Fuel Systems Explained
We’ve all been there, or will be there some day. Heading our on a beautiful sunny day, cruising out the channel towards our favorite weekend spot, and suddenly the engine coughs, sputters, and dies. Instantly a pleasant day trip has turned into a hassle at best, and a full-blown emergency in far too many cases.
An expensive tow to the nearest shelter could be in order, or an anchor needs to be set, or sails prepared. Recently, a beautiful 55’ ketch was washed up on the beach of San Diego when they suffered a poorly timed engine failure, and were unable to sail off of a lee shore. How can we avoid this problem?
There are several likely causes for unexpected engine failure. First and foremost, let’s get this out of the way: Is there fuel in the tank? Always check your fuel before going out, and consider carrying spare fuel if you can safely do so. ‘Nuff said.
OK, now to the underlying issues. Basically, the mechanics say a gas engine needs three things to run. Spark, compression, and clean fuel. The same techs also say a diesel engine needs three things to run properly. Clean fuel, clean fuel, and clean fuel. Clearly we have something worth looking at here. We will leave spark and compression to a future article, and focus for now on how to keep our fuel clean, with an emphasis on marine diesels. This article will focus on how to keep clean fuel flowing into your engine.
Clean fuel starts at the dock. Do you trust the source of your fuel? In general, the more fuel that a fuel dock or gas station sells, the less likely you are to find contamination. Diesel sitting still for a long time will grow algae in the bottom, which will spread to your tanks, and muck up your filters in no time. I have been recently informed that the diesel algae is in fact a form on anaerobic bacteria, flourishing in the petroleum in the absence of oxygen. I’ve been calling it algae my whole life, and will continue to do so. Suffice it to say, there are living creatures in the diesel tanks, and you don’t want them.
Tip #1: Buy a Baja filter! Prefilter every drop of fuel before it goes into your tank.
A Baja filter is a funnel that you put into your deck fill as you gas up. It has a series of filters that remove a good portion of the gunk from the fuel before it ever gets into your boat. This is especially important in other countries: I’ve gotten water in my tanks from a dock in the Bahamas, sand from a fuel truck in Greece, and primordial algae sludge from Mexico. If you pre-filter, not only do you get rid of most of the problems, you can see what’s going into your tanks as it goes in.
OK, the fuel is on board, now what? Consider treating it in your tank with a biocide additive. This keeps algae from growing in your tank. Be careful though, if you already have growth, all that the biocide will do is kill it and make it float up to the surface. The biofilm that normally sticks to the bottom and sides of the tank is now dead, floating and ready to be sucked into your engine. Often the addition of a biocide to an older boat causes more problems than it solves. If you must kill the bugs, go for it, but consider having your fuel polished (pumped out of the tanks, cleaned, and returned) if it causes issues. And scrub the tanks out with a sponge on a stick while they’re empty.
So now we have a clean tank, so we’re not causing any new problems at least. Where do we look now? Next stop is the fuel filters. Most problems will be seen in the primary filters. These will be external, not built onto the engine. The best primaries are made by Racor, although there are increasing numbers of cheaper made Sierra filters appearing lately, and the occasional Fram unit on older boats. A few other brands exist, but are less common on American boats.
Tip #2: Keep spare primary filters on board and easy to find!
The primaries should be somewhere easy to access. You will have to change them, and it’s better to learn about them now than when the engine dies in the shipping channel. Locate them. If you don’t find a primary, check again. If you still can’t find one, go to the store, buy one, and put it inline between the engine and the tank. If you have a diesel engine, do not install anything on the return line, or allow it to confuse you. Diesel engines do not burn all their fuel, so part of it will flow back into the tank. Every diesel has two lines, one going to the engine, and one returning to the tank. The regular line should be easy to locate – it will go to a filter. The return line will come from the top of the injectors, generally, and go directly to the top of the fuel tank with no interruptions.
Tip #3: Make it easy to service your primary filter!
Click here to see a better installation - note the valves before and after the filter to allow you to prevent air getting into the system while working.
Click here to see the best installation - this is a dual filter system that makes it possible to switch filters without shutting off the engine, the best possible setup in an emergency.
So how do we change the primary filters? There are two types of filter, a spin-on element, and a drop-in element. Spin-on filters simply unscrew, usually with the help of a filter wrench, and screw on a new one in its place. Drop in filters have a canister that opens at the top, and the old element is removed from the canister and replaced with a new one.
|Dropin Filter||Dropin Elements||Spinon FIlter||Spinon Elements|
Simply changing the filter is not enough, though, if you have sediment in the bowl at the bottom of the filter. A good practice is to drain out all the fuel into a glass jar before changing the element. Take a good long look at the fuel in the jar: is it black? Does it smell organic? Is there sand or water? This will give you a good indicator of the quality of the fuel in your tank. Also, fill a spin-on element up with clean fuel before screwing it back on, or top up the canister for the drop-in filter. The less air gets into the system, the better, so really topping off the filter before putting it back together will pay off.
Now the primaries are clean, what else could be a problem? A much neglected area is the secondary fuel filter.
Tip #4: Don’t ignore your secondary filter!
Most basic engine service jobs do not bother with the secondary, leaving it for the “annual” or more advanced service. It needs to be replaced, if not as frequently as the primary. The secondary filter provides a second level of protection for your engine. It is usually a finer filter (maybe 2 microns) than the primary, and it makes the fuel clean enough to go through the injectors without clogging. For removal, remember what you’ve learned with the primaries, just remove the filter, fill the new one with clean diesel, and replace it. Like an oil filter, a fuel filter will seal better if you rub a tiny bit of old oil around the seal, and make sure the seal fits properly and snug.
Now you’re finished replacing things, but there’s one more step if you have a diesel engine. You have to make sure the air is bled out of the lines that you’ve just been messing with. If you have a valve before and after the filter, and you worked only with the valves closed, and you filled the element with clean fuel before you replaced it, you’re in good shape. Most diesels will push a little air through while running, just run the engine a while and make sure it’s OK.
If you let air in the lines, or didn’t fill the filters, you will need to do an engine bleed. Remember, none of this applies to a gas engine! The correct way to bleed a diesel is in your service manual (you got the service manual, right?), but here are the basics. First, figure a way to get fuel moving through the lines. Most diesels have a lever on the base of the fuel pump that can be manually pumped, giving you pressure in the lines. Some installations have a small electric “lift pump” that can be used for this instead. The dangerous way is to have someone turn over or run the engine, which engages the actual fuel pump to give you pressure. Stay away from moving parts!
Now you have pressure: just crack open a bleed point (generally by loosening a fitting a tiny bit) and let diesel/air spray out until no more air comes through, just clean diesel. Start at the first bleed point, on top of the secondary filter usually, and work your way through to the injectors last. You will make a mess, but when you’re finished, the engine will be air-free. If you think there’s only a little air in the lines, and the engine is running OK, you may be able to get away with only bleeding at the injectors.
Now you have given the diesel everything it needs: clean fuel, clean fuel, and clean fuel! For further issues and troubleshooting, get the service manual and/or a good mechanic. Best of luck!
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