Diesel problem?

Bill Roosa · May 24, 2013

the fuel shutoff is the key to understanding this one. I'm willing to bet that the fuel shutoff is always partially engaged. When the wakes hit the fuel shutoff goes to "full on" and the engine starts actually getting the fuel it needs to run (really fast at this throttle setting).
I'd suggest he clean the injector pump for starters.
As a general rule engines don't speed up unless the problem goes away or some part of the fuel system "sticks open".

Don S/V ILLusion · May 24, 2013

Bill may be correct (he usually is) but this is one of those problems impossible to diagnose on the Internet unless you are asking us to play the multiple guess game. About the only clue is that something is wrong in the fuel system.

kloudie1 · May 24, 2013

It is the governor's job to control speed .. If given enough fuel, a diesel will speed up until it does a rapid disassembly (!) or it runs out of air and begins to smoke. My "in the dark" guess is that something in the governor is very sticky.. either the rotating weights or more likely, the linkage or springs.. causing it to not tell the injector pump to slow down. A wake would cause a little load increase which would make the governor ask for more fuel. Then the sticking is keeping it from decreasing back to the speed it was before.. Thing to remember about our diesels is that the "throttle" lever is actually setting an RPM target for the governor. The governor moves the injector pump rack to add fuel (or subtract) so that the diesel goes to that RPM setting which is dictated by the "throttle" .. Pulling the fuel cutoff decreases fuel and slows the engine.. the vibration eventually shakes the sticky part back to normal and everything is back in equilibrium until something changes the load and the RPM is no longer at setpoint.. then the governor tries to act to get back to setpoint. Control it by fuel shutoff or blocking air. Not a good idea to run it that way because it may launch itself if unloaded when it decides to run away.

higgs · May 24, 2013

Bill: Why would a wake cause the fuel shutoff to suddenly go to full on?

Chris Patterson · May 25, 2013

On the Yanmar, the fuel shutdown lever is spring loaded. If you took the cable off, and shutdown the engine with JUST the lever on the side of the pump, it will spring to open/run as soon as you release it. So the "pounding" would make it spring back to its usual "full fuel" starting point.
I don't know that I would recommend this to someone that has never been into a pump before, but removing the top of the pump, and a liberal shot of luber-goo will sometimes "unstick" them..
I swear that I would try a new filter first. When in doubt about a diesels performance, the filter should always be the first thing on the agenda, THEN try other fixes.
(It'll make 'em do strange things Kemo-Sabe)..

txtowman · May 25, 2013

Trash in the tank will do that, water in the fuel will do that, especially slime in the tank.
If this is the case, the sludge or debris is partially blocking the intake fuel line in the tank causing higher than usual throttle settings. Then the wake dislodges the debris and opens the line so you get more fuel.
A pressure gauge on the fuel line after the filter will help diagnose this type of problem, it basically splits the fuel system in half.
If you have adequate pressure at this point your tank, filter and pump are generally working properly. And the pump and injectors should be the problem. If not the problem is between the filler neck and the gauge.

The first thing I do to a diesel when I get it is to install this gauge. Some mechanical pumps make the gauge pulse, so you need to add a needle valve between the source and the gauge. A lot of filter housing have unused ports available to install the gauge.

I hope I explained it right, so you understand it.

anchorclanker · May 25, 2013

The governor has a maximum fuel set point. If the engine overspeeds it has to be getting more fuel than it should. If it is actually overspeeding as noted on a tachometer, you dont have a fuel restriction/delivery problem, its getting plenty of fuel. In fact in that situation it is getting too much.

So we need more data. Was the engine underpowered and the wakes broke something loose momentarily allowing the engine to actually get the correct amount of fuel? Or was it actually overspeeding and totally unresponsive to throttle lever position? The first cause would most likely be filters, lines, etc, but could also be within the governor. In the second case it IS something in the governor.

weinie · May 25, 2013

anchorclanker said:
The governor has a maximum fuel set point. If the engine overspeeds it has to be getting more fuel than it should. If it is actually overspeeding as noted on a tachometer, you dont have a fuel restriction/delivery problem, its getting plenty of fuel. In fact in that situation it is getting too much.

So we need more data. Was the engine underpowered and the wakes broke something loose momentarily allowing the engine to actually get the correct amount of fuel? Or was it actually overspeeding and totally unresponsive to throttle lever position? The first cause would most likely be filters, lines, etc, but could also be within the governor. In the second case it IS something in the governor.

If the max fuel set point has previously been raised to compensate for another problem like a dirty fuel filter or clogged fuel pickup, then I could definitely see the engine over revving.

If not, then, If its revving high, but not going past the maximum rpms as indicated in the specs, I would check the throttle lever, cable, and linkage.

If it is revving past the manufacturer's maximum rpms, it's definitely got to be the governor.

AXEL · Jun 6, 2013

Hopefully is just oil in the crankcase and not oil mixed with water (sea or fresh). But that would be another problem...

Steve Dion · Jun 6, 2013

The over-filling of the crankcase is a definite problem on a diesel. The engine will suck the oil from the crankcase. The only way to shut her down is to starve it of air.

One of the previous posts indicated that the fuel shutoff is spring loaded. I have had two Yanmars (GM & JH) and neither of these engines had a springloaded fuel shutoff.

Pulling the fuel shutoff (if it is down below) is actually a good way to prevent anyone from starting your engine if you are not around.

Chris Patterson · Jun 6, 2013

Pulling the fuel shutoff (if it is down below) is actually a good way to prevent anyone from starting your engine if you are not around. -quote-

Two weeks ago, my compadre leaves his slip in heavy wind. The shift cable broke. The shut-down cable was ALREADY nonfunctional. As the boat was tearing hell out of the marina, nobody knew how to shut the engine down except the owner/helmsman. He was a little busy at that moment. He finally got below to kill it, but wow. I put in new throttle, shift, and shutdown cables in it afterwards, that was accessible FROM THE HELM!

(The pump shutdown is spring loaded. As is the throttle).

RichH · Jun 6, 2013

shemandr said:
First, it's not my engine. But, I was helping a friend move his boat from the winter yard to his slip and the engine was acting up. So this is posted as a learning experience for me and I hope others.
First symptom was a varying of engine RPM without any throttle adjustment. There was some light smoke. It seemed like the engine was speeding up - not slowing down. No stall. The owner slowed the throttle and used the fuel shutoff to control the varying RPM. It seemed like this had happened before. The engine, A Yammie 2 cylinder, returned to normal operation for another hour. After going over a wake the engine acted up again pretty much the same way. Bad fuel? But why does it speed up? It returned to normal operation and ran normally until the next wake (20 minutes). This time it ran so fast I thought it was running away and there was a lot of smoke which was dark and smelly. I was starting to tear the engine cover off to block the air intake to stop the engine. But using the same reducing the throttle and feathering the fuel shut off, the owner regained control of the engine and it ran fine for the rest of the trip. The owner was bringing a mechanic in to look at the engine so I'm out of it - although I wouldn't be a lot of help anyway.
For learning purposes does anyone have an explanation of what was going on?

A good possibility is that the fuel delivery system isnt designed correctly and 'stores' water in the tubing, and releases small 'slugs' of water as during rough seastate or large wake, etc. The water, depending on how much will change the 'combustibility' of the fuel by changing the equivalent 'cetane number' ... a small bit of water in the correct proportions will increase the cetane number and you can get 'better' combustion if its the 'correct' amount of water.

Heres the situational problem - you must be able to remove water from the VERY LOWEST part of the fuel delivery system, such as the 'sump' of a filter bowl. If that sump is not at the VERY lowest 'elevation' other portions of the fuel delivery system will take over the function of the lowest level and become the 'sump', water can accumulate even if the oil is flowing through the 'sump' as the water will reverse flow backwards to the sump against the flow of oil. water in diesel fuel will always seek the lowest level and will 'reverse flow' through oncoming oil to do so. Due to 'rough seas, etc. sometime a bit of water or a small 'slug' of water will get carried through along with the oil and the fuel lift pump or high pressure pump 'chops it up' or returns it to an emulsion of oil/water mixture .... etc. etc. etc.
Look carefully at the run of tubing between the first filter in a multi-filter series and verify that the hose/tube in that section is equal in elevation or higher than the inlet/outlet of the lowest oil filter (with sump) in the series .... all the way to the engine. 'Nutherwords' a properly designed oil delivery system should be such that 'from' a filters 'water sump' all the way to the engine, the tubing should be constantly increasing in 'elevation' .... never in any section become lower in elevation. If there is such a 'low elevation' run of tube, be suspicious that it can occasionally trap water, occasionally release a small 'slug' of water and if it gets as far as the injectors can change the 'combustibility' of the diesel oil. Worst case scenario is that a 'larger' slug of water can get carried through and come in contact with a very hot injector tip and destroy it.

Water injection was common in the mid part of the 20th century for racing vehicles and aircraft as it raised the cetane number for diesels and raised the octane number for gasoline engine; ... it was abandoned due to the difficulty to accurately control under varying conditions and varying engine loads, etc.