5.7l mercruiser engine bogging down..

[21seapro]

I have a 2000 254 Larson cabrio with a 5.7L mercruiser with a 2 barrel carb. The problem I'm having is ill be on the boat cruising at 3500rpm and all of a sudden the engine will fall flat on its face and I would have to cruise the rest of the way home in idle. (1500rpm). I found out if I remove a fuel/water sep and reinstall it the boat will run great again for only about 1-3 min if that. I repeat the process again it goes again for another 1-3min. HElP please.


before the boat went into the water I replaced fuel pump, fuel/water separator, water pump, rebuilt carburetor, drained 5 gallons of fuel from the bottom of the tank, plugs, wires, cap, rottor; and Bravo 3 outdrive.

when I had the problem for the first time I replaced fuel water separator, drained more fuel from tank, and fuel hose. And i still have the problem. When the boat sits overnight I could run the boat at idle for hours when its running at 3500 rpm it will run great for about 20 minutes and that's when the problem starts.

 

[UncleWillie]

Re: 5.7l mercruiser engine bogging down..

Sounds like a Fuel Tank VENT problem.
When you open the fuel line it draws air into the tank.
When running the fuel can't leave the tank unless some air come in to replace it.
If you open the Fuel Filler CAP do you get the same results?

 

[alldodge]

Re: 5.7l mercruiser engine bogging down..

Could be your gas tank vent or cap and your creating a vacuum in the tank but this usually takes a bit longer then 1-3 minutes. But when you fist start out it takes 20 minutes which sounds closer. When it does it again loosen the gas cap and see if the problem goes away.

Could also be the pick up tube is clogging up with junk on the bottom of the tank, or some other kind of restriction. When your under load you need a lot more fuel then idelling or sitting in neutral. Try blowing air (low pressure) 5-10 psi back through the line to the tank.

 

[21seapro]

Re: 5.7l mercruiser engine bogging down..

I cleaned up the pick up to before but when in the water.
when I'm on the way now released a few cap when I have the problem the problem still cars. I disconnected the fuel vent to the tank and blew through a rubber hose and it air just came out the other end of the tank. Is that normal? I'm going to try putting eventos on it and just holding it up on the way and see what happens then.

 

[alldodge]

Re: 5.7l mercruiser engine bogging down..

Its normal for air to to come out the fill neck for the vent hose. The vent hose just allows air to enter the tank as fuel is used. Did you tryto blow air through the fuel line back to the tank? Maybe there is a restricton in the lick up.

 

[21seapro]

Re: 5.7l mercruiser engine bogging down..

this is where I am. Best bank I completely bypassed the vent holes and just went to get out into the open app by replacing the tube to the vet.the bullring great forget about 20 minutes and then the problem was still there. I started to drain some more fuel for my tank and saw that it was very watery so I'm going to start by draining the complete tank and putting new fresh fuel in it.

the other question I have is if it was bad fuel why would it take 20 minutes for to happen?

 

[21seapro]

Re: 5.7l mercruiser engine bogging down..

so I drink water fuel out of the tank. Fuel definitely had a lot of water in it. I put 20 gallons of for fresh fuel93 octane. I took it out and still have the same problem. When I went to check the fuel filter I noticed that the fuel tank was trying to suck in air. so released the fuel/water sep and put it back on and the boat ran great again. When I got back to the dock I removed event from the tank to filler neck and found it clear. I also removed the 90 degree elbow that the vent go to get screwed into the tank also clear.what I don't understand is where does it go into if it goes to fill neck and the cap is not a vented cap. HELP ME PLEASE....

 

[alldodge]

Re: 5.7l mercruiser engine bogging down..

I'm not fully understanding what your saying is the fuel tank had a lot of water in it still. So you drained the entire tank and add 20 gallons back. From here on I'm going to go from what you said to what I think you meant.

(You said) When I went to check the fuel filter I noticed that the fuel tank was trying to suck in air
(Me) How did you determine it was trying to suck in air?

(You) So released the fuel/water sep and put it back on and the boat ran great again
(Me) You took the fuel/water sep off and what happened? Did you fill it back up with fuel, empty it out, what happened?

(You) When I got back to the dock I removed vent from tank to filler neck and found it clear. I also removed the 90 degree elbow that the vent go to get screwed into the tank also clear.
(Me) I take it you removed the vent hose coming from the gas filler hose and the gas filler hose going back to the tank and it was clear, Is this correct or did you mean something else?

(You) What I don't understand is where does it go into if it goes to fill neck and the cap is not a vented cap
(Me) I think your asking how is the tank vented if the cap is not vented. There is another small hose either coming from the tank or tee'd off from the main gas tank fill line to a small vent port mounted near the gas cap. The vent can be chrome or plastic, and and can be round, square or other shape.

 

[NHGuy]

Re: 5.7l mercruiser engine bogging down..

Partially open the gas cap when the problem occurs. If you hear a sucking sound you have a restricted tank vent. Then it will run well for a time til the vacuum is too great for the pump to overcome.
BUT, you also have water in the carburetor float chamber. Or a dirty carb. Or all 3.
You are smart to watch the fuel water separator. Get 3 or 4 cartridges, don't reuse them. And check their contents each time you change one. Pour the contents in a clear glass jar and watch for water, it will sit on the bottom if there is any. Keep monitoring the fuel til it's clear and pure.

 

[cantaris]

Re: 5.7l mercruiser engine bogging down..

Is it possible that you have fuel pump lines reversed? Another possibility for water in tank is if tank is corroded on bottom then any water in bilge could find its way into the tank. Or fuel vents getting water sprayed into them.