Testing for “off road” or “red dye” diesel?

Oaklevel wrote:
So they didn't disconnect the lines to feed the fuel?

Basic diesel fuel system: (See last paragraph)



Because diesel engines don't require spark plugs and hence no electrical system to run them they are inherently more reliable especially in a marine environment where salt water and electricity do not do well together. But because they don't require an ignition system they do need a high pressure injection system. Injectors and the injection pump are the heart of your diesel engine and if either fail so will the engine. Because of the high pressures required both the injectors which fit into the cylinder head and the injection pump which delivers a precisely measured quantity of fuel at high pressure to the injector, have to be engineered to extremely high tolerances. Neither are able to be serviced or repaired without special equipment. Any slight mismatch in sizing or any scratches may result in them not being able to build up sufficient pressure and without this, fuel cannot be introduced into the engine. It is for this reason that special emphasis is placed on cleanliness in the fuel system as foreign particles entering the injection pump can very easily damage it. Whereas in a gasoline engine some dirt can be perhaps tolerated (though not desired) in your diesel engine it can, and often will, result in failure of the engine with perhaps expensive bills. It is for good reason that these engines are equipped with several filters to clean any impurities from the fuel.

Firstly it is important that the fuel be relatively clean when you put it in your tank and in areas of the world where less attention is paid to this than here in the U.S. it is sensible to use a funnel with a filter gauze in it to stop the larger pieces of debris entering your tank. From the tank, fuel normally passes to a primary fuel filter which is often combined with a water separator to eliminate any water from the fuel (water won't burn but will cause rust!). This primary filter will probably stop particles of dirt larger than 10-12 microns (1 micron= 1 millionth of a meter).

After the primary fuel filter comes the lift pump which is usually a mechanical pump mounted on the engine which operates at low pressure sucking the fuel up from the tank and passing it on down the system. Because this is a mechanical pump and only works when the engine is running you will sometimes find an electric pump installed between the primary filter and lift pump. This electric pump should normally only be used to pump fuel when the engine is not running such as when changing filters and trying to eliminate air from the system (air won't burn either and can often cause a blockage in the injector pump). After the lift pump comes the secondary filter which is normally mounted directly on the engine. Fuel passing through this filter is further screened for even finer particles of dirt before it is passed on to the injection pump.

The injection pump passes the fuel at high pressure to the injectors in a precise sequence and with a timing to match the position of the pistons in the cylinder. It normally operates efficiently and without failure for many thousands of hours so long as the fuel entering it is kept clean. The fuel itself lubricates the pump as it is being passed through.

When fuel reaches the injector it compresses a precisely set spring within the injector allowing fuel to spray in to the cylinder. The exact design of the cylinder head, piston top and of the hole or holes in the injector determine the efficiency of the engine, the aim being to distribute the fuel as evenly and as quickly as possible. In the injector a small quantity of fuel is allowed to lubricate the mechanism and this fuel is then returned either to the injection pump or to the secondary filter via a line connected to all the injectors.

Because both the injector pump and injectors use the fuel as a lubricant more fuel is delivered to them than is necessary and the unused fuel is eventually returned to the fuel tank along a return line.