[marcberoz]

I have a 4 stroke, 6hp Nissan outboard that stalls out at low RPMs. The cause is an air bubble that grows within the fuel filter. When the bubble gets so large that it only touches the bottom of the filter element, I stall out. At higher RPMs, I also get a bubble in the fuel filter, but it never gets large enough to stall me out. The bubble never goes away. I can squeeze the bulb endlessly and get the bubble to shrink until there is only a sliver of air in the filter. As soon as I quit pumping the bulb, the bubble begins to very slowly grow again. I replaced all the spring clamps with screw clamps in the engine. I've used thread tape where the fuel line screws into its fitting on the the tank. I've also added an additional screw clamp to all the connections on the supply line between the tank and outboard itself. What I have not replaced is the barbed female connector on the engine. Before I go ahead and do this (which seems somewhat complicated due to how the connector is mounted on the engine), am I overlooking something? I also don't understand why the engine doesn't quit as soon as the air bubble touches any part of the filter element. Can anyone educate me about what is going on?
Marc

 

[RichH]

Suggest you inspect the VENT on either the integral fuel tank (if so included) or on the remote tank. Many of the new remote tanks have self closing vents which 'supposedly' open to allow air into the tank but do get stuck and then develop HIGH vacuum in the tank. When the tank is a vacuum it is very easy to suck air through many of the intervening connector in the line between the tank and OB. Solution: poke a needle sized hole in the remote tanks 'valve' or remove that damn air valve ... or simply leave the tank cap loose.

Note: if you leave the tank hose connection in place with the OB not working and the tank gets hot, the developed pressure in the tank will FLOOD the carburetor .... get rid of that valve.

All this is California 'CARB' compliant ... and really does NOT work in 'real life'.

 

[ReefMagnet]

Air can be sucked in past the the clamps or crimps on the fuel hoses. The other suspects are the quick connect fitting seals. Had it happen to me. The seal won't let fuel leak but allows air to be drawn into the fuel line. Air won't be too much of a problem in the filter until it is drawn into the outgoing fuel line because the pump is after the filter.

I'd try a new fuel line assembly between the tank and engine if all else fails. Then I'd carefully inspect and swap out suspect hoses.

 

[marcberoz]

Rich:
I'll check the tank vent but I don't suspect it is the problem. My external tank has a knob on the top of the filler cap and I believe that when loosened, it directly allows air to enter the tank - nothing fancy at all. I think my tank is about 15 years old.

ReefMagnet:
It is the quick connect fitting at the outboard end of the fuel supply hose that I am concerned about. Is there any way to renew the seal of this fitting without actually replacing the fitting? Again, I suspect that the quick connect fitting on the outboard will be a PITA to replace.

 

[capta]

Thread tape is unacceptable on fuel lines, ESPECIALLY gas. Go to the auto parts store and pick up a little bottle of Indian Head shellac. Use that at any connection point you think may be leaking, though it is really designed for threaded fittings, not hose barbs. If this doesn't solve you problem, just get a new fuel hose as the hose itself may have a pin hole. Even the fuel filter itself could be the source of the air; you've just got to keep trying things until you solve the problem.

 

[RichH]

ReefMagnet's suggestion is equally valid. Ive replaced my leaky 'quick connect' after about 4 years. You can buy these from any online Tohatsu site.

 

[TakeFive]

Where is the fuel filter where the air bubble appears? If it's in the fuel line, then the air leak would be upstream of it - not at the entrance to the motor.

I spent 5 years trying to fix a air leak on my 50 HP motorboat. Every time I went to full throttle, I'd lose power and could see air sucking through the glass filter enclosure. I only use that boat on vacation once a year, so each year I'd try something else. I replaced all the quick connects, primer bulb, hose nipples, etc. It turned out to be either a little crack in my fuel line by the disconnect at the tank, or the little plastic tube that goes from the top of the plastic threaded part (where the disconnect nipple screws in) down to the bottom of the tank. The latter part was just friction fit, and I sealed it with some Permatex gasoline-proof gasket sealant (brown colored stuff). I fixed both things at the same time (during the fifth year), so I don't know which was the culprit. But it's worked like a charm since.

Like others said, you'll have to just replace things one at a time. But don't overlook the things that fixed my motor.

PS - While I was doing all this I converted the quick connect fittings to Honda design, so my larger gas tank and hoses could be moved to my C250 if I need more fuel capacity. That's come in handy a couple of times on cruises.

 

[marcberoz]

Capta:
I'll take off the tape & get some shellac. I guess the gas just dissolves the tape?
I replaced the fuel lines on either side of the bulb. The next thing I'm going to do is replace the quick connection at the outboard end of the fuel supply hose. I'll replace the priming bulb if that attempt doesn't work.

TakeFive:
The fuel filter is located under the outboard's cover. I replaced the fuel lines in the motor 2 years ago after the problem 1st began. I've also changed fuel tanks in that time. Maybe there is a crack in the quick connect mounted on the motor. I'll check.

 

[MarkSF]

Note: if you leave the tank hose connection in place with the OB not working and the tank gets hot, the developed pressure in the tank will FLOOD the carburetor .... get rid of that valve.

The new tank should have a valve which limits pressure in the tank to about 5 psi. This shouldn't cause flooding, as it's less than the priming bulb can generate. Maybe the float valve is leaky?

 

[barefootnavigator]

It may be your fuel pump, put the tank at the same level as the engine and see what happens.

 

[marcberoz]

Update on today's effort.
Tried to purchase a quick connector for the outboard end of the fuel supply hose at West Marine but they did not have anything that would work on a Nissan. While at the boat, I saw there was a little play between the 2 parts of the quick connector located at the gas tank. Ran the motor while pressing the 2 parts of this connection together with my hands. Seemed like the motor was running longer before it stalled out. Hope that is the problem. I'll replace these connectors in the next few days & let you know how it turn out.
Thanks for your help.
Marc

 

[marcberoz]

I've made progress. I replaced the fuel hose, bulb and end connectors. That didn't work so I replaced the fuel line in the engine from the fuel hose connector to the fuel filter. Now I can idle without stalling but the fuel filter does not stay full. In fact, there is so little fuel in the filter, I don't understand how the motor keeps running. In spite of this, the motor will idle for 15 minutes or more. Don't know why, but at least the motor keeps running.

 

[Flybyknight]

Air can be sucked in past the the clamps or crimps on the fuel hoses. The other suspects are the quick connect fitting seals. Had it happen to me. The seal won't let fuel leak but allows air to be drawn into the fuel line. Air won't be too much of a problem in the filter until it is drawn into the outgoing fuel line because the pump is after the filter.

I'd try a new fuel line assembly between the tank and engine if all else fails. Then I'd carefully inspect and swap out suspect hoses.

He's correct, that happened to me with a BRAND NEW Johnson quick connect fitting.
Richard

 

[Flybyknight]

Thread tape is unacceptable on fuel lines, ESPECIALLY gas. Go to the auto parts store and pick up a little bottle of Indian Head shellac. Use that at any connection point you think may be leaking, though it is really designed for threaded fittings, not hose barbs. If this doesn't solve you problem, just get a new fuel hose as the hose itself may have a pin hole. Even the fuel filter itself could be the source of the air; you've just got to keep trying things until you solve the problem.

That's true! You need a pipe thread compound there, not tape.
Richard

 

[marcberoz]

Re: 6hp Nissan outboard leaks air into fuel line - End of the story

Finally got the thing fixed. The problem was the male connector on the fuel line at the engine. Replaced it and now the motor works fine. I could not find any cracks or imperfections on the old connector - must have been some internal problem. So total cost of repair was about $90 for parts I didn't need and about $28 for the part I did need. Not complaining though. It's nice to have a motor I can depend on. Didn't realize how uptight I had been getting as I approached my mooring ball and had to throttle down. Thanks for all your help.
Marc

 

[socal c25]

$90. + $28. is the price you paid for a sound piece of mind when it comes to your fuel system. Now that is complete you can focus on something else, sailing perhaps?