Yanmar Engine runs 4h and revs drop and then stops

[zeseb]

My Yanmar 4JH4-TE runs just fine during 4 hours at sea but after it starts to slow down then stops.
Temperature and pressure are fine. After 30 minutes, I can start the engine without any problem.
I changed the fuel filters and still doing it.
The tank feeds the fuel by gravity (Day Tank) and it looks clean fuel.
I noticed the following:
1- The fuel return line is connected to the CAV filter not to the tank, hence the temperature of the fuel in the filter is very hot after 4h
2- The breather line on the day tank was full of fuel and some fuel was getting out of it when pumping some fuel from the main tanks up to the day tank
3- The breather line is also connected to the pipe filing the day-tank like a T-connector.
Is this setup wrong?

[Lepke]

One point of a return line is to circulate and pre heat the fuel in the tank and also run it thru the filters multiple times. That helps keep various fuel components properly mixed and allows the filters to remove more water if present.
The fuel transfer pump should either fill the day tank thru it's own tank fitting or be taped into the main fill point. The transfer pump going into the breather tube doesn't allow air an easy escape out of the tank when filling. It could pressurize the tank and cause leaks elsewhere. Most likely out the breather port.
The high temps could be expanding fittings and hose connections, allowing the pump to draw air. Fuel is also supposed to cool the the injectors.

[undercutter]

It is imperative that there be no restrictions on the return line from the engine. This should in no way be fed through any filtration system.

Every fuel tank should have its own vent to atmosphere to avoid pressurizing the tank(s).

How long have you owned the boat? Has this problem just started or has it been occurring since you acquired her?

[RaymondR]

The facts that:

the engine runs OK for about 4 hours before shutting down,

the fuel gets hot,

it starts again after about a 1/2 hour shutdown,

tends to suggest that the fuel is getting hot enough to flash off vapors which prevent liquid fuel getting to the injector pump. After a cooling off period the pump can again get suction and the engine run.

Post a diagram of the system and we may be able to suggest a solution.

[zeseb]

I bought it like that around 1 month ago. And I don't think this a new setup. The engine is around 3000hrs now.
So it's impossible to know if that's a new problem or not. At least, the former owner didn't mention it.

[sailormed]

Originally Posted by zeseb

............. At least, the former owner didn't mention it.

1. He is honest and never used the engine for 4 hours.
2. He is not honest and knew abou it.
In any case there is nothing to lose by asking him.

[paulajayne]

Check the fuel tank vent.
Is it clear?
If not tank is running into vacuum and stopping fuel getting to motor.

[undercutter]

Originally Posted by RaymondR

The facts that:

the engine runs OK for about 4 hours before shutting down,

the fuel gets hot,

it starts again after about a 1/2 hour shutdown,

tends to suggest that the fuel is getting hot enough to flash off vapors which prevent liquid fuel getting to the injector pump. After a cooling off period the pump can again get suction and the engine run.

Post a diagram of the system and we may be able to suggest a solution.

Or a restriction on the return side. I agree, a diagram is required.

The OP could also try running out of a five gallon pail with both the pickup and return lines running into the pail adding fuel if required. This would eliminate a fuel tank feed or return problem.

[zeseb]



[zeseb]

Well, I've just posted a diagram showing the Main tank, the pump bringing the fuel from the main tank to the day-tank, the pre-filter (CAV)
(After I have no idea what it is between the pre-filter and the injectors (No real knowledge with diesel engines, and my boat is stuck far from my home) but the fuel falls by gravity into the pre-filter which gets hot as told), then the injectors with the return line back to the pre-filter.
You will notice the T connectors on the Day-Tank which has the breather line and the fuel feed from the main tank.

The pre-filter is something like https://www.ssldieselparts.co.uk/ima...mbly%20004.JPG

On the top, there are 4 connections, 3 are used;
2 inlets; return line + fuel from the day-tank
1 outlet; to feed the engine

[zeseb]

As you can see on the "Diagram", I drew (Well, I am not a Diesel expert to be honest), you can see:
1- The Main Tank at the bottom of the Hull (250L)
2- The Pump bringing up the fuel from the main tank to the Day-tank (70L) via a T-connector which is also connected to the Breather line (A plastic pipe)
3- The Pre-Filter (https://www.ssldieselparts.co.uk/ima...mbly%20004.JPG) which has 4 connectors and 3 are used:
3a- 2 inlets: The fuel from the Day-Tank falling by gravity and the fuel from the return line coming from the injectors
3b- 1 outlet: The fuel going out in direction with other filter etc and then injector (No idea what's between the pre-filter and the injector, I do it by memory - my boat is stuck in a marina 1000kms away from home)
4- The injectors
5- The return line back to the pre-filter (CAV filter which gets very hot after hours - I suppose because of the fuel going through the engine)

[Lodesman]

Originally Posted by zeseb

My Yanmar 4JH4-TE runs just fine during 4 hours at sea but after it starts to slow down then stops.
Temperature and pressure are fine. After 30 minutes, I can start the engine without any problem.
I changed the fuel filters and still doing it.
The tank feeds the fuel by gravity (Day Tank) and it looks clean fuel.
I noticed the following:
1- The fuel return line is connected to the CAV filter not to the tank, hence the temperature of the fuel in the filter is very hot after 4h
2- The breather line on the day tank was full of fuel and some fuel was getting out of it when pumping some fuel from the main tanks up to the day tank
3- The breather line is also connected to the pipe filing the day-tank like a T-connector.
Is this setup wrong?

The filling line connected to the breather sounds like a fuel spill in the making - I would separate those two pronto. It is most likely that the increased ambient temperature (from the engine running) causes the fuel in the fill line below the T-fitting to expand, block off the breather tube (vapour lock) and starve the engine of fuel.

The return line should go back to the day tank. IME, the engine draws about twice as much fuel as it uses, so a lot flows back to the tank. I'm not sure how it would work to stop the engine, but it would seem to be unnecessarily restricting the flow of fuel to have it try to shove it's way into the filter.

[zeseb]

This is exactly what we were talking with the mechanic. The plan is to create a new hole in the tank for the breather line only and to separate from the fuel inlet.
And also, having the fuel return into the day-tank instead of the pre-filter.

[Cavalier]

Fuel return needs to be moved to terminate directly in the day tank, as it shouldn't run into the filter (back pressure). The day tank should have a dedicated and separate vent line. The main tank should have its own dedicated and separate vent line.

[Steven UK]

Originally Posted by paulajayne

Check the fuel tank vent.
Is it clear?
If not tank is running into vacuum and stopping fuel getting to motor.

+1 on this , you could test this by running the transfer pump when it started to slow down. Though I think you would be having problems filling the day tank also.