Troubleshooting the auxiliary diesel engine of a sailboat

If air gets into the high pressure fuel injection pump, it will stop an engine or prevent it from starting.
Steve D’Antonio

Last month I reviewed some of the reasons an engine or generator set might not start. This month, I’m going to dig deeper into why an engine can start but not actually start.

When an inboard diesel engine starts up, several things happen. Air is drawn into the cylinder or combustion chamber through the intake air manifold / filter, passing through the intake valves as the piston is drawn into the cylinder, creating a vacuum. When the piston returns, the inlet valve closes, putting the air under pressure. On a common gas engine, the compression ratio is about 10 to 1, and the compressed air is mixed with fuel vapor. On a diesel engine, the compression ratio is much higher, around 20 to 1, and the fuel is not mixed with air until after compression.

While higher compression increases the thermal efficiency of an engine, in the case of a diesel it is mandatory. As the air is compressed, its temperature rises and, in the case of a diesel, it is hot enough to ignite the fuel when it is injected into the combustion chamber by the injection pump and the fuel. ‘injector. If a diesel has insufficient compression, the air will not be hot enough to ignite the fuel. For this reason, a conventional diesel smokes on start-up; the combustion chamber is still relatively cool, preventing complete combustion, and this white smoke is made up of tiny droplets of unburned fuel. Thus, low compression can be a reason for difficult (or not at all) starting. This could be due to worn piston rings, stuck or improperly adjusted valves, or a starter motor that turns too slowly (between 150 and 250 RPM is required for a diesel engine to start). Faulty glow plugs – either the spark plugs themselves or the solenoid that controls them – can also make starting difficult or nonexistent.

Sailboat Engine Parts
The lift pump supplies low pressure fuel to the injection pump; if it fails, the engine will not start.
Steve D’Antonio

Another aspect of this is the injection of the fuel. For conventional pump-line-nozzle (i.e. non-electronic) injection systems, fuel passes from the tank to a coarser primary filter (typically 10-30 microns), typically via a rubber hose , often to a water separator with a transparent bowl. From there it passes to a low pressure (or lift) pump, after which it flows through a steel line, where it then enters the finer secondary fuel filter (in the range of 2-7 microns ). This is then the high pressure injection pump, and finally the injectors, still via steel pipes. In this sequence, the most common causes of failure are clogged filters, a faulty sump pump, or air in the lines.

Your primary fuel filter must be equipped with a vacuum gauge; without one, you’re essentially flying blind, with no way of knowing how hard your sump pump is working to suck fuel through that filter. So if you don’t have a vacuum gauge here, I highly recommend that you install one; it is a valuable troubleshooting tool, and it will alert you when this filter needs to be replaced. If, during start-up, you observe the rise of the vacuum gauge, there is a restriction somewhere between the lift pump and the filter; of course, the element itself should be checked and changed if in doubt.

Read more: Monthly maintenance

If the vacuum remains low, less than about 3 inches of mercury, move on. The vacuum gauge will not alert you of restrictions that occur after the sump pump, including the secondary filter. However, if you change this filter when you change the main filter, this is unlikely to be the culprit. Regarding filters, it should be noted that some sump pumps include their own built-in filter or strainer, which can get clogged.

Sailboat Engine Parts
Supplying fuel to this pump using a separate fuel supply is one solution. Some piston rings are used to seal the space between the piston and the cylinder so that compression can be generated. If these are worn out, an engine may be difficult or will fail to start.
Steve D’Antonio

Air can enter a high pressure fuel system in several ways, but most commonly through primary filter housings, low fuel consumption tanks, and plumbing connections between the tank and the sump pump. . Air will only be drawn in where a vacuum is present, between the tank and the lift pump, so focus on inspecting these areas – at rest they could leak fuel. Make sure everything is tight. Then try to purge the high pressure system to purge any air that is present.

Finally, on older engines, it is possible that the injection pump and the injectors wear out, and / or become clogged with carbon respectively. If the engine has more than 3000 or 4000 hours, the wear on the pump reaches a point where the fuel pressure is low enough to prevent combustion. This is often preceded by easy cold starts, where the fuel viscosity is higher, and difficult hot starts, where the fuel viscosity is lower. In addition, if the injection pump uses an electric solenoid-type stop piston, these may fail or jam. Some engines activate the piston to make it work; others energize it to stop. It’s an easy solution and worth checking out.

Steve D’Antonio provides services to boat owners and buyers through Steve D’Antonio Marine Consulting (

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