Symptoms
The engine does not start correctly at times, has long startup times (regardless of the engine temperature), it is slow to accelerate in the upper speed range. There is a tendency to misfire while driving at a constant speed.
Cause – insufficient pressure in the supply system
"False" air is sucked into the air supply through leaks in the system (fuel does not have to escape through these leaks). It could also be: pinched fuel line, clogged fuel filter, with the wrong fuel (excessive addition of gasoline to diesel fuel, in winter), obstructed outflow of fuel to the tank, damaged fuel pump in the tank, damaged distributor injection pump or CR high pressure pump. In the case of the Common Rail system, the pressure in the fuel tank may be too low (the fuel pressure sensor in the tank sends erroneous signals or does not work at all). In the case of an injection system with unit injectors – tandem pump pressure relief valve does not close.
Troubleshooting and fault elimination
Visually inspect the fuel system for leaks: rigid and flexible cables (including hose clamps), fuel filter with preheating valve, fuel level sensor. attention: leaks often appear after service work! If you suspect leaks, you can place a piece of transparent tubing in the discharge line. Air blisters up to the size of a fingernail are not dangerous. To check tightness, pressurize the supply system, always using a pressure limiter (max. 0,5 only)! Supply systems that are not self-venting must first be vented manually. Filters and lines must be completely filled with fuel. Introduce compressed air at the fuel tank through the fuel supply line, plug the return line in front of the tank. If there are no obvious leaks, to check, whether the pressure is maintained for at least 15 my (no pressure drop is allowed).
Then we check the fuel pressure in the supply system: in the VE pumps, the pressure is checked at the fuel outlet to the tank, at the "out" screw (guide value at idle >4,5 only, increase to approx. 8-9 barium). In an injection system with unit injectors, the pressure in the outlet line of the tandem pump must be approx.. 4000 rpm at least 7,5 only. When it is smaller – disconnect the return line. If the pressure is still below now 7,5 only, then the tandem pump is defective. Whereas, when, after disconnecting the return line, the pressure is not less than 7,5 only, tandem pump is ok, but the unit injectors are internally leaky (o-ring that seals the inlet against the drain). The pressure in the fuel tank of the Common Rail injection system is checked by the self-diagnosis method using the measurement values or with a high pressure tester, when attached to the pressure sensor bore. In addition, the low pressure circuit must be checked, that the discharge and return pressures are as required (attach a standard pressure gauge).
Gasoline in diesel fuel: too much gasoline (e.g. >10%) leads to the formation of vapor bubbles and, consequently, to misfiring, until the engine comes to a complete standstill. Usually it is like this, that by the time the roadside assistance service arrives, the engine will have time to cool down and the fault cannot be found. For this check, a sufficiently long test drive must be carried out.