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MB Paul
MB Paul, Diagnostic Technician
Category: UK Car
Satisfied Customers: 4316
Experience:  Qualified Mercedes Benz Diagnostic Technician - 20+ years experience - Workshop Foreman for Mercedes
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My neighbours Mercedes Benz c220 cdi Diesel will not start.

Customer Question

My neighbours Mercedes Benz c220 cdi Diesel will not start. This has gone on for many months and has beaten the MB dealers and all assorted geniuses who tried to repair it. The engine turns over perfectly, but evidently the fuel is not being injected. We have tried all the usual causes. The car drives perfectly when it occasionally starts. On a nunber of ocassions, when mechanics were working on the car - it started afterwards, and was OK for a few days. Heater plugs, injectors etc have been changed. One theory was that there was a voltage drop to the CPU, so a relay was fitted. This did not help. Regards, XXXXX XXXXXXXX@XXXXXX.XXX 00353872366388
Submitted: 5 years ago.
Category: UK Car
Expert:  MB Paul replied 5 years ago.

Hi Robert,


It sounds like most of the usual items have been changed, I would first of all check for fault codes to see if any codes have been stored which may indicate a problem with a component.


Usually this kind of fault is down to low pressure in the fuel rail which will prevent the engine from firing up - this can be confirmed with a diagnostic too reading the pressure sensor while the engine is cranked over - if it is not reaching the required pressure set by the ECU then the engine will not fire up.


All the low pressure lines should be inspected - you have said the O rings have been replaced but have all the seals been replaced on all hoses? If not then I would ensure all are replaced.


The fuel filter can cause problems & we have many problems with non genuine filters installed, so would ensure a brand new Mercedes Benz fuel filter is installed.


Injectors are another problem - were new injectors installed or used ones?


There are also seals on the fuel regulator valve at the end of the fuel rail - if this has not been inspected then I would replace the seals on this.


If all of the above checks out ok then the next step would be to have the pressure from the high pressure fuel pump checked - Mercedes has a special tool to connect up to check this.


There are of course many other issues that can cause the problem but would have all of the above checked first.


Let me know how you get on.


Best regards, Paul

Customer: replied 5 years ago.
All the above items/issues have been checked. If there was anything obvious, the car would not start and run perfectly - occasionally. As there may be a voltage drop to the ECU, a relay was fitted and the car worked fine for 8 weeks afterwards. Then it resumed its old ways. However, a friend of mine hit that relay with a screwdriver as the car was being turned over.....and it started! That was yesterday. As the relay is sealed and does not look new (fitted by "professionals"), we are going to fit a new one on next Tuesday. Perhaps bad contacts. I apologise for taking up your time....but perhaps the above will be of use to you. Now if only the MB could be as reliable as my Nissan Primera. The MB dealers are rip off merchants. Most mechanics I dealth with were silly, but thought they were hearth surgeons. XXXXX XXXXX.
Expert:  MB Paul replied 5 years ago.

Hi Robert,


You would think that it would not be like that intermittently but in my experience most problems where the engine cranks over but does not fire up it is down to low pressure in the fuel rail! It almost always only causes non start issues & unless severe the engine will run & maintain rail pressure just fine once it is running.


The only way to be sure is to actually monitor the rail pressure when the fault is present, which can obviously be difficult if the fault does not occur often.


Usually if there is a voltage issue to the CDI ECU then the engine will not actually crank over as the ECU sends a signal for the starter to turn.


Has the vehicle been checked for codes? Another issue for cranking over but not starting can be a faulty camshaft sensor on these engines - but this will usually store a code.


Have any codes at all been recorded do you know?


Has the garage been able to reproduce the fault?


Have all seals on the fuel lines & on the regulator in the rail been replaced?


Are any air bubbles present in the fuel lines or are there any signs of fuel leaks?


No need to apologize & if I can help in any way I will be glad to! It can be frustrating for both the owner & techicians when a fault is intermittant!


Best regards, Paul

Customer: replied 5 years ago.
Dear XXXXX, As the car is now starting - provided that the fitted relay is given a thump (!) beforehand, we will fit a new relay on Tuesday. The MB dealership could not find any stored codes which account for the problem. Frankly, they are not that bothered, as they still get paid by the hour for not performing. The car has been checked out by the the usual assortment of local "geniuses", but the relay fitting, worked for 8 weeks. I will update you on Wednesday re. progress. Regards, Robert.

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