Ok, weird thing that's been happening lately with my 89. Every time I go to unscrew the gas cap I notice a huge discharge of air/fumes from inside the gas tank. It literally blows the gas cap out of my hand. I've done some research and could really only narrow it down to a vent valve on the tank or a vented gas cap. Not sure with which I should try first. I've also looked into the vapor canisters and all the smog equipments but I was told from a fellow mechanic that I would need to worry about those if the gas cap was like a vacuum and sucking air in.


I figured I'd repost this in here since this section more reflects my series.

On an update I replaced my gas cap and still have the same problem.

I had did some research and found out the gas tanks "Rollover" through the emissions system on a purge valve on my intake manifold. At least thats what the Haynes manual says.

 

Some pressure buildup is normal, particularly during the hotter months and partially because our vintage recirculates the fuel, which causes some unnecessary heating and related vaporization of fuel. What you describe seems excessive.

Not familiar with our specific vapor recovery system (only because I haven't yet had to work on it) - but suggest you're on the right track with your thoughts - either a blockage in the line to the vapor recovery canister, a problem with the recovery canister itself, or a non-functioning vacuum valve/line which is supposed to pull the vapors into the engine for combustion.

On the canisters I am familiar with, there is a filter at the bottom which tends to foul and plug up over many years. However, a plugged filter media would seem to suggest excess vacuum would be pulled on the tank while engine running.

Essentially, the carbon in the canister is intended to absorb the tank vapors when the engine is not running - vapors that are released as pressure builds in the tank and relieved through the canister, then the canister releases these vapors as the engine pulls air through the canister.

Good luck, and keep us posted. The solution would be one to file for future reference.

 

I've seen this posted about, a few times. Nobody's posted a fix (or what was wrong). Would be nice to know. Is it a single or dual tank set up?

 

I have a similar problem. Here is the thread: https://www.f150forum.com/f10/emissi...-system-51416/

Well I failed emissions testing 2 times. The first time I took my truck I passed everything but the evaporative line test to the charcoal canister. They stick a line in my tank that pressurizes the tank and check for pressure at the charcoal canister. There was no pressure. That weekend I got under the truck and started looking around and trying to pressurize the tank. To make a long story short I found a "T" on the line that was covered with plastic to keep pressure from going thru and then I found the same thing on the the top of the front tank. I then pressurized the tank and there was flow to the canister. I then took the truck to get emissioned again. Now I failed the fuel cap pressure test. All the pressure escapes to the charcoal canister so there is no build up.

There is suppose to be a 1 way check valve on the tank so no fuel escapes if the truck is upside down. I just bought the truck so there was no mention of why the previous owners may have done this. Plus they live in a non emissions city so the didn't give a F.

Can someone confirm any of this and possibly help me find parts and or part numbers. I have all data but I've been looking everywhere for check valves and I find ****. TIA!

Sorry to reply back late but I usually use my iphone when online and typing long replys back is no fun. This law is stirring quite a contraversy. I rather not get started on it right now. This tread would turn into something else.

I've been doing alot of checking. No service writters seem to know what is right. I talked to one and they said the roll over valve is suppose to close when it gets to a certain amount of pressure but how is that suppose to happen? Pressure doesn't build that fast to close a valve. I checked my rollover valves and they're fine.

There are no leaks in my lines. I tested from my charcoal canister to the t fitting for both tanks. Pressure builds and comes back out the line because it can't go anywhere else.

So that brings me to what the second service writter said. He told me that the charcoal canister is suppose to hold the pressure. I blocked off all holes including the caps and it did hold but once I put the caps back on they didn't. So I bought a new canister and caps thinking this was the problem. I tested it and it did the same thing the old one did. The caps aren't solid so it doesn't close the hole off totally. I'm guessing incase to much pressure builds it can go somewhere. So other then that I'm stumped. I'm just thinking if I plug the holes in the charcoal canister I'll pass but I'd really like to know what I'm missing.

Do any of you have anything else between the t from the tank to the charcoal canister? Purge valve, anything? I don't and in all data it shows a purge valve but doesn't show the location. The purge solenoid is after the canister so its not the problem but regardless it stays in the closed position when off I've already checked.

So problem is after the t for the fuel tanks and before the canister. If possible can someone take a pic of their charcoal canister line. I'd like to see if theres a valve that I'm missing. Thanks everyone!

 

ok too much reading but on the top of the tank there is a vent that has a tube that goes to the front of the truck that gets cleaned then fed into the engine if thats cloged the good old chem of expanding and contrating will aply if no vent if you ever leave a gas container out you know what i mean no depending on the year i dont know if you have a presurized system or not but just make sure the vent is not cloged or the tubing cloged from carbon or some forign matter

 

You have to have a restriction. Take off the hose that t's to 1 line and the line off the canister. Try to blow air thru it and see if it's restricted. If it is then somewhere in that line it's closed up.

 

????

 

One possibility would be to unplug the hoses that comes from the tank at the charcoal canister at the front of the truck to see if pressure gets released before undoing the cap. On my '89 F150 it is just to the left of the radiator if you are facing the radiator from the front of the truck. My guess is that you won't get any flow.

I haven't investigated this, but I just worked on my tanks. On the top of the tank, there is a little valve-like part that sits in a rubber gasket. That is where the hose from the charcoal canister connects to the tank. I would guess that the purpose of that valve is to release pressure from the tank to go through the canister and on to the intake manifold or wherever the other hose goes to. If you have two tanks, there is a Y in the line so that both tanks have the same deal; that is, the lines from the tanks to the canister join and the Y and go on to the canister.

If the valve is supposed to open to relieve pressure (and I cannot imagine what else it would be doing there), then if the valve is frozen or otherwise malfunctions and the rubber gasket is any good, then a lot of pressure would build up in the tank. When you open the tank by the gas cap, it would serve the same function as the valve opening except the vapor goes into the atmosphere.

My two cents. I hope it is helpful.

 

May have been helpful four years ago.