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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 20376
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have a Mercedes C250 TD, S reg estate. I drove it back from

Customer Question

I have a Mercedes C250 TD, S reg estate. I drove it back from where I bought it from which was a 7 mile journey. When I bought it the engine was cold and it drove home without fault. 3 days later I tried to start the car and it wouldnt fire. I have been turning it over many times and it just will not fire. It is not even trying to start and there is enough fuel in it to get it going. Easy Start has been sprayed into air intake and it still will not fiire. Even the AA couldnt get it going. The oil fiiller cap was removed while cranking over and a hand placed over the hole and hardly any suction was felt. Any suggestions? Thanks
Submitted: 2 years ago.
Category: Mercedes
Expert:  Matt replied 2 years ago.

I'd suggest doing a compression test next to see if the engine is OK and hasn't slipped cam timing somehow

if thats OK then I'd check that you're getting full power to the glow-plugs, they pull a lot of current so take care when checking but see if the relay is clicking when the ignition is turned on first and also check the fuse if this is OK check to see if the engine earth strap is also OK.

If the plugs haven't been changed in the last 20K then they may have to be replaced.

You can test the glow-plugs with a multimeter by measuring the resistance across the plug ( between the power connection and the body) it should be around 4-8ohms if its tending to infinity / open circuit the plug is faulty or by using a clip on current clamp, either on the supply wire to glow plugs or onto engine earth, with glow plug light on you should see 15-20amps for each glow plug fitted, 4 cylinders would be approximately 60-80amps.

Also its worth checking that the electric pump that feeds the main fuel pump is working correctly, it should run for a few seconds then switch off when you first turn the ignition on. If the connections to this are corroded or damaged then the pump could stop at any time. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse, ideally measure the voltage at the pump and check its getting 12V

Also if its not injecting at all then check the connections to the crankshaft sensor, unplug it and check for corroded contacts and broken wiring, and consider fitting a new sensor - do the same for the cam position sensor

If the EGR valve is stuck open then this too can effect starting so try disconnecting it to see if this helps or changes anything

Could also be an injector leaking fuel back through the overspill pipe, so not getting enough fuel pressure to tell ECU to fire the injectors, this is trickier to find and you’ll need the garage to do a ‘run back’ test to identify which one is the issue

Lastly it’s also worth considering fitting a new fuel filter as if only slightly blocked this can lower fuel supply pressure and choke the main pump, the pump will be more sensitive to this when its cold as diesel thickens considerably with low temperatures.

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  • 9 dollars, 2 hours of my time, and I drove away. Your diagnosis was right on the mark. Thank you so much. Phil
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