Engine Cuts Out - Ford C-Max Forum
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post #1 of 4 Old 09-28-2011, 12:23 PM Thread Starter
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Hi we have found that our C Max Zetec 2008 1.8TDCi diesel that we bought at the beginning of this September has an alarming tendency for the engine to die as we slow down after driving for some time (about 2 or 3 hours) at speed on the Motorway. As we slow to stop (for a line of cars, traffic lights on a slip road, or a peage, the engine simply cuts out and is reluctant tow start again. No warning dashboard lights show. This happened on A6 in the south of France and we had to be towed off the motorway, and also on a slip road coming off the M20. A Ford garage in France could find no fault nor the dealer who sold us the car. Has anyone else experienced a similar problem. This is scary and we are not crazy about getting in the car again!

C Max 2008 Zetec 1.8 TDCi 5 dr diesel
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post #2 of 4 Old 09-28-2011, 03:29 PM
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If it only happens after slowing down after a long period at steady (Motorway) speed, it would seem to point to fuelling. With conventional mechanically controlled diesel engines, the mechanical governor in the fuel pump would cut the fuel off completely "on the over run" when the engine was slowing down, reinstating it when the engine speed dropped below a predetermined value, generally about 850 rpm or idle speed. Modern electronically controlled engines, both diesel and petrol, do the same and, with my 2.0 TDCi the fuel "cut in" point when the car is moving is about 1000 rpm and is quite noticable as the car will quite happily trundle along with no throttle input after that, in the lower gears at least, given the right conditions. It would appear that your engine is having its fuelling cut, but not reinstated when it's slowed down. It could be an ECU fault or, more likely, a problem with the engine speed sensing element which may be giving false readings after having been at a high temperature for a prolonged period. If the engine is OK after it has cooled down, and in normal day to day use, this may well be the case. It's not unheard of for electronic components to throw a hissy fit due to prolonged heat exposure and then be perfectly normal again when they've cooled down. This may be why no fault is showing up when the system is interrogated by the diagnostic machine. If the speed sensor is telling the ecu that the engine is going faster than it is, it will continue to cut the fuel, as that's what it's programmed to do. This is a bit of a stab in the dark but that's how it seems to me.

2008 2.0 TDCi Zetec
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post #3 of 4 Old 09-28-2011, 05:19 PM
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The ECU compares this voltage to that in the fueling map for the particular
engine speed and load and adjusts probably the injector time accordingly.
Depending on which way the voltage changes (presume increase for
accelerating) it will increase the fuel for the accelerator pedal being oushed
down and vice versa so I don't think there is an exact "cut in point"

I think I would check the resistor to the accelerator pedal

The other thing that caused a similar problem with our previous petrol
focus was the road speed sensor having an intermittent fault that allowed
everything to work normally till you slowed to a stop and then it cut out

54 Plate 1.6 TDCI LX ~aspiring to be something else
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post #4 of 4 Old 09-28-2011, 08:03 PM
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I was going to mention the road speed sensor as a possibility, but wasn't sure if it actually fed info to the ecu on the 1.8 engine. Again, if this particular sensor is telling the ecu that the vehicle is travelling faster than it actually is, then the fuelling will be decreased below what it should be. It is, after all, merely a very sophisticated electronic engine governor!
Regarding the fuel "cut in" point, It's just something that I have noticed when coasting out of the last of the 40 mph limit in 5th gear and into the 30 mph limit. Because it's slightly downhill I can actually feel the engine begin to drive again at about 1000 rpm. I then have to change into 4th, because the engine appears to not want to drop below 1000rpm it would have me speeding, even with my foot not on the accelerator! It does this in other gears as well so I assume it's normal. My last two cars, both Citroen 2.0 HDi s, behaved in a similar fashion.

2008 2.0 TDCi Zetec
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