Throttle Problem

[Robertsalkeld]

This is my first time
My boat is an Offshore 48 moored in Shoreline Marina, Long Beach California (Polar Bear)
My problem is with the starboard engine, a Cat 3126.
When I bring the throttle down to idle it sometime stalls.If I bring it down suddenly it always stalls.
A mechanic upped the idle speed as a temporary fix.He is so busy I don’t know when I will see him again.The temporary fix did not work.

Any ideas?

 

[Comodave]

Welcome aboard.

 

[rgano]

Another welcome.

A number of ideas come to mind:
Air ingestion into the fuel line - check ALL the connections from the fuel tank
Heavily clogged fuel filter
Unlikely, but check air filter
Clogged or partially clogged fuel tank air vent if that engine is fed from only one tank

 

[Lepke]

A remote possibility is the throttle linkage is too tight or out of adjustment and binding.

Diesel engines are controlled by a governor. The governor is part of the injector pump in most engines. Normally a diesel will idle w/o issues because of the governor. When you move the throttle lever, you're actually telling the governor to run at a specific rpm and it sends more or less fuel to the injectors to do that.
My experience with Cats is with bigger engines. I don't know your engine well and don't have a manual. Some injector pumps use the engine's oil and some have their own internal supply. In either case, the oil needs to be clean and the correct viscosity. Otherwise the governor can be slow when responding to changes, more noticeable at low speeds.
If both engines share the same fuel supply, then it's not a fuel problem and probably not a filter problem if the engine runs fine at higher rpm. It's also not a fuel pump problem or blocked pickup tube or fuel line. Don't start changing fuel filters if you don't know how to bleed air from the fuel system or you'll have new problems for the mechanic to figure out. And if you use a typical marina mechanic, he won't figure out he's got an air problem until he replaced a few hundred $ in unnecessary parts.
You should have a owners manual and should buy a service manual even if you don't plan on doing your own work. By having the reference, you will sometimes find solutions to problems mechanics miss. If you belong to boatdiesel.com, you may be able to download a pdf of the manual, free. Their forums are a good place to ask questions about problems like yours.

 

[Island Cessna]

I am also not familiar with this specific engine, but it appears from BoatDiesel that the 3126’s are all turbocharged and aftercooled.

Being an old guy I was active in the early days of TA engine development and all of them had some sort of system to delay the increase of fuel rate after throttle increase in order to allow the turbo to accelerate and provide enough air to avoid over fueling.

As Lepke says your engine will have a governor. When you reduce “throttle” this Governor will be shutting the fuel off completely until the engine gets down to idle speed. It sounds like whatever method your engine is using to prevent the injection pump from over fueling on acceleration is reacting too slowly, or sticking, and the engine dies before the fuel rate is restored. Changing the idle speed would have no direct effect on this.

Your engine is electronically controlled and I have no clue where to look for the problem or solution. (refer to “old guy” comment above) In the old mechanical fuel control era the first place to look would be the pipe connecting the aneroid fuel limitation system to the intake manifold. Today’s equivalent would be the manifold pressure sensor.

While waiting for the mechanic’s return I would retard the the throttle as slowly as possible. If this works it will help your mechanic isolate the problem.

Bill