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Roadside Diesel Repairs

How to be a MacGyver mechanic for a broken-down diesel

Economy and reliability are great reasons to own a heavy-duty diesel 4x4, so even when the conditions are bad it's easy to feel a bit over-confident when driving an old reliable diesel-powered truck. Compared to a computer-controlled gasoline engine with its electronic spark ignition system, a mechanically injected diesel is downright simple. The lack of a distributor, wires, spark plugs and some sort of electronic control means that you have considerably fewer parts to fail. Then what makes a diesel decide not to run?

Suffer the Fuel

Fuel. Ninety-nine percent of the time a diesel engine stops running or runs badly without any previous warning is due to a problem with the fuel itself or its delivery. Air bubbles in the fuel, water in the fuel, gasoline in the fuel, clogged fuel filters, fuel that is too cold or a faulty fuel shut-off valve are all good reasons for a diesel to shut itself off. Fortunately, all of these problems can be remedied on the side of the road if you have the knowledge and some simple tools.

The first tool you need is either a good manual for your vehicle or a long intimate acquaintance with all the workings of your truck. Other tools that you should carry with you are whatever wrenches and screwdrivers will allow you to access and disconnect your fuel lines. You should also carry a new set of fuel filters and the tools you need to change them, some duct tape, a bottle of water-dispersion chemical additive, and maybe a piece of wire for hot-wiring that pesky fuel shut-off valve. The only special tool that we suggest carrying is a hand-operated vacuum pump. It is the same kind you may have used to bleed brake fluid. They're also great for bleeding air or flushing contaminates out of a diesel fuel system when you're on your own.

Shut-Off Solenoid

One of the first things to check is the fuel shut-off solenoid. This little valve opens when you turn on the ignition to allow fuel to flow through the injector pump. Its primary job is to turn off the fuel when you turn off the truck, so if this valve were to lose power while running it would shut the engine off in less than a second.

When the ignition is turned on you should hear a little click in the vicinity of the injector pump. If in doubt, just run a wire from the battery to the valve and try to restart. Make sure you know in advance which wire is which! Obviously the wires and terminals differ from one brand to another and even from one truck to another.

Contamination

More commonly, the first sign of trouble develops not long after refueling your truck. This would indicate in most cases some sort of fuel contamination. Keep in mind that the fuel filters, lines, and pumps can hold as much as a gallon of fuel. This means that your truck can go anywhere from five to 20 miles after a refuel before trouble develops.

If you feel that you have some sort of fuel contamination, then the first order of business is to find out what the contaminant is. Water is the most common trouble. Water can build up over days, or if there is enough water in your fuel tank, it can shut you down right away. It can get there by any number of means. Condensation can build up from high humidity, and slowly contaminate your tank until there is enough to start working its way through the fuel lines. A heavy rain can push past the seals on your fuel cap, or even worse, past the fuel tank covers at the gas station.

Fortunately, your diesel has a fuel separator. In almost all cases it is part of one of your fuel filters, and it even has a drain on it just to evacuate the water. To remove the water from the separator of most trucks, simply open the valve and let the water run out until you see clean fuel. Some earlier Fords have a separator that is not part of the filters, but it behaves in a similar manner. For some Chevrolet models, it is necessary to open upper and lower valves on the filter to allow the filter to drain completely, then close the lower valve and crank the engine with the fuel shut-off solenoid disconnected until fuel comes out the top valve.

Disconnecting the fuel shut-off solenoid allows the low-pressure pump to push fuel without the engine starting since the injector pump is inoperative. At this point, the air and water are bled out, the top bleeder can be closed, the wire for the shut-off solenoid reconnected and the truck restarted.

Total Drain

However, if you are anything like us, the sound of draining fuel or pumping fuel all over the engine might not sound so great. It is also a task that is hard to accomplish on your own, since it is not always possible to see the filter or drain valves when you are at the ignition switch. Sometimes gravity alone will not pull all of the water out of the separator. For these reasons we like to use the previously mentioned vacuum pump to suck the water or contaminated fuel out of the drain valve openings. Simply use the brake bleeding attachment for the pump and it will pull any air, water and contaminated fuel into a clear and sealed container. This method is much cleaner, provides better results and it does not wear out your batteries with excessive cranking.

If a large amount of water is determined to be in the tank, then you will need to find a way to get it out. Some trucks have a drain plug on the bottom of the tank. Since the water sinks to the bottom, then you can remove the plug (carefully!) and watch the water run out. When you start to see clean diesel, put the plug back in. Some lucky Chevrolet owners will find a petcock valve in the rear wheelwell area. If you have this system, you can run a hose and siphon out the water.

Even after having accomplished these tasks, there is still going to be some water in the tank. It is a good idea to use a water-dispersing additive to completely remove the rest of the moisture. Often these chemicals work well in an emergency to help disperse even large amounts of water. It is a good idea to carry a bottle of this additive in case you find it impossible to remove the water through a drain plug or tube on your tank. Keep in mind, however, that the water is still in your tank, even if the chemical is able to disperse it through the diesel fuel. Either drive to where you need to go, and then drain the tank, or use the tank of fuel up quickly to avoid corrosion of internal injection parts.

Culprits

Space doesn't permit us to cover every possible source of mechanical failure on diesel, but cold weather, gasoline contamination or air bubbles can be other culprits. If you do find yourself on the side of the road with an old diesel that doesn't run, this article should give you a head start on what to expect and how to repair it by yourself with a minimum of expensive tools or a degree from a technical school. Preparation and a good mental map of where your fuel system components are located can turn you into a roadside MacGyver.