Mercruiser 8.1 engines losing fuel at crusing speed

[Menace53243]

I have a Bertram 33 repowered with Mercruiser 8.1. Fuel injected. The boat will start absolutely perfect, run fine and not miss a beat until 20 minutes of run time and then an engine will then starve for fuel. It will start slowly and then continue where the engine will then not run over 1700 RPM. The engine will run all day long at 1700 without missing a beat. Occasionaly I can bring back up to speed and it will run for a longer period of time. If I let the boat sit for some time, it will run perfectly until again 20 minutes into run time. I have the following done to the boat. Two marinas have worked on it as well. Scheudled now to check codes.

1. Fuel filters changed after only a few hours of running. dumped into glass jar and all is clear.
2. New fuel lines from pick up to fuel separator.
3. Fuel pick up has been taken out several times, cleaned clear.
4. Vent appears to be clean.

Thank you for reading.

 

[alldodge]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

I have a Bertram 33 repowered with Mercruiser 8.1. Fuel injected. The boat will start absolutely perfect, run fine and not miss a beat until 20 minutes of run time and then an engine will then starve for fuel. It will start slowly and then continue where the engine will then not run over 1700 RPM. The engine will run all day long at 1700 without missing a beat. Occasionaly I can bring back up to speed and it will run for a longer period of time. If I let the boat sit for some time, it will run perfectly until again 20 minutes into run time. I have the following done to the boat. Two marinas have worked on it as well. Scheudled now to check codes.

1. Fuel filters changed after only a few hours of running. dumped into glass jar and all is clear.
2. New fuel lines from pick up to fuel separator.
3. Fuel pick up has been taken out several times, cleaned clear.
4. Vent appears to be clean.

Thank you for reading.

:welcome: to iboats

Has anyone disconnected your knock sensor on the side of the block? If they haven't give that a try the next time it happens. If it comes back and performs as before get your timing checked. Also pull the plugs and above all else if it does solve the problem don't leave the sensor unplugged. If the motor finds a knock it will retard the timing to protect it. Everytime the engine knocks a piece of your engine is going out the exhaust.

 

[Menace53243]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

:welcome: to iboats

Has anyone disconnected your knock sensor on the side of the block? If they haven't give that a try the next time it happens. If it comes back and performs as before get your timing checked. Also pull the plugs and above all else if it does solve the problem don't leave the sensor unplugged. If the motor finds a knock it will retard the timing to protect it. Everytime the engine knocks a piece of your engine is going out the exhaust.

Thank you for the help ! I will certainly check this.

 

[alldodge]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

By the way, how do you know it's starving for fuel?

 

[Menace53243]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

Thank you for the help ! I will certainly check this.

Well, that is a good point and I don't know that its fuel. What it sounds and acts like sure represents fuel. But I am definitely open to any ideas. I haven't solved it yet so it certainly is still a mystery to me. Again, thank you for response.

 

[alldodge]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

Well, that is a good point and I don't know that its fuel. What it sounds and acts like sure represents fuel. But I am definitely open to any ideas. I haven't solved it yet so it certainly is still a mystery to me. Again, thank you for response.

Are you feathering the throttle or is it full forward and still no speed?

 

[Menace53243]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

The problem is at cruising speed. 3200. It will run for 20 min at that RPM and then one motor will die out. it is not a particular engine but random. It will switch between port and starboard. One engine will die out, not stall but die out, surge again back to crusing speed then die out again.

 

[MikDee]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

I have a Bertram 33 repowered with Mercruiser 8.1. Fuel injected. The boat will start absolutely perfect, run fine and not miss a beat until 20 minutes of run time and then an engine will then starve for fuel. It will start slowly and then continue where the engine will then not run over 1700 RPM. The engine will run all day long at 1700 without missing a beat. Occasionaly I can bring back up to speed and it will run for a longer period of time. If I let the boat sit for some time, it will run perfectly until again 20 minutes into run time. I have the following done to the boat. Two marinas have worked on it as well. Scheudled now to check codes.

1. Fuel filters changed after only a few hours of running. dumped into glass jar and all is clear.
2. New fuel lines from pick up to fuel separator.
3. Fuel pick up has been taken out several times, cleaned clear.
4. Vent appears to be clean.

Thank you for reading.

Has the fuel pickup screen in the tank been checked? and has the the anti-siphon valves (one way valves) off the fuel tank operation been tested?

 

[UncleWillie]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

The problem is at cruising speed. 3200. It will run for 20 min at that RPM and then one motor will die out. it is not a particular engine but random. It will switch between port and starboard. One engine will die out, not stall but die out, surge again back to cruising speed then die out again.

Do you have One fuel tank or Two?
If One, One fuel Pickup or Two?
To confirm the vent, Open the Fill Port for a second and see if you get another 20 minutes.

 

[alldodge]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

The problem is at cruising speed. 3200. It will run for 20 min at that RPM and then one motor will die out. it is not a particular engine but random. It will switch between port and starboard. One engine will die out, not stall but die out, surge again back to crusing speed then die out again.

Do you have an engine synchronizer ?

 

[Menace53243]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

No engine Syncronizer.

 

[alldodge]

Re: Mercruiser 8.1 engines losing fuel at crusing speed

No engine Syncronizer.

Then don't bother with the knock sensor since we now know you have twins. You have a fuel issue, check what UncleWillie mentioned and let us know