Pressure in the outboard tank causing issues?

[ShawnL]

I have an older Mercury 9.9 (2-stroke), which I had worked on when I bought it (with the boat). After having it serviced, it's always been great. Until this weekend. Started it, let it idle for a couple of minutes and then dropped our mooring. As soon as it had any throttle it would stall. Ended up having to drop anchor in our mooring field so we wouldn't end up on-shore or hitting another boat. One thing I noticed is that the tank (slightly old premium gas) under quite a bit of pressure. I took the cap off the external tank and it let out a lot of pressure.

It was a lot warmer than normal here this week (90 deg instead of 70s). I'm wondering if all the tank pressure was the culprit. Normally we attach the line to the motor, pump the bulb until it's firm, hit the choke and fire up the engine. It always starts on the 1st or 2nd pull. Then take the choke off and let it idle while we're setting up. We may have rushed things a little this time, not letting it idle for more that 2-3 minutes, but that's the only different thing, other than the pressure in the tank.

Once I got it restarted (~ 10 pulls and messing with the choke) it seemed fine. We were able to slowly motor around the mooring field for a while until we felt confident it was running ok. Then went out for a sail. When we're day sailing we leave the external tank attached and just use the kill switch when the sails are set. When it was time to come in, it started on the 2nd pull. Normally it's the 1st.

I'm not sure what the cause of the initial failure was -- wondering if it was the fact that the tank had a lot of pressure in it and was sending too much fuel to the engine, flooding it. I'd like to head it off though -- we're on a mooring and can't sail into or out of it. If I need to service the engine, I'd need to row out and remove the motor. Then again, it could be the fuel. It's from the fall. Though it was premium, non-ethanol and had some sta bil in it, which has always worked for me in the past.

 

[Don Guillette]

Start with the simplest first - dump the old gas and fill with new premium. When the tank is empty check the screen and see if there is any crud in the tank bottom. While you're at it pull the spark plug and clean or replace.

 

[Project_Mayhem]

Are you using ethanol fuel from a gas station or ethanol free gas from a marina? Ethanol is terrible for your motor and using higher octane than necessary can lead to carbon deposits.

Any sealed container, especially with liquid in it will expand and contract with temperature. On a hot day, my gas jugs will look like a roundish balloon. Nothing to be concerned about.

The first thing I suspect is that your motor wasnt getting fuel. I'm guessing some air got into the fuel line if this is the case. Many things can cause the symptoms you described.

 

[Timm R Oday25]

Your 6 gallon tank ,if it's a standard outboard tank should have a vent built into the cap . Our caps can be turned to vent or closed.
The fuel pump on your outboard and the float in your carb determine if the engine will be flooded .
We have a red plastic fuel tank that had the vent plugged . I went to remove the filler cap and was instantly sprayed in the face and chest with gasoline vapers !! Not a good experiance .. Next time .. if there is any stalling out try making sure the vent is open and the quick disconnect fuel fitting is fully seated .

 

[ShawnL]

It's an old tank. No vent. I'll make sure to crack the fill tube and let out any pressure next weekend when we go out and see how it works.

 

[thinwater]

a. If the pressure pushed gas past the needle valve, the needle valve is shot. It should hold 30-40 psi easy. But I do not think this was the problem. You would have had a LOT more trouble starting and there would have been gas seeping out of the overflowing carb. Been there, done that. I'm pretty sure you just mucked up the choke settings some how. It happens to all of us occasionally. Really. No big deal. Unless the problem recurs I would think no more about it.
b. You can install a demand valve on the tank (see below). These are required in new installations. A simple cut-off valve also works. But not the problem.
c. I doubt the gas is a problem, but leaving gas over the winter is poor practice. Pour it into a car.
d. There is nothing terribly wrong with e10. It works in millions of cars and I have used nothing else in outboards for 30 years.
1. Keep it dry. the tank should be closed and kept out of the sun when not in use.
2. use a good anti-corrosion additive. Many are snake oil. Stabil 360 marine, Merc Store-n-Start, and Biobor EB are proven in independant testing for ethanol corrosion.
Demand Valve

The OP said "No vent. I'll make sure to crack the fill tube and let out any pressure next weekend when we go out and see how it works."

Obviously, portable tanks are ALWAYS vented in use. Maybe you misspoke. If the tanks build vacuum that will make the motor cut out and not restart.

 

[Davidasailor26]

Normally we attach the line to the motor, pump the bulb until it's firm,

Are you still able to pump the primer until it’s firm? If so the needle valve in the carb is working. That needle would also prevent any excess gas from building up in the carb. Even if the tank is under pressure it wouldn’t be as much as the primer bulb could make.

 

[jibes138]

Check two things. This weekend they both failed on my little 20hp outboard skiff.
1. Pull the pickup tube out and see if the pickup tube is shot. Mine was cracked and leaking air.
2. Check your primer bulb. Mine had a split in it and also leaking air into the fuel.
Lastly I just found out Yamaha recommends replacing fuel pump every three years (another source told me 3-5 years). I guess I’ll buy one, it’s been about 15 years. Maybe the Merc has similar needs for fuel pumps.
The pickup tube in my fuel tank looked like tygon tubing, clear plastic tube. West marine brand 3 gallon tank, certainly not premium fuel line tubing. This is an unseen item so unless you pull the fitting you’d never know it was shot. Probably a good annual inspection item.
New tanks cost a fortune now, I tried searching for a pickup tube replacement assembly, no luck. Good fuel line attached to the old fitting won’t fit through the hole in the tank. I’ll try to open up the hole without wrecking the threads.
I also found a filter screen plug in the pickup hose. If that gets clogged you could be restricting fuel flow. I have a canister water separating filter on the boat so I disposed of the filter plug.
Good luck with your fix!

 

[Project_Mayhem]

d. There is nothing terribly wrong with e10. It works in millions of cars and I have used nothing else in outboards for 30 years.

Are you saying it's safe to use fuel with up to 10% Ethanol added?

 

[Timm R Oday25]

E10 is safe to use in 2 stroke motors . If used in a timely manner . The problem starts when the gas sits for a period of time . Then a process occurs which is known as "phase separation. " . The alcohol ( the ethanol ) starts to absorb moisture from the air . When there is enough ,the ethanol being denser than the gas molecules separates and sinks to the bottom of the tank . The pickup tube for every tank is at the bottom . The engine demands fuel and sucks up this water and alcohol . The engine of course stops running .

 

[thinwater]

E10 is safe to use in 2 stroke motors . If used in a timely manner . The problem starts when the gas sits for a period of time . Then a process occurs which is known as "phase separation. " . The alcohol ( the ethanol ) starts to absorb moisture from the air . When there is enough ,the ethanol being denser than the gas molecules separates and sinks to the bottom of the tank . The pickup tube for every tank is at the bottom . The engine demands fuel and sucks up this water and alcohol . The engine of course stops running .

Close. Once the water concentration tops about 0.5% (depends on temperature) The ethanol/water mixtures forms a separate, denser phase that is not gasoline soluble. The big difference is that the water renders the ethanol insoluble in gasoline. Ethanol must be at least 96% to be soluble and about 98% to be reliably soluble.

EPA. Memorandum re. phase separation, 1995

 

[east coaster]

I draw your attention to US Boat advice of 22 May 2023 regarding the use of fuel with ethanol in all recreational vehicles.

 

[Project_Mayhem]

So when does corrosion and hose separation due to ethanol occur?

 

[ShawnL]

Just an update. Went out yesterday and filled the tank with fresh gas. It looks like my tank is so old that it doesn't have a vent, at least that I can see. Attached the hose to the outboard, pumped it up until it was firm, and it started on the 3rd pull (almost started on the 2nd). We let it warm up for ~ 5 minutes and we were off. A couple of hours later, there was a front moving through, winds were picking up and a couple of larger sail boats started heading in. A quick look at the weather radar showed a thunderstorm approaching, so we dropped the sails fired up the motor (2nd pull) and headed in at 5 knots.

Not a shred of difficulty with the motor this trip. So. it looks like it was just a fluke -- older gas, not enough time to warm up, might have flooded it trying to re-start in a hurry, etc.

 

[JoeWhite]

Shawn, to be sure, pull the spark plugs, clean them and install them. They may have collected carbon deposit due to unburned fuel.

 

[Timm R Oday25]

Shawn thanks for taking the time to let us know how things turned out .

 

[thinwater]

So when does corrosion and hose separation due to ethanol occur?

Corrosion. Ethanol draws water if the cap or vent is open. Sometimes a smidge of salt sneaks in. The dissolved water is enough to create a weak electrolyte, and with aluminum and brass connected, galvanic corrosion starts. The aluminum corrosion products (white) clog the jets. The solutions are keep the gas dry (close the cap and vent) and using an anti-corrosion additive, such as Biobor EB.

Hose separation. Some pre-ethanol hoses are not compatible with ethanol. It's probably just old. Newer gasoline hose should be fine, but sometimes hoses fail because they are low quality, and the flexing each time you tip the engine does not help. Does the hose have to make repeated sharp bends? (note: there are special requirements for hoses in enclosed spaces).