If this is the G9T common rail diesel engine, there are a number of areas of concern including, the T.D.C. sensor, the glow plug system, the E.G.R. (exhaust gas recirculation) system, the low pressure fuel pump, the high pressure fuel system, fuel cooling system, as well as the usual electronic computer systems and sensors, so if the engine management warning light comes up on the instrument display, you must get a diagnostic done first, so you know what fault codes are being recorded. Whilst the diagnostic computer is connected, you should also take a look at the running data for the engine, assuming the engine will run. You need to look at the fuel pressures in particular, because these engine require an extremely high fuel pressure and if the fuel pressure is low, it will immediately cause problems.
In an old conventional diesel with mechanical injectors, normal low pressure supply and expensive mechanical injection pump with internal cam, the fuel pressures were in the region of 185 p.s.i. and as the injection pump approached the timing point and reached those pressures a particular valve was opened, and that particular cylinder injector needle would lift off its seat and spray the diesel fuel into the cylinder at the timing point which would then be ignited by the very hot air from the high compression. Once running, the diesel would run without any electrics until the fuel supply was cut off. With these new extremely high pressure computer controlled diesels, the system is considerably different.
First you have a low pressure pump to deliver the fuel to the high pressure pump and this does nothing more than raise the fuel pressure to anywhere between 300 bar and 1400 bar! Yes that is bar not psi. So we are looking at around 440 psi to 21,000 psi (in fact in the smaller K9K diesel it can be 1600 bar or 23,500 psi at the top end) These high pressure pumps no longer have to be accurately timed to the engine crankshaft as all they are doing is supplying the common rail with a constant supply of high pressure fuel, and the release to the right cylinders at the right time, is now controlled by the injection computer and its sensors. In achieving this extremely high fuel pressure the fuel gets heated and has to be cooled, so the fuel is continually circulating through a cooling system with a temperature sensor on the fuel filter housing that routes the fuel as necessary. So if it is too cool as when first started the fuel will not pass through the cooling pipes, but once it reaches the optimum temperature it will now circulate through these cooling pipes.
The injectors are complex electronic things now, coded to the computer and cylinder so you cannot switch them around at all, and unlike the simple spring loaded needle valve type, the latest electronic ones even use piezo-electric stacks to do the lifting of the needle off its seat, which actually allows up to five discrete openings per cycle to smooth the power produced. In the old mechanical system it was that one large injection that produced the distinctive diesel knock from the impact of the large burn. With five discrete injections the power produced is spread over a timed period and is both quieter and smoother. However you can imagine that to get five discrete injection spread around the timing point in a minuscule amount of time requires absolute computer precision combined with the very high pressures. So electrical power is always necessary to keep these modern diesels running all the time.
They require 250-300 bar simply to idle, so if the fuel pressure is too low they will either not start at all or run very erratically and often cut out again. The low pressure pump is often suspected if the fuel pressure is low, as it is not delivering sufficient fuel to the high pressure pump. Another possibility is the high pressure valve on the common rail if it cannot maintain the correct pressure in the common rail. A third possibility is excess fuel leak off from the top of the injectors and there is a tool for measuring the amount of the leak off. A faulty fuel temperature sensor causing either overheating of the fuel or incorrect routing of the cooling system will cause problems. Finally if the high pressure pump starts to break up, the hardened pieces will get into the fuel system and the whole fuel system has to be replaced! You can now see why a diagnostic is the first step to give you some idea where the problem lies.
If you have a misfire, with the old mechanical system you would slacken each injector pipe in turn to find out which cylinder was not producing the power. This was the equivalent of pulling a H.T.Lead off a spark plug in a petrol engine. YOU MUST NEVER DO THIS with a common rail diesel: a) because of the extremely high pressures involved and b) because all it would do is drop the pressure in the common rail and would prove nothing.
Easy checks and possible repairs
Sometimes difficult starting or erratic running is just caused by too big a TDC sensor gap which needs resetting. You have to be able to get underneath the vehicle as the sensor is right at the bottom of the flywheel, but resetting it is easy. The sensor is held by a spring clip that digs into the side of its body. You press the clip inwards to release the sensor and then push the sensor in until the end touches the flywheel. Now slowly release the clip to hold the sensor in this new position. Once the engine is spun over the gap will set automatically.
Sometimes the sensor can be faulty or its wiring or plug connection is the cause, but again fitting a new sensor is easy as you release the clip exactly the same as in the resetting procedure and withdraw the old sensor and fit the new one. If the wiring or plug to the loom is faulty, replacing those are the same as any other electrical wiring, but make sure the polarity is kept correct, and don't necessarily follow the colours on the wiring insulation as they have been known to be the wrong way around!
If the glow plugs are indicated as faulty, these are quite simple to check if you have an ohmmeter. Simply pull the electrical connection off the top of the glow plug (which is a push-on clip on connection) and test the resistance from the top positive connection to earth. A good plug should be around 1 ohm (± 0.1). Anything else means you need to change it. Check all four and replace only those necessary.
Back to top
