Fuel and exhaust systems – diesel engine
General information
The fuel system is what most sets diesel engines apart from their gasoline powered cousins. Simply stated, fuel is injected directly into the combustion chambers, and the extremely high pressures produced in the cylinders is what ignites the charge. Unlike gasoline engines, diesels have no throttle plate to limit the entry of air into the intake manifold. The only control is the amount of fuel injected; an unrestricted supply of air is always available through the intake.
There are two major sub-systems in the fuel injection system: the low pressure (also known as the supply or transfer) portion and the high-pressure injection (delivery) portion.
The low pressure system moves fuel from the fuel tank to the injection pump. Fuel is drawn from the fuel tank by the pump which is a part of the fuel pump module. It’s then pumped through a fuel filter/water separator and then into the high-pressure injection pump. A bypass system allows excess fuel to return to the tank.
The fuel injection pump supplies high-pressure fuel to a fuel rail, which is attached by high-pressure fuel lines to the fuel injectors. The injectors are electrically actuated, in firing order sequence, by the ECM.
Fuel system problems are by far the most frequent cause of breakdowns and loss of power in diesel-powered vehicles. Whenever a diesel engine quits running or loses power for no apparent reason, check the fuel system first. Begin with the most obvious items, such as the fuel filter and damaged fuel lines.
The fuel system on diesel engines is extremely sensitive to contamination. Because of the very small clearances in the injection pump and the minute orifices in the injection nozzles, fuel contamination can be a serious problem. The injection pump and the injectors can be damaged or ruined by contamination. Water-contaminated diesel fuel is a major problem. If it remains in the fuel system too long, water will cause serious and expensive damage. The fuel lines and the fuel filter can also become plugged with rust particles or clogged with ice in cold weather.
Diesel fuel contamination
Warning: The pressure in the high-pressure fuel lines can reach extremely high pressure (as much as 23,000 psi), so use extreme caution when working near any part of the fuel injection system with the engine running.
Before you replace an injection pump or some other expensive component, find out what caused the failure. If water contamination is present, buying a new or rebuilt pump or other component won’t do much good. The following procedure will help you determine if water contamination is present:
a) Remove the engine fuel filter and inspect the contents for the presence of water or gasoline (see Tune-up and routine maintenance).
b) If the vehicle has been stalling, performance has been poor or the engine has been knocking loudly, suspect fuel contamination. Gasoline or water must be removed by flushing (see below).
c) If you find a lot of water in the fuel filter, remove the injection pump fuel return line and check for water there. If the pump has water in it, flush the system.
d) Small quantities of surface rust won’t create a problem. If contamination is excessive, the vehicle will probably stall.
e) Sometimes contamination in the system becomes severe enough to cause damage to the internal parts of the pump. If the damage reaches this stage, have the damaged parts replaced and the pump rebuilt by an authorized fuel injection shop, or buy a rebuilt pump.
Storage
Good quality diesel fuel contains inhibitors to stop the formation of rust in the fuel lines and the injectors, so as long as there are no leaks in the fuel system, it’s generally safe from water contamination. Diesel fuel is usually contaminated by water as a result of careless storage. There’s not much you can do about the storage practices of service stations where you buy diesel fuel, but if you keep a small supply of diesel fuel on hand at home, as many diesel owners do, follow these simple rules:
a) Diesel fuel ages and goes stale. Don’t store containers of diesel fuel for long periods of time. Use it up regularly and replace it with fresh fuel.
b) Keep fuel storage containers out of direct sunlight. Variations in heat and humidity promote condensation inside fuel containers.
c) Don’t store diesel fuel in galvanized containers. It may cause the galvanizing to flake off, contaminating the fuel and clogging filters when the fuel is used.
d) Label containers properly as containing diesel fuel.
Fighting fungi and bacteria with biocides
If there’s water in the fuel, fungi and/or bacteria can form in warm or humid weather. Fungi and bacteria plug fuel lines, fuel filters and injection nozzles; they can also cause corrosion in the fuel system.
If you’ve had problems with water in the fuel system and you live in a warm or humid climate, have a diesel specialist correct the problem. Then, use a diesel fuel biocide to sterilize the fuel system in accordance with the manufacturer’s instructions. Biocides are available from your dealer, service stations and auto parts stores. Consult your dealer or a diesel specialist for advice on using biocides in your area and for recommendations on which ones to use.
Cleaning the low-pressure fuel system
Warning: Diesel fuel is flammable, so take extra precautions when you work on any part of the fuel system. Don’t smoke or allow open flames or bare light bulbs near the work area, and don’t work in a garage where a gas-type appliance (such as a water heater or a clothes dryer) is present. Since diesel fuel is carcinogenic, wear fuel-resistant gloves when there’s a possibility of being exposed to fuel, and, if you spill any fuel on your skin, rinse it off immediately with soap and water. Mop up any spills immediately and do not store diesel fuel-soaked rags where they could ignite. When you perform any kind of work on the fuel system, wear safety glasses and have a Class B type fire extinguisher on hand.
Water-In-Fuel (WIF) warning system
1. The WIF system detects the presence of water in the fuel filter when it reaches excessive amounts. Water is detected by a probe located in the fuel filter that completes a circuit through a wire to a light in the instrument cluster that shows a gas pump and water drops.
2. The WIF system includes a bulb-check feature: When the ignition is turned to ON, the bulb glows momentarily, then fades away.
3. If the light comes on immediately after you’ve filled the tank or let the vehicle sit for an extended period of time, drain the water from the system immediately. Do not start the engine. There might be enough water in the system to shut the engine down before you’ve driven even a short distance. If, however, the light comes on during a cornering or braking maneuver, there’s less water in the system; the engine probably won’t shut down immediately, but you still should drain the water soon.
4. Water is heavier than diesel fuel, so it sinks to the bottom of the fuel tank. An extended return pipe on the fuel tank sending unit, which reaches down into the bottom of the tank, enables you to siphon most of the water from the tank without having to remove the tank. But siphoning won’t remove all of the water; you’ll still need to remove the tank and thoroughly clean it. Warning: Do not start a siphon by mouth – use a siphoning kit (available at most auto parts stores).
Removing water from the fuel system
5. Disconnect the cables from the negative terminals of the batteries (see Engine electrical systems).
6. Remove the fuel tank (see Fuel and exhaust systems – gasoline engines).
7. Remove the fuel pump module, then drain the tank into an approved container.
Dispose of the fuel following local regulations.
8. Thoroughly clean the fuel tank. If it’s damaged or otherwise unusable, replace it. Clean or replace the fuel pick-up filter.
9. Install the fuel tank and add fuel.
10. Open the fuel filter drain valve and attach an extension hose to it that empties into a container (see Tune-up and routine maintenance).
11. Perform the fuel system priming procedure by turning the key to the Start position momentarily. This will energize the transfer fuel pump for a 30-second interval. Repeat this until you’re satisfied that clear fuel is coming out.
12. Replace the fuel filter (see Tune-up and routine maintenance).
Gasoline in the fuel system
Warning: Gasoline and diesel fuel are flammable, so take extra precautions when you work on any part of the fuel system. Don’t smoke or allow open flames or bare light bulbs near the work area, and don’t work in a garage where a gas-type appliance (such as a water heater or a clothes dryer) is present. Since diesel fuel is carcinogenic, wear fuel-resistant gloves when there’s a possibility of being exposed to fuel, and, if you spill any fuel on your skin, rinse it off immediately with soap and water. Mop up any spills immediately and do not store diesel fuel-soaked rags where they could ignite. When you perform any kind of work on the fuel system, wear safety glasses and have a Class B type fire extinguisher on hand.
13. If gasoline has been accidentally pumped into the fuel tank, it should be drained immediately. Gasoline in the fuel in small amounts – up to 30 percent – isn’t usually noticeable. At higher ratios, the engine may make a knocking noise, which will get louder as the ratio of gasoline increases. Here’s how to rid the fuel system of gasoline:
14. Perform the same procedure as for removing water from the fuel system (see Steps 5 through 12).
15. Try to start the engine. If it doesn’t start, purge the injection pump and lines: Place rags around the fuel rail. Loosen the fuel line fittings a little, just enough for fuel to leak out. Depress the accelerator pedal to the floor and, holding it there, crank the engine until all gasoline is removed and diesel fuel leaks out of the fittings. Tighten the fittings. Limit cranking to 30 seconds with two or three minute intervals between cranking. Warning: Avoid sources of ignition and have a fire extinguisher handy.
16. Start the engine and run it at idle for 15 minutes.