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 Troubleshooting
kfz-tech.de/PDM4
Yes, we start with tests in the high pressure area, although you might prefer to check the low pressure first, following the structure of such a system. At least with an electric pump, you can
listen or, for example, measure the pump relay to see whether it is even running. You could also look at the return flow, but this will not help you identify an unfavorable control of the delivery
pressure.
If the delivery pressure is easy to read with your instruments, then do it. 3 to 5 bar can be achieved depending on the data sheet. What kind of defect is assumed? Quite simply, the common
rail diesel does not start despite the starter motor turning briskly. Compression does not usually disappear that easily. So it was still there yesterday, so you don't actually need to measure it at
first today.
If you insist, then only the starter current during the individual compression strokes. We not only want to get to the cause quickly, but also (dis)assemble as little as possible. Definitely not glow
plugs or injectors. But also don't open any lines if possible. And if so, then only beyond the area from the fuel filter to the injectors, the pressure limiter or the return line(s).
Why are you so cautious? Firstly, the effort involved, secondly, the necessary pressure gauges up to 2,500 bar and thirdly, the protection against contamination by even the smallest dirt
particles. There is hardly anything worse when it comes to repairs than making further mistakes in the process. Even if these particles are stopped by a sieve and cloggs before they reach
one of the injectors, you have to search endlessly.
So far we haven't achieved anything, but now we are heading straight for the object of our desire, the rail pressure sensor. This is supposed to fix everything, and is best read out via the
diagnosis. The engine that won't start doesn't care whether the low pressure is generated electrically or mechanically and how difficult it is to measure it. It wants, let's say, at least 180 bar of
pressure in the rail, which must not drop during injection due to insufficient delivery.
Shall we bet that you have already chosen the right place to start your troubleshooting? If these two conditions are met and the engine still doesn't start, then either the rail pressure sensor
itself is telling you something that isn't there, or the fuel in the rail isn't reaching the respective combustion chamber. We could also check the signals from the injectors now, but we're opting to
look at the return flow quantities.
Why open the pipe system now and not stick with the electronic measurement? Firstly, you can't do anything wrong with the return lines and secondly, the incoming signal says nothing about
the actual opening or the tightness. So what do we expect? Regular, slightly dripping returns, incredibly small quantities. If nothing is dripping somewhere, then perhaps nothing is getting there
or the injector is open. Incidentally, you can even use this method for a short time with a Piezo system with 10 bar back pressure.
The rail pressure sensor remains for now. If we don't believe the diagnosis, we can look at its signal directly. Does it really go from zero shortly before starting to the value that is ultimately
displayed? We can use the oscilloscope to check whether it makes any movements that would indicate a defect. Of the pins, the one that is considered is the one that gives 0 V to the engine
ground. The signal voltage should be at least 1 to 1.5 volts, depending on the manufacturer.
The only remaining possibility is the much more likely assumption that the required rail pressure is not reached during starting. Pressure could either not be generated or could be lost.
Assuming the latter, the high-pressure valve is suspected. Fortunately, it is relatively easy to check that it is closing by checking its return flow. Since its governed values are far from being
reached during starting, no fuel should leak out here.
Before replacing it, however, you should check whether a signal is being received at all, because if this is not the case, a valve like this can open even at low pressure during starting. If you
have not found the fault yet, it could be a bit more expensive than initially assumed. A last resort before replacing the high-pressure pump would be to check its activation piloting. You could
also inquire about possible partial purchases for your pump, such as the quantity control valve, and check these.
But before you go to all this trouble and spend a lot of money: it could be that the high-pressure pump is not getting enough fuel. The simplest solution, for example for a petrol injection, is to
look beyond the pressure circuit. You would have to get the return of the low-pressure circuit, only that one. If enough fuel is getting through here, then the only thing that can be wrong is the
pressure control.
Regardless of whether the pressure in the high-pressure circuit is regulated or not, you could install a simple check valve or even temporarily bend the line and see if the engine then starts.
Otherwise, the amount per minute without starting the engine would have to be determined and compared. If this is not achieved, the fuel filter, the lines and/or the low-pressure pump come
under suspicion. If this is achieved, then you have our regrets because the high-pressure pump is very likely defective.
| There are also repair kits for common rail pumps. |
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