Yanmar 2GMF looses RPM after a few hours of motoring.

[Runaway2]

We sail a 1985 Pearson 303 and estimate our Yanmar 13 horse to have roughly 1800 hours on it. A new concern has developed after we have been motoring for a few hours. We normally run between 2500 and 2700 RPM averaging 6 knots in standing water. I normally can throttle up to 3000 to 3200 RPM and get 6.5 but I don't hold that for long. Lately, when I throttle up after running for hours, it will briefly reach 2900 but than bog down to 2600 or so regardless of the maxed throttle. If I sit in our slip allowing the motor to cool before shutting down, I can push it up to 3200. When we are first motoring out for the first few hours, we can do the same.
Engine condition:

The engine has always emitted a sooty black exhaust that often coats our transom but has done so for 5 seasons. That means it's running too rich and probably could use injector cleaning and possibly a fuel pump servicing. I change the oil regularly and the level remains steady. I add diesel fuel treatments along with biocide, seal conditioners, cetane boosting, and water treatment to remove condensation. I originally thought the engine room wasn't getting enough air and it was getting too warm but I ventilated, checked and changed the air filter, and tested and still the same symptom. I have changed out the fuel filter and the water impeller is pushing water through with no clogs plugging the strainer. Engine temp hovers appropriately at 150-160 degrees once warmed up. Could the fuel pump be to blame?

 

[Runaway2]

C

 

[kloudie1]

sounds like you've done a good bit of troubleshooting.. Tight valve clearances can sometimes cause your symptoms.. Tank dip tube (fuel pick up tube in tank) screen plugging can cause symptoms similar as well..
Probably not related to the symptoms, but it looks like you have a bit too much prop pitch .. Have the RPM readings been verified by a hand held tachometer?

 

[Cayennita]

The black smoke usually indicates injectors problem. I had a similar problem and went away after replacing one of the injectors (front) for a rebuilt unit that I keep as a spare. Also, I tighten the lines going from the injector pump to the injectors while the engine was hot.. For some reason one line was allowing air in when the engine was hot.

 

[Steve Dion]

If the engine is over propped/fouled you will also emit black smoke (too much fuel).

 

[seadaddler]

FUEL

But he said 5 seasons like this and the problem is new so one thing mentioned is all these additives is doing some thing and going some where,as already said check fuel tank pickup and also check inside fuel tank,maybe while motoring the tank is stirring things up into fuel pickup.

 

[Fred Villiard]

Did you check the fuel tank vent? A clogged vent could cause to loose power after running awhile.

Fred

 

[Ken13559]

mixing elbow fouled up.

 

[sailaway78]

You said fuel filter

You said you changed the fuel filter. Did you just change one filter? Should be two on there. One on the engine and a primary somewhere in the line. Sounds like from what you have stated, it is a slight fuel starvation problem. Fuel filters, or tank pickup would be first place I would look. Other suggestions are also good places to look.

 

[timebandit]

I like cheap fixes and do not like guessing so I would check to see if it is a fuel starvation problem by just waiting untill it happened and open the filler cap. If the engine runs normal that look for a partially clogged vent.

 

[sailaway78]

On the money

timebandit is the first to come up with the very first thing you should check. We all know this, but no one else mentioned it. That is certainly the very first thing to check.

 

[Fred Villiard]

I did

Fred

 

[kloudie1]

Yup.. That is a simple and easy and kinda probable.. Good job, Fred.. and Time Bandit

 

[Fred Villiard]

An easy way to check is when it starts to lose power is open the fill cap and see if the engine picks up.

 

[Runaway2]

Thanks for the tips everyone. They are good advice and I will follow.

I did some motoring last weekend and noticed something. When I noticed the rpms start to starve to 2600 (I was SOG 6.5 that dropped to 6) and hold, I cut throttle, shifted to neutral, then reved the motor and it would go to 3400 rpm. It seems that when I hit a stong head current (i'm guessing i was as my speed paddle wheel is broken and I only have GPS SOG), the resistance works the prop harder and perhaps forces a drop in rpms. This would be similar to if you drove your truck up a steap hill and the engine was giving you everything, even though you have your foot to the floor. I suspect that is the case.
The other factor is that I added 9 gallons of diesel before heading out. Perhaps the fuel is a source of the problem as I didn't notice the problem nearly as much after 7 hrs of motoring.

This is the first season I really hit the Columbia river with many hours of motoring. The 2GMF in the Pearson 303 has historically been known to be slightly underpowered for a 6 ton boat as I have read in the rags and perhaps that could be the problem? I never noticed this when motoring from Boothbay Harbor, ME to Falmouth but perhaps the currents were never as strong as the Columbia river currents.

Thanks for all your thoughts everyone. :bang:

 

[Runaway2]

sounds like you've done a good bit of troubleshooting.. Tight valve clearances can sometimes cause your symptoms.. Tank dip tube (fuel pick up tube in tank) screen plugging can cause symptoms similar as well..
Probably not related to the symptoms, but it looks like you have a bit too much prop pitch .. Have the RPM readings been verified by a hand held tachometer?

It's the original prop as best I can tell. I confirmed the pitch was appropriate. Thank you though.

 

[Runaway2]

The black smoke usually indicates injectors problem. I had a similar problem and went away after replacing one of the injectors (front) for a rebuilt unit that I keep as a spare. Also, I tighten the lines going from the injector pump to the injectors while the engine was hot.. For some reason one line was allowing air in when the engine was hot.

I'll look into some injector servicing. Sounds like that will help clear the air.

 

[Runaway2]

Did you check the fuel tank vent? A clogged vent could cause to loose power after running awhile.

Fred

I have put that check on my list. I have not checked the fuel vent. Thank you.

 

[Runaway2]

An easy way to check is when it starts to lose power is open the fill cap and see if the engine picks up.

I'll do that next time Fred. Thx.

 

[RAD]

Clogged filters will exhibit these symptoms cause the engine cannot get enough fuel when under load, try some fresh fuel and new filters and if the boats been sitting be aware that you might have algae growing in the tank and that can be a real pain in the behind cause right after a filter change the filter will clog again
I would replace the filters and pull the fuel line from the tank and insert it into a portable fuel tank and take the boat for a ride into some current and see what happens