General principles of diesel fuel injection diagnosis

This overview follows the VEHICLE diagnostic process outlined in an earlier post:

First - law #1 :  Make sure the simple, cheap, easy things are right before moving on!
V: a VISUAL check is the first step.
E: check the EASY things next.
H: HOOK up a scan tool for problems on a system monitored by the ECM.
I: ISOLATE the problem to a smaller area within the system.
C: CHECK each item only once.
L: the LAST action is to always verify the repair.
E: EVERY problem has a simple solution.*
*simple doesn't necessarily mean easy to find. And rarely a problem may have multiple causes. But most difficult problems are simple ones hiding behind layers of distraction.

• Always do a visual inspection before a diagnostic attempt is made. Check for fuel and oil leaks, loose or disconnected wires or connectors, holes or splits in air-to-air components, a plugged air filter, and make sure there is fuel in the tank. Quite often a problem will be something simple, and this step will save time and effort. Especially on HEUI engines, check oil level and smell for fuel. This step will also help determine if you are dealing with a fuel system problem or some other issue. For instance, a low power complaint can often be found to be a split boot or loose or broken clamp in the charge air system.
• Check for codes, even if the engine light isn't on. Even a stored code can give a clue to the problem, but you can’t repair merely from code information.
• Check filters. Spin off the primary filter and dump it into a clean container to check for water and contaminants. If it is empty or very low on fuel, there may be a restriction or leak between it and the tank. 
• Don’t bother running the engine on ether, as all this proves is that the engine has some compression. If the engine was running recently, you already know that. If a HEUI system is run on ether with a low oil level, the high pressure oil pump will be damaged.

• Check the fuel pressure with a gauge. You must know you have fuel pressure to spec, with no air, to proceed with diagnosis. The fuel pressure check is one of the main tests to do to confirm or eliminate a fuel problem causing a no start. Many wiring and sensor issues can cause a crank but no start.
• Check for air being sucked into the system. You can install a piece of clear hose in the return or remove the return line somewhere and run it into a bucket of fuel. An occasional bubble is ok, but a steady stream of bubbles means air is entering the suction side of the system. On a low pressure system you may be able to install your clear hose after the transfer pump, but it must be installed after the suction side. Air can be sucked in a bad filter, loose fittings, pinholes in lines, or a tank pickup tube. Usually these won’t leak fuel but you may be able to pressurize the tank a little and find it.
• On HEUI systems, always check engine oil level. If the oil is significantly low, the engine won’t start. ICP (injection control pressure) must be checked, and typically about 500 psi is needed to start the engine. The only realistic way to check is with a scantool on the datastream menu. Low ICP should set a code, but the problem could be the pump, sensor, or an internal leak like injector o-rings. Sometimes if you remove the valve cover and run or crank the engine a leak can be found. If the harness to the ICP regulator on the pump is disconnected, the ECM will try to run the engine with a substitute value. If it runs, the high pressure sensor or circuit may be faulty.
• On HEUI systems, check for oil in the fuel. If the fuel is black, o-rings on one or more injectors are bad. A high engine oil level with fuel contamination is bad injector o-rings or injector. 
• On light duty vehicles especially, smell the fuel when you remove a filter. Drivers sometimes don’t want to admit, remember, or realize that they filled up with gasoline. 
• To get any further than this, you must know exactly how the particular system works and what each component does. For some problems a wiring diagram will be needed. You also need to know when to give up and seek professional help. Replacing parts and hoping to get lucky can easily cost more money than paying a shop to fix it.

  Thanks for reading, keep watching for an upcoming post on ABS

 Bruce