Gas Tank Pressure

[Zmax]

Need some help with a problem. My old Mercury was running great until last weekend. Started acting like it was choking out and then dead. Had to let it set for a while and she would take off again. When I got home I fired her up for a flush and it ran great for a while before choking itself out. I discovered the tank was under a lot of pressure. That little epa cap did not let any of it out. Where I'm not sure is where does this pressure come from? If the motor is sucking fuel out of the tank how can that much pressure accumulate. I've never had that pressure build up before now.

The shop that generally works on the old girl can't get her in until early to mid May and I have a trip to Moses/Potholes later this month. Any help appreciated.

[Amx]

Depending on the brand of motor, there could be two lines between the motor and the tank. If so one is the fuel line from the tank, and one is a pressure line from the motor. Crankcase pressure from the piston going up and down pushes air pressure into the tank, that pressure then pushes the fuel from the tank to the motor. Sounds like something is clogging the fuel line from the tank to the motor. Remove the line from the motor, and from the tank. Blow air thru the fuel line and see if anything is clogging it. If so, clear/fix it. If nothing is clogging it, then the problem is in the carbs, or something on the motor, like maybe a small fuel filter. Clean what's not working.

[Onmygame]

The fuel line needs air - plain and simple, could be a clog in the hole at the knob on top the gas cap.

Also look to see if the motor is processing water properly - with a small stream coming out of the upper back of the shaft. If not, you might need to change the impeller. Could be bogging down after warming due to no water cooling the motor.

Google your problem, and you should find discussion in forums that might make troubleshooting easy.

Oh - and don't forget Youtube for helping you figure out how to fix these things.

Others will be along with more - the best of luck.

onmygame

[Toni]

How old is the tank? I am assuming it is portable. Plastic?

[Zmax]

It's a 6gal portable tank that sets down in the battery/tank compartment. I've looked it over and I have but one line going to the motor. The way it was explained to me and has worked up until now was you pump gas to the motor with the gas bulb to prime and from there it sucks fuel out of the tank. There were times when we forgot to open that vent cap and the motor created too much vacuum. opened the cap and problem went away. Put a new tank in about a year ago which solved most of the problems I had up to that point. Have done numerous searches on YouTube concerning this with no luck. Will attempt a google search.

Question, let say if the pressurization thing continues could a person just leave the gas cap loose enough to vent it off. That stupid little epa vent is doing me no good.

[Zmax]

Forgot. She's pumping water real good. Installed a new impeller this spring.

[Amx]

Ok, so your set up has an actual pump at the engine, which still most likely works on that same crank case pressure. It pulsates a diaphram that pumps the fuel up to the carbs. If there is pressure in the tank, then maybe the diaphram is split/punctured, or the heat from something is pressurizing the tank. Yes try it with the fill cap loose, or even off if you are in the driveway using a garden hose to the motor for cooling like when flushing.

[Sideburns]

The epa doesn't want any vapors (hydrocarbons) escaping. All gasoline in a sealed container (above a certain temp) will produce a pressure, ethanol does makes this vapor pressure worse. The pressurized balloon shaped gas cans scare the heck out me, and I'm sure that adding fuel pressure to the fuel pump's designed outlet fuel pressure could cause flooding.

Venting by loosening the cap will cause some spillage, but otherwise should correct fuel pressure. I would recommend running ethanol free fuel though, because ethanol is a water magnet.....

If your problem is indeed excessive fuel pressure, there are small adjustable inline fuel pressure regulators available. Might be a permanent solution.

[Sideburns]

BTW, most of the fuel symptoms similar to yours turn out to be restrictions or blockages in the fuel system, in my automotive experience.

[TrackerPro16]

If it runs fine with the cap loose or off I am going with either a plugged vent hole or corroded and stuck vent in the cap. Had the same thing on one of mine. For a temp cure I put a nick across the gasket so it would vent, then got a new cap at my convenience. IF you have one line going to the engine the tank is building pressure up on it's own from sloshing fuel and or warming up.
BUT, usually you would not get that much pressure at home in a barrel so I would ask, are you SURE it is pressure and not vacuum? Also are you also SURE it a 'single' line going to the engine and not one line with TWO ports? You said "old" so I presume it has a carburetor. If that is the case the float needle SHOULD hold against extra pressure, but 'could' get lifted if pressure got too high. Seems odd but it can happen.
Give a little more info on the engine if you could. 2 cycle, 4 cycle, HP. Depending on where you are I may be able to come over a take a look.

[Toni]

For those who don't have newer portable tanks. They do not vent. Therefor they can't have plugged vents.

[Sideburns]

But, ... There's this one way v__t valve that... nevermind, I dont think its wise to argue with Toni!

Lets call it a built-in, one way air inlet valve in the cap. If it plugs, the tank will have a suction on it, like you experienced with your old tank before you cracked the bleeder.

A good test would be to reproduce your symptom, let it die, and immediately crack the float bowl drain to see if you have fuel in the carb.

Another good trick is to shoot a shot of starting fluid in the air inlet when the symptom is present, and see if it makes it run better or worse. If it takes off, you've just diagnosed a fuel starvation problem.

[Bodofish]

Since it only has one line going to it, it's not a closed system. So I'm going to have to say it's a problem with the fuel pump. Stuck check valves making it pump both ways. Put in a couple of strategically placed check valves in the squeeze bulb and the next thing you know, you have a pressurized fuel tank. A good fuel bulb squeeze will give any OB enough gas to run for a while, well long enough to puff up the gas tank.

[hewesfisher]

I'd try running it with the fuel cap off or at least loose enough to let air in. If it runs fine, problem identified. Your "old Mercury" was not designed to run from a pressurized tank or one under vacuum. If your tank comes under vacuum, it may be sufficient the pulse pump cannot overcome it, conversely, if your tank is pressurized, it may be sufficient to force the needle off the seat/s and force fuel into the bowl.

Newer, EPA mandated tanks are not always the best when used with older engines. Might help if we knew a little more about the old Merc, i.e., year, hp, 2 or 4 stroke...

[Zmax]

I did take her out for a test run. Once she started choking out I loosened the cap reliving the pressure. She fired right back up and had no further problems. Will just have to fish with a loose cap until my appointment with the boat doctor. Thanks for all the ideas, gave me a few more angles to look at this thing from.

[Amx]

So the tank vent is clogged. Or the wrong cap, or the wrong tank for that motor. If it was running fine before, and 'just' started happening, then the vent is clogged/dirty, or something.

[hewesfisher]

I did take her out for a test run. Once she started choking out I loosened the cap reliving the pressure. She fired right back up and had no further problems. Will just have to fish with a loose cap until my appointment with the boat doctor. Thanks for all the ideas, gave me a few more angles to look at this thing from.

You're welcome. Your tank is not under pressure, it's under vacuum, and fuel flow stops once tank vacuum exceeds the pulse pump's ability to pull fuel from the tank. Got to have a functional vent.

[Bodofish]

I did take her out for a test run. Once she started choking out I loosened the cap reliving the pressure. She fired right back up and had no further problems. Will just have to fish with a loose cap until my appointment with the boat doctor. Thanks for all the ideas, gave me a few more angles to look at this thing from.

You're welcome. Your tank is not under pressure, it's under vacuum, and fuel flow stops once tank vacuum exceeds the pulse pump's ability to pull fuel from the tank. Got to have a functional vent.

Yep, Easy mistake big difference Pressure or vacuum...........