jim.

 

Interesting. I've owned this since it was new and do not recall ever having a problem filling the tank. I'll keep this in mind for future reference.

I had a class C motorhome on a Dodge chassis years ago that had that problem. The fill tube was almost at the same level as the top of the tank so if there was much of an incline, it wouldn't fill. I figured that out when I ran out of fuel not too long after I filled (or so I thought) the tank. I ignored the fuel gauge 'cause I knew I had filled the tank.
Thanks
Jim.

 

The new cap didn't fix it, btw.