1968 Mercruiser 120 fuel delivery mystery

[paul ages]

Ok, five bucks to the first person to solve this mystery.

The boat is an 18' 1968 Fiberform Continental with a Mercruiser 120 I/O, fuel tank in the bow. When I take off from the dock, I can run WOT for about 5-10 minutes, then the engine dies from fuel starvation. Eventually I figured out that when I disconnect the fuel line from the fuel pump on the supply side, there is a loud PSSSHH and then fuel flows again. From there on out it's progressive... I can go less and less distance between the purgings.

At first I thought I had venting issues in with the tank and a vapor lock was occurring, but the problem persists with the fuel cap removed altogether. Then I thought maybe it was the water separator/ filter and bypassed it. Then I thought maybe it was old fuel line collapsing under pressure and replaced the line. Finally I gave in and bought a new fuel pump (not really sure how it could be the problem)... but the problem is still there.

So my question is this: How does air get in the fuel line at all if it goes straight from the pickup at the bottom of the tank, into the fuel line, and to the pump? Would a crack in the pickup tube do this? It seems to me that if air was getting sucked in along with the fuel it would get progressive, as the pump would be having to suck on all that air stacked up in from of the fuel... or maybe not. I'm running out of ideas, and I really just want to go fast in my boat for more than 10 minutes.

Help, anyone?

 

[NHGuy]

Re: 1968 Mercruiser 120 fuel delivery mystery

Is the hissing pressure or vacuum? If vacuum check for an obstruction in the tank pickup or the anti siphon valve.
You could put on a small temporary tank to check, if it runs OK on a temp tank something is hinky in the built in one.
If the hissing is pressure releasing I'd be surprised.

 

[paul ages]

Re: 1968 Mercruiser 120 fuel delivery mystery

I have a fuel pressure/ vacuum gauge I've been meaning to install inline to find out, but with the new pump I can't see how it could possibly be pressure. It doesn't have an anti-siphon valve, nor a screen at the bottom of the pickup if I remember right. I've considered sediment in the tank, but I drained the whole tank into clear containers this spring (expecting to see water separation from the ethanol), but the gas was clear and clean. Sure, there could still be sediment that only gets picked up when the tank sloshes around a bit, but the timing is almost exact every time. From one ramp, I can say the EXACT place it's going to happen within a couple hundred feet.

I really suspect a crack in the pickup tube or an air leak where the fuel line connects to it. I feel like it was a bit better when I filled the tank up all the way earlier in the summer (kind of a painful endeavor). I can't see how else the air would even get in the line. I think I'll try disconnecting the fuel line and sticking it straight into a can of gas.

 

[kd7isf]

Re: 1968 Mercruiser 120 fuel delivery mystery

I know you said it doesn't have one, but it really sounds like a deteriorated antisiphon valve to me.
Are you sure you replaced the fitting that allows you to connect the rubber hose to the tank?
That fitting IS the antisiphon valve.
I'd replace it with a similar size brass fitting and see if the problem goes away -- if so, get a new antisiphon valve...

 

[paul ages]

Re: 1968 Mercruiser 120 fuel delivery mystery

Aha! I didn't know that's where the anti-siphon valve is... if I disconnect the fuel line, fuel definitely continues to siphon out freely. Should it?

 

[paul ages]

Re: 1968 Mercruiser 120 fuel delivery mystery

Ok, now I get it that the anti siphon valve prevents fuel from siphoning back into the tank for easier starting.

 

[FreeBeeTony]

Re: 1968 Mercruiser 120 fuel delivery mystery

From what I understand it actually prevents it from flowing into the bilge in the event of a break in the line........

 

[paul ages]

Re: 1968 Mercruiser 120 fuel delivery mystery

I think I found the problem... and I can't believe it has hounded me this long. I pulled the pickup from the tank and discovered that it is longer than the tank is tall. Just below the fitting, there is a kink in the tube which allows it to fit in the tank, being too long. I think the kink has been restricting the flow just enough to cause the problem when the engine is in high demand. It answers all of the questions. :


I'll cut it to the correct length and take it out soon. Fingers crossed!

 

[FreeBeeTony]

Re: 1968 Mercruiser 120 fuel delivery mystery

Sounds like you found the problem.......but I would replace the tube.