Fuel Filter Mystery - Solved!

Spray is powered by a single diesel engine. Specifically a Ford-Lehman 135 horsepower, 6 cylinder, naturally aspirated diesel engine. That engine must run reliably, not only to get us where we are going but also to keep us out of trouble. An engine stall at the wrong time (like when the wind and surf are towards the rocky beach) could be catastrophic.

 

Fortunately, Spray's diesel is a simple and robust design. To understand the simple part you need to know how a diesel differs from a typical gasoline engine. In each cylinder of a gas engine a mixture of vaporized gasoline and air (mixed in a carburator or more recently in a fuel injection manifold) are compressed by the rising piston, and when the piston is at the top of its travel, and the fuel/air mixture is fully compressed, a spark is emitted by the spark plug which explodes the fuel/air mixture to create the downward power stroke on the piston.

 

In a diesel there is no spark plug and no 'premixing' of fuel and air. Instead only air is compressed by the piston, and really compressed a lot, such that the air instantly heats to over 800 degrees F when the piston is at the top of its travel. At that point diesel fuel is injected at super high pressure into the cylinder and as it hits that hot air it immediately explodes to create the power stroke. Simpler, as it needs no electric system to fire spark plugs (no ignition coils, no distributor, no spark plug wires, no spark plugs), but the diesel fuel injectors, and the fuel pumping system that creates the high injection pressures needed, must be well designed and built.

 

It's an old saying for marine diesels that the 3 major sources of problems are :

1) Fuel

2) Fuel

3) Fuel

 

This is because the diesel fuel injectors shoot the high pressure fuel through a very small hole (essentially invisible) into the cylinder, and if that hole gets plugged, then that cylinder won't fire and the engine runs poorly, or not at all. So any contamination in the fuel such as sediment or water can cause problems. Mariners cruising under diesel power worry about contaminated fuel, and take steps to both prevent contamination and to filter out any contaminants that do exist.

 

Spray is typical in that she has both a primary fuel filter that filters the fuel as it exits the fuel tanks, and secondary fuel filters that re-filter the fuel after the primary and after a stage of fuel pumping. The primary filter gets the most attention. By far the dominant brand of primary diesel fuel filter on boats is Racor, so its common to hear a boat owner say 'its time to change the Racor' or similar when the replaceable filter element in the Racor filter becomes clogged with contaminants.

 

How do you know when to change the Racor element? The best way is to install a gauge that measures pressure drop across the filter. A new element allows easy flow of fuel so there is little pressure drop, while a clogging filter will have the pressure drop increase. When you see that happen you know 'it's time to change the Racor ! On Spray that means 1 hour labor and $10 for a new element.

 

But Spray lacks the pressure gauge needed for detecting a clogging filter, so what now? We have 2 options :

1) We can replace the Racor element periodically, hopefully before any clogging occurs. Spray has an engine-hour meter so we can replace the elements after every say, 200 hours of engine operation. This method usually works but it means you probably change filter elements more often than needed.

2) We could continue to run with a given filter element until a problem occurs. This happened to me 4 years ago as I (and BIL Dennis) were heading into Lake Okeechobee from the west. The symptom of a clogging Racor started about 100 run-hours after changing the element, and was as you might expect : RPM would drop periodically, then resume to normal. It occurred more often at higher power settings than at lower. the diesel was not getting reliable fuel flow. We ran in that mode for a couple hours until we stopped for the night. Changed the Racor element the next morning and then continued across the lake without incident. [I later figured out that the problem was using a too restrictive 2 micron element. I now use 10 micron.]

 

It looked like a similar thing happened twice on this southbound trip. The first time was the day we approached Charleston SC. It was a cold day with strong NE winds so we were driving from the enclosed salon. Very now and then the RPM would drop and I would bump the throttle up to restore the desired speed. This happened several times but we made it to our destination marina in Charleston OK. Since the Racor element had almost 200 hours on it I did change it and the problem did not reoccur ..... until last Wednesday when we were traveling southwards through the Palm Beach area. If you check the blog entry for that day you can see that it was a nasty day, with rain and high NE winds. The RPM drops were same as in SC, and bumping up the throttle again fixed things for a while. We made our goal of Lake Boca Raton without incident.

 

So I figured it was again time to change the Racor element, but I'd wait until morning to let the engine room cool. Checking my logbook I noticed that this element had only served for 90 hours of runtime, which seems too low for clogging the filter. So what was going on?

 

Later that night as we swung at anchor in the still strong winds, an idea came to me. We left the next morning without changing the Racor element and did some tests that showed everything running fine with no RPM drops.

 

So what was the problem? Why did we see symptoms similar to a clogging Racor twice on this southbound trip?

Any guesses?

I'll post my answer in a day or two.

 

 

 

 

Posted with Blogsy