fuel system problem diagnosis part 2

       Hello, all, and welcome to part 2 of diesel fuel problem diagnosis. Last time I covered some of the more common fuel injection systems, now I would like to extend this information to diagnosing common problems.
Probably a good place to start is with some common principles. The most important concept governing a fuel system or any system for fluid delivery is that pressure is not generated by the pump, but by flow vs. orifice size. At a given flow rate, a smaller orifice or opening will increase pressure, and a larger flow rate for a given orifice size increases pressure. Think of putting your thumb over the end of a running garden hose. The ultimate limit of flow and pressure is the pump, where the gear or diaphragm size multiplied by the pump speed limits the flow, and ultimate pressure is limited by the sealing ability of the pump’s components. What this means is that low system pressure is often something like a stuck regulator, and not necessarily a bad pump. A regulator is merely a ball or a washer held against an orifice by a spring. The strength of the spring regulates the pressure in the system.

fuel system check valves/regulators

The three most common general fuel system problems are: low fuel pressure, air intrusion, and flow restriction. The following paragraphs will deal with these in turn.
Check valves should be checked whenever there is a pressure or flow problem not due to air intrusion. It is often surprising what can be found stuck in a valve. Small pieces of the inside lining of rubber fuel hoses, bits of Teflon tape, gobs of silicon sealer, bits of paper, and pieces of plant matter are common. Some check balls are made of a hard rubber-like material, and these can be found swollen and sticking open. Probably fuel additives and gasoline along with age, helps break down the material that lines and other components are made of. System pressure must be checked with a gauge, but on high pressure common rail systems fuel pressure must be checked with a scantool. Many high pressure common rail systems incorporate the transfer and high pressure pumps in one component, making it impossible to gauge the low pressure part of the system, but there may be exceptions.

        Another thing to keep in mind is that a system won’t develop rated pressure if there is air mixed with the fuel, nor will it feed the engine the proper amount of fuel. Air can be sucked in at loose, damaged, or improperly installed filters, loose or damaged lines or connectors, worn primer pumps, and broken fuel tank pickup tubes. If two tanks are present, a malfunctioning diverter tee can empty one tank causing that side to draw air. A vacuum gauge plumbed in to the suction side of the system should draw around 2-4 inches of mercury at idle. Any less means lots of air is getting in, any more would be a restriction. Probably a better way to check for air is to install a piece of clear hose somewhere on the suction side and look for a steady stream of bubbles. On systems that return to the tank, the return line can be extended and put into a bucket of fuel and watched for air. Occasionally an injector will stick open and engine compression will pressurize the whole fuel system with air. Lots of air.

        Flow restrictions are probably more common than air intrusion. Of course, fuel filters should be the first thing checked, but don’t forget the screen or separator type primary filter. Pre-2004 Internationals, for instance, have a small plastic bowl by the spin-on filter that never seems to get cleaned out. Besides plugged filters, nylon fuel lines can be kinked, elbows can collect debris, and the aforementioned check balls can restrict flow if swollen. One thing that can drive mechanics crazy is an intermittent restriction caused by something in the tank that occasionally gets sucked against the pickup, shutting off flow. The little foil bottle seals on fuel additive bottles are famous for this. One sign of a restriction is if the filter is dry or very low after the engine dies. Of course, lots of air getting sucked in could cause the same thing. If you think there is a restriction between the tank and the engine the fuel line can be unhooked at the primary filter and run to a bucket of fuel. if the engine runs now, you know where to look for the problem.

Keep in mind that it is very difficult to diagnose problems with modern engines without good software to display codes and run tests. Even having an active code to give a clue to the nature of the problem is usually only a start. For instance, a common code that could be generated for a no start on a HEUI injected motor could be for low injection control pressure.

      There are numerous problems that could generate this code, including:

• low oil level 

• a worn engine oil pump that can’t keep up with the demands of the high pressure system

• internal oil leaks like broken injector o rings

• a failed ICP (injection control pressure) sensor or bad sensor circuit

• a failed ICP regulator or circuit

• a failed or worn high pressure pump

         More than once I have seen a HEUI engine with significant external oil leaks run low enough on oil that the high pressure reservoir runs out of oil and the engine stops. The owner or a mechanic will then repeatedly crank it trying to get it to start, maybe running it on ether. High pressure oil pumps will not tolerate being run dry for very long, and will soon burn up with this kind of treatment. Oil level must be the first thing checked for a no start on one of these engines, especially with a low ICP code.

Keep watching this site. Next I will outline a general system of diagnosis for fuel system problems, and then I will post on ABS problems.