On the 1991 - 1999 Monteros, is the gas cap supposed to make a "hissing" noise when you unscrew it? I believe these caps are not vented, and require other lines and some type of confusing fume trap box to meet EPA standards. From my experience, if there is a sizeable hissing noise and air being suddenly allowed to enter the tank (when unscrewing the cap), then the venting system has a problem. The poor fuel pump will be working harder and harder as the vacuum increases when the fuel tank gets lower and lower as you drive. Although the cap says that "Loose cap may turn on check engine light", I have tried to leave it a bit loose to prevent that vacuum from building up.

In the meantime, are there any "How To" threads that addresses this issue, and what to do to solve the problem?
If not, any suggestions/comments?
Thanks.

 

Hissing sound is not normal. As fuel is consumed, the liquid level is reduced and it creates negative pressure (slight vacuum) in the tank. Most likely an issue with the gas cap. Most gas caps have one way valve built into them. This allow air to enter the tank but not to escape. Check that it is not stuck or clogged with dust. Another possibility is a line to/from charcoal canister got plugged or pinched. Start with the gas cap, its simpler

 

Hunter.......And I am not holding a Rocky Mountain Rattler in my other hand while unscrewing the cap. I could be wrong, but I thought that these caps did not allow any air back to the tank......I will check. But if no air is allowed back from the cap, then how is the vacuum pressure relieved? Would it strictly be relieved thru that carbon cannister which routes the vapors back to the engine to be burned?

I just took some pics of that wascully gas cap. Looking close up into the cap, there is a large spring and what "appears" to be a rubber center which might be designed to open and close. Seems the spring and valve would not be needed if it was designed to not allow venting. I will poke and pry at it some, maybe spray some penetrating oil up inside it, and see if that valve does open.

 

I'm pretty sure that plastic caps should allow one way air flow - in but not out.Be careful prying around so that you don't break anything you don't intend to...
Just as an example, I have an older vehicle that uses a metal cap with rubber seal. That cap is not vented and tanks "breathe" through a fluid separator and then the charcoal canister. When my fluid separator was plugged, I was getting a massive vacuum in the tanks after running in the winter, or pretty big outflow of vapors on very hot days.

 

Well, when I was looking on eBay for a replacement cap, all the ones that claimed to be "an exact match" claimed that they were not vented. But, seeing the spring and the center "flap" in the one on the Monty, Im siding with you and think that it should be a "vented" cap.

What kind of Mitsubishi do you own? I bet they used this same cap on alot of the 90s models.

 

Mine is 2001 full size. I think when they say "vented' that literally means there is a tiny hole in the cap that allows air to come in and out freely. Mitsubishi caps (as are probably all newer vehicles) are not vented as in not allow vapors to freely vent to the atmosphere. So if your tanks heats up and develops positive pressure, the vapors have to vent through charcoal canister. At the same time I'd expect that it will allow one way air to come into the tank to replace liquid gas that's been used up.

 

OK, just went out to the car to confirm my suspicion. The vent cap on my 2001 Montero is definitely one way only. You see the "device" in the center. If I put my mouth on it and suck air - it makes warbly sound and air flows (into the tank). However, if I try to blow air out - no luck - will not let pressurized fumes out of the tank. I'm sure you should have the same setup. I wonder if your cap is broken since it is missing the center portion of the valve. But then it makes me wonder why it would develop negative pressure in the tank instead of freely venting air in and out.

 

Hunter.......OK, so you put your mouth on the cap and sucked in air, and it made you feel all "warbly"......must be that super hi-test youve been running in the tank. ha ha.
Good test you did......so, the cap allows some air to enter, but prevents the fumes from escaping.
I will see if I can do a similar test with my gas cap.........will repost and let you know how "warbly" I got......and if I dont reply, that means that I got so warbly that I "kicked tha bucket". haa ha

Interesting when I compare your cap with mine.....maybe I do have a center plastic piece missing. I will check the other Monty caps also for comparison.

 

Warbly, not wobbly..... All I can say, don't smoke while you're performing this very scientific test

 

Thats a Big 10-4 from my Monty's back door.