Skip to main content

Q&A - Diesel Engine Failure

Have a question? E-mail it to or send it to SoundingsEditorial, 10 Bokum Road, Essex, CT06426

Q: What’s the most common cause of diesel engine failure?

A: In my experience, the answer has been clear: fuel problems. Many things can cause problems with your fuel. Some of the most common are purchasing “bad” fuel, letting fuel sit too long in your tank, letting it accumulate water by condensation or leaks around the fill pipe or tank vent, and the introduction of air into the fuel stream going to the engine.

One way of having a reasonable chance of getting good fuel is to buy from docks that pump a lot of fuel all the time to a regular, repeat stream of customers, such as local workboats. Higher prices don’t necessarily mean better fuel. If you have any doubts, ask the supplier about the fuel filters in the system.

Also, consider using a filter as you fill the tank. I occasionally use the Racor RFF8C filter funnel. It has a Teflon-coated stainless filter that Racor says removes most water and other contaminants from fuel. (As with any product, heed the instructions and cautions.) The only drawback with any such prefilter is that it slows the filling process, particularly if you’re taking on a large volume. But I’d rather have long filling times than the consequences of bad fuel. (The old Baja-style filter was quite slow, and I’ve seen some fuel docks that refused to let people use it because it took so long. It also requires a lot of room to store.)

Obviously, high-quality filters between the tank and the engine are critical, but it’s necessary to regularly check and change the filter elements. A fuel filter pressure gauge will help you determine the state of the elements. I carry at least six spare elements on board because, if I have a problem, I know I may need to change an element many times before getting back into port.

If fuel sits in your tank for too long, particularly if it hasn’t been treated by a quality fuel additive, gum deposits will build up from fuel deterioration and biological or bacterial contaminants in the tank. In the advanced stage they may look like puddles of tar on the bottom of the tank and in other areas. If portions of this material break loose from sloshing or other causes, they can stop up the suction end of the fuel line. Even if there are no deposits, the contamination may be suspended in the fuel. Most of this will be removed by good filter elements, and this is indicated by a messy, black slime on the outside of the elements. If your filters are becoming clogged more and more frequently, consider having your fuel “polished” and your tanks cleaned by a professional.

As mentioned, it helps to use good fuel additives to diminish this threat. Over the years I’ve been happy with Biobor JF, and recently I’ve found Star brite’s StarTron to be helpful. (This is based upon my subjective observations, since I’ve conducted no scientific tests with either.) These products use different methodology, and you should read and heed the labels.

An often unsuspected problem with fuel is the introduction of air — even small amounts — into the fuel supply. Symptoms include “hunting” — as when the diesel speeds up and slows down on its own — stalling out with higher RPM, not reaching normal RPM for the throttle position, and hard starting. (Keep in mind that these also can be symptoms of other problems.)

Air can be introduced by any break between the tank pickup and the fuel supply suction pump. It can be introduced by failures in the pump itself. A joint, tight for years, may begin sucking air because of vibration. A line may develop a pin hole. A gasket may go bad and allow air to enter the stream. Sometimes air comes in around or at the fuel filter. For example, if you don’t replace the old cover gasket with a new one when you change the element, and if you don’t do the job correctly, you may allow air to enter. Because of the many places that air can get in, this can be a particularly difficult problem to troubleshoot.

Obviously, there is much more to be said about this and other aspects of fuel for your diesel. I’ll go into this subject more deeply in a future issue.