98 windstar fuel system help

I own a 98 windstar 3.8. My gas mileage has went downhill over the past year. We did a tune up on it and also changed out the o2 sensors and tps. That did not help. We have noticed that when we take the gas cap off that there is a significant amount of pressure released. We also noticed that the fuel tank was being sucked flat, not completely but it has a pretty good bow in it. Now we unscrew the gas cap before we drive so that it doesn't build up as much pressure. The van has not thrown a code and it does not have any codes stored in the PCM. I would think that this issue should throw a code. When we purchased the van 4 years ago we could go 120 highway miles on a little more than an quarter tank of gas. Now the same trip uses almost three quarters of a tank. Any suggestions?

I think that the negative/vacuum pressure in the tank is limited by the cap itself. It includes a vacuum relief that dumps in fresh air as needed. I would replace the cap with one from Ford. The primany source of fresh air is valved thru the carbon cannister ... so something must be awry here also.

I don't know how it plays into other things ... but there is a tank pressure sensor that is part of the in-tank pump and gauge assembly.

It is curious that you have not had a MIL. I have no idea about the variance in fuel economy.

Has you checked the pressure regulator for an external leak?

Replaced the gas cap with a new gas cap from ford and still the same results. I have not found any leaks. Any more ideas would be greatly appreciated. Thanks.

I'm guessing that you don't have a leak....because if you did.....you would not hear the "whoosh" when you remove the gasoline cap.
I agree with you that if the vapor recovery system was not working, you 'should' have a Check Engine Light and a code stored that indicates the problem.
A good diagnostic at a shop might be in order.

Ok we put the van up on the ramp outside and played around a little just to see what happens. We took down the charcoal canister and its rusty but still solid. Then we unplugged the canister vent solenoid that is located on the canister and the negative pressure was released out and the gas tank resumed its normal shape. This makes the van throw a code. So we plugged it back in, cleared the code and after a minute or two the negative pressure returned and the tank sucked flat again.

My understanding of how the EVAP system works is that the fuel tank pressure sensor sends to the PCM then to the canister vent solenoid which controls when the canister is released into the manifold. So if my understanding is correct, this would leave me to believe that the fuel tank pressure sensor is not operating correctly or the evaporative emission valve is plugged. My husbank seems to believe that it is the canister vent solenoid that is not operating correctlly. Any suggestions are appreciated.

I think you are close to finding the problem. Is there any way to remove the canister to check and see if it is the problem ... perhaps plugged solid?

I have had to replace the vent solenoid on other models before.

All of the lines have been checked.

My '98 Windstar has NO pressure or vacuum relief when I remove the gas tank.

Should I hear this (like I do with my wife's Corvette?)?

(The '98 has 240,000+ miles and I've noticed gas mileage going down since the tranny rebuild about 3,000 mile ago)

Bkimbel

I have seen on my 98 Windstar that the air line on the charcoal cannister that allows air into the cannister can become plugged by dirt. This is the rubber hose that goes from the cannister to an opening very near the gas cap. If you open the fuel filler door and look inside, towards the front of the van, you will see the opening. The problem is that over the years, dirt entering that opening rolls downhill in the hose and eventually accumulates enough to block the hose. I had the whooshing sound upon gas cap removal, as well as a "farting" noise which is the gas cap vent operating to limit the vacuum in the tank. Disconnecting the hose at the cannister and blowing it out resulted in a fountain of dirt and corrected the problem.