Fuel vacuum problem

[dutchman1018]

Hi all,
First post here so go easy on me....I'm a regular on my construction one, I get the rules.

So, I have an Arrowglass Coho, 19', 100hp mercury OB, below deck aluminum tank, no fuel pump.

I've replaced every single part of the fuel lines, etc. from tank to motor with the exception of the bracket that the fuel filter is on and the fuel stem that is inside the tank....(waiting to hear back from tank manufacturer to see if they have one).

The problem I'm having is that their seems to be a lack of venting (restricted perhaps) because the primer bulb would become completely sucked flat after it ran. Today I replaced the anti-siphon valve at the tank and it cure that but still seems to run itself dry....I'm stuck on a vacuum issue but perhaps I'm wrong.
As mentioned, the only two parts not replace are the fuel stem (inside tank) and the filter bracket.
Is there something else starving the motor of fuel or am I missing something?
Any help is appreciated and I'm sure I missed important facts somewhere.

Thanks,

Bob

 

[Bifflefan]

Re: Fuel vacuum problem

Welcome to iboats.

First, there is a fuel pump, somewhere. It has to have one. But if your sucking the bulb flat, that pump is fine.
Second, there should be a vent by the tank filler or possibly a vented cap on the tank. Either way there must be a way for the tank to vent or you pull a vacum and suck the bulb flat and then it will stop running sometime.

Some where the vent is plugged or you have the wrong cap on the tank.

 

[dutchman1018]

Re: Fuel vacuum problem

Welcome to iboats.

First, there is a fuel pump, somewhere. It has to have one. But if your sucking the bulb flat, that pump is fine.
Second, there should be a vent by the tank filler or possibly a vented cap on the tank. Either way there must be a way for the tank to vent or you pull a vacum and suck the bulb flat and then it will stop running sometime.

Some where the vent is plugged or you have the wrong cap on the tank.

Thanks for the welcome!

Fuel pump must be on the motor?...fuel sucker?...

The vent line is new, the vent cap is new and all clear of obstructions.

It purrs like a kitten for a bit then dies. What I failed to note was that after I replaced the anti-siphon valve, the bulb is no longer flat, but it's still starved of fuel....maybe it's an inline filter....replaced it an hour ago,I'll fire it up and see.

 

[Bifflefan]

Re: Fuel vacuum problem

Thanks for the welcome!

It purrs like a kitten for a bit then dies. What I failed to note was that after I replaced the anti-siphon valve, the bulb is no longer flat, but it's still starved of fuel....maybe it's an inline filter....replaced it an hour ago,I'll fire it up and see.

Then another guess is the bulb is just ready to be retired, you may also have a leak in the threads at the anti-siphon valve since it doesnt flaten after you changed the valve.
If you let it run till it dies and pull off the fuel tank cap, does it suck in air?
If it does you dont have a vent and need one. Or run it without the cap in the driveway and see if it still dies.

I suppose it could be the pump getting weak. I dont know where your is, but your manual will tell you. Im fairly sure its on the motor.

 

[gss036]

Re: Fuel vacuum problem

If that did not fix you problem, then take a look at the bottom of the tank. You may have to remove the pick up tube from the tank, no big deal. Some times there is enough junk accumulated that it will clog. There may be a screen on the end of the pick up tube. You can try to blow air back through the fuel pick tube, if that fixes it for a little while, then you have found the problem. Yes, there is a fuel pump on your motor. It consists of a diaphragm and gaskets. If it can suck your primer bulb flat, then it is working. I am assuming you installed the primer bulb going the right direction, there usually is an arrow showing the correct direction for fuel flow.

 

[dutchman1018]

Re: Fuel vacuum problem

giving it a rest for the night....post more later.

Thanks already.

 

[a70eliminator]

Re: Fuel vacuum problem

Did you put a new primer bulb on with your new fuel lines, it sounds like it could be a probelm with the check within the sqeeze bulb. It seems like it would take more than just a few minutes to pull the tank and all into a vacuum, so i'm thinking it's just in the feed line.

 

[dutchman1018]

Re: Fuel vacuum problem

Sorry, posted my exit before I read the last reply.

Yes, bulb is in correct direction, fuel pick up is clean, but has a moving part that makes me suspicious....more later.

Thanks

 

[Chris1956]

Re: Fuel vacuum problem

The antisiphon valve is typically a check ball and a spring, usually located in the fitting next to the elbow on the fuel tank. It could have a screen, and it could be inside the fuel pickup tube. It is best to find it and clean or remove it for testing.

If you open the fuel tank filler cap a bit, does that fix the problem? If so, the fuel tank vent is bad. If not, it is the fuel tank pickup, or fuel line, filter etc.

 

[dutchman1018]

Re: Fuel vacuum problem

The antisiphon valve is typically a check ball and a spring, usually located in the fitting next to the elbow on the fuel tank. It could have a screen, and it could be inside the fuel pickup tube. It is best to find it and clean or remove it for testing.

If you open the fuel tank filler cap a bit, does that fix the problem? If so, the fuel tank vent is bad. If not, it is the fuel tank pickup, or fuel line, filter etc.

Anti-siphon is new and doesn't have a screen, has a tank vent and everything is new, fuel filter is new, lines are new. I replaced the inline filter last night but with all the messing around, had to put the battery on the charger.

If all of this stuff is new and I've eliminated the bulb flattening....it's something else. I'll price the filter bracket today and if reasonable, replace that and see if it cures it.

 

[NHGuy]

Re: Fuel vacuum problem

1st I'd try running til it quits and then open the fuel filler. If it gives a big sucking sound of vacuum releasing you have a continuing vent issue. Since you replaced and/or checked the vent it's unlikely, but it needs checking. And if it is a vent issue pull the vent tube off the tank inlet and check for an obstruction right at the tank vent inlet. If it runs out of gas and there's no vacuum you might have a fuel fill issue in your carb(s). They have floats that drop down which then allows the carb float chamber to fill. All it takes is a tiny little thing to screw them up.
You should also consider running it on a remote tank before you buy any more parts, that will determine if it's in the tank or toward the fuel delivery system. Remember you could have junk in the main tank which gets into the pickup when running and then falls away when left to sit.
Or if you already suspect it is that; remove the sending unit, siphon out the fuel, then inspect or clean out the tank. Once you have the fuel out you can screen it for debris. You say you changed the fuel filter, check it also for debris, It might be saving your carbs while starving them of fuel.
Good luck, you are getting there. You will figure it out!

 

[109jb]

Re: Fuel vacuum problem

how does it run with the fill cap off of the tank. If it is a venting problem that will give you a big vent.

 

[dutchman1018]

Re: Fuel vacuum problem

Just ran it again right now and it stayed steady for a while, and then the usual....died.
Could very well be the carbs....don't think it's crud in the fill stem.

I'm going to do what I advise others on my construction forum and hire a professional who knows what they are doing.

My guess is that it's something simple and I'm missing it. I'd rather pay someone a few bucks to diagnose properly so I can get out on the water safely.

I'll post the news when it's fixed to keep sharing the knowledge.

Thanks for everything thus far.

 

[dutchman1018]

Re: Fuel vacuum problem

Hey All,

Reviving this in case it helps someone else.

I'm not sure exactly what fixed the vacuum problem, but most of the diagnosis pointed at the anti siphon valve. I replaced it with a simple barbed connection at the tank and voila.

Other notables found during the issue were a cracked oil line from the oil injection tank and lastly, when I thought I had it nailed.....a leaky gasket at the fuel pump. While priming the motor with the cover off, I saw gas coming out of a corner of it and realized it was sucking air at high RPM...part of the overall issue I guess.

So there you go. Using good forum policy, post the answer so others can benefit.

On to boating and fishing!