Fuel tanks,  lines, fills, grounds,  pickup tubes, and gauges  by Jim Cross


Fuel Tanks:


Fuel tanks are as diverse as the vessels in which they are installed.  They can be built in, molded into the structure, or secured in place. They are found manufactured from various materials, copper, aluminum, steel, black iron, stainless steel, Monel, fiberglass, rubber,  plastic, and probably other materials I have not come across. The contents of a fuel tank is, more often than not, just plain fuel. In some cases there are other surprises found in the fuel tank. Water is one of the most frequently uninvited guests, along with various bacteria, and mixtures of rust and slime. Most fuel tanks are designed  so that the fuel pickup tube does not extend all the way to the bottom, thereby missing the heavier water or other debris which may be lurking along the bottom of the tank. There are types of bacteria which form huge colonies and have the appearance of a jelly like material. These globs of material remain suspended in the fuel tank at various levels. Once in a while the blob will come up under the pickup tube and cut off the fuel supply to the engine. Of course, this invariably happens in rough seas, miles from port.

Fuel tanks are very difficult to inspect. As most of them can not be inspected from the inside, a peripheral external inspection is all that can be performed. Because of the location of most tanks, three or four sides can not be inspected. So, in a large vessel with two or three fuel tanks mounted in the bilges, a lot must be left to the imagination. I have also had the occasion to note fuel not drawing from one of two full tanks.


Example:  A vessel with two fuel tanks usually is equipped with a manifold which permits switching engine fuel feeds from either or both tanks.  This is a very handy system to have aboard when fuel is running low in one tank. In this example, the vessel is about to empty its primary tank and the operator decides to switch to the auxiliary tank, which is full. A few minutes after the switchover the engines quit and it appears that they have run out of fuel. Re starting fails and it is determined that fuel is not reaching the engines. Imagine having a full tank and not being able to use it.

Here’s what happened next. The auxiliary tank was checked with a dip stick and was full. The filters, manifolds, and fuel lines were cleaned and checked for leaks or obstructions, and no problems were found.  The fuel pumps were tested and found to be working properly.  After everything was tested and the engines were still not getting fuel, it was suggested to switch back to the tank that was almost empty. The engines again started and sprang to life without hesitation.  It happens that the pickup tube had rotted completely away in the tank that was full.

The design of all marine fuel tanks only allows for fuel line connection from the top of the tank. So, a metal tube, called a pickup tube is fitted to the top of the tank and the fuel line is connected at this point.  Usually the pickup tube is fabricated from quality non corrosive metal tubing which is rigid and extend almost to the bottom of the tank. Some space, at the bottom of the tank, is set aside for water and other debris which collects over time. The tube on this tank was fabricated from an aluminum tube secured to a bronze cover plate. The tube corroded away at the bronze to aluminum connection due to electrolytic action.

It is imperative to check out fuel supply systems, including manifolds during the sea trial.

On a recent sea trial I conducted, the vessel owner, who was operating the vessel, mentioned that one tank was about half full and the other was empty. He mentioned this after I asked to try the manifold to check the feed from both tanks.  As I spend most of the time during the sea trial in the engine compartment, I noticed that both tanks were accessible. They were mounted outboard of the engines within easy reach. The tanks appeared original and were about twenty years old. They had been painted some time ago and were well secured. I tapped on the tank that was supposed to be empty and found that it was almost full. After we returned to the dock, the owner went off for coffee and I continued my inspection which included the control station wiring. I was interested in why the fuel gauge indicated “empty”.  The back of the control station panel was easily reached from a main salon hatch. Imagine my amazement to find a disconnected wire at the fuel gauge. So, I slipped the connector over the terminal, and, sure enough, the gauge read full. It looked like the owner knew he had problems with the tank and tried to defeat the inspection by disconnecting the fuel gauge. I showed the disconnected wire to the prospective buyer and let him know that there was probably a problem in the fuel tank. When the owner returned we confronted him with the disconnected wire and he said he did not know about any disconnected wires. We asked if we could re-connect the wire to see if the gauge would work, and he said sure.  After re-connecting the wire the gauge sprang to life and indicated an almost full tank. So, we again asked to run the engines and check out the fuel manifold system. The owner, with a red face at this point, reluctantly said yes and we tried the manifold system which, of course, did not permit fuel to be supplied from the almost full tank. The owner admitted that he had run out of fuel on that tank some time ago and had the gauge disconnected because it was giving him a false reading. One might wonder why he wouldn’t just add fuel to the empty tank.

This is another example of the owner trying to hide something from a possible buyer.


Another interesting situation that I observe on occasion is the presence of huge bacteria colonies in diesel fuel tanks. The colonies present themselves as gelatinous blobs that float at various levels in the fuel, some close to the bottom and some in the middle. In this unusual case I was operating a 1982 42 Atlantic Offshore with twin 3208 Cats. The sea trial was to take us to Newport, a distance of about twenty miles. The sea was almost calm for about half the trip and the vessel performed better than expected at all engine RPM’s.  About half way the wind and sea picked up a bit, small sea, about two feet or so.

I had already checked out the fuel delivery system before getting underway, and it was in great shape. As soon as we hit chop of about two feet the starboard engine went back to an idle while the throttle lever was at half throttle. Incidentally, the wind and waves began to increase as soon as the engine decided not to operate properly. We decreased speed and went along for a while on the port engine,  which would have been a pain without the auto pilot. Both fuel gauges indicated full,  and I knew the delivery system was in good condition, so what could cause the engine to idle. I left the throttle lever in the same position it was in when the trouble began. Believing that the remainder of the trip was going to be a drag I just sat back and watched the sea grow. Five or ten minutes went by and the engine was back to full RPM. What magic is this, I thought.  Another few minutes went by and the same engine went back to idle again. I was navigating from the Sound into the Bay then to Newport harbor. As I approached the harbor the engine revved up and shut down a couple of times, and I thought how nice it would be to have both engines running as docking around the million dollar yachts is a bit tricky in all this wind.

Well, luck was with me, and as soon I reached the harbor and the waves went away the engine came back to its normal operating mode and docking was not a problem. I let the new owner know about the details of the day and he was upset that I did not pick up on this problem during the previous days original sea trial, which was conducted without a wave in sight. As he was with me the first day, he admitted, the vessel operated as it should and I could not have noted a problem if there were no symptoms. I had a real interest in this problem, so I decided to find out why and what. After several calls to local Marine Diesel mechanics  who suggested everything from water in the filters to clogged injectors, I talked the new owner into another sea trial, no charge, and I would remain in the area of the fuel manifold so that I could change tanks when the problem arose. Sure enough, as soon as we hit some mild chop the engine RPM dropped back to an idle. I switched tanks via the fuel manifold and the engine picked up speed and remained steady all the way back to the dock. The new owner had the fuel tank in question cleaned and the cleaning crew discovered several large blobs of what looked like black Jello. It appears that the blobs were floating at a level where they could get under the pickup tube which had a screen to prevent large particles from entering the system, and partially shut off the fuel flow.

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