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Mike, UK Car Mechanic
Category: UK Car
Satisfied Customers: 39198
Experience:  Hi, I am a Car/ vehicle technician of 34 years. 29of -running my own repair/diagnostic business.
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I have a 320Cdi 6 cyl Merc..diesel..fitted

Resolved Question:

I have a 320Cdi 6 cyl Merc. diesel that consistently uses more fuel than it ought to. I have already fitted a new Air Flow meter. Interestingly the only time it gave good consumption was when the outside temperature was very high (28 degC).
What should I look at next?
Submitted: 7 years ago via AskTheMechanic.
Category: UK Car
Expert:  Mike replied 7 years ago.
Hello and Welcome to justanswer...

There are of course many reason that any given vehicle can have poor fuel consumption and it doesn't take much to send it off...

Reasons can be from any one of the sensors affecting the engine management system which has a detrimental effect on running, even un felt - but enough to cause trouble with mpg... to problems with the brakes on the vehicle slightly binding on one of the wheels - to problems in between with the tyres... pressures , tread , even tyre type... and even to usage, terrain , where you live - how its used, driven etc etc..

A lot of the above you can probably rule straight out but do think about them as there may be one or more having an adverse effect..

Going back to sensors on the engine - most will put the engine management light on if there is a fault with any one of them - but one of them won't always - and that is the ECT sensor... This is the Engine coolant temperature sensor..not the one for teh gauge - but the one that sends signals to the cars Ecu.. main computer - re. the temperature of the engine.

The Ecu then sends commands to the fueling and ignition systems according. If the sensor is faulty and thus sends incorrect , or no info to the ecu as above, the fueling etc will be incorrect and thus do as you describe resulting in poor mpg.

If you remember the old auto and manual choke systems.. (on petrol engines - but the principal is the same ) works like that , only electronically.. If and when the choke was in the wrong position for the temp of the engine, the engine performance would suffer and if it were not adjusted right , slightly "On".. then increased Mpg ....- exactly the same.

So, without the aid of plug in diagnostics at the moment and the results thereof - its well worth renewing that sensor. Its usually located on the front right of the engine head - under the plastic cover... I will give you a pic / drawing below which shows its location for you ... identified by a red dot !


Other than the things mentioned above which should all be considered - I would suggest renewal of that sensor and try it from there... IF you were still to have trouble after that , then proper diagnostic tests would need to be done to check the rest of the engine management system through.... but the Ect sensors are a common issue....


Here is the drawing / location for you for the Ect sensor..






I Sincerely XXXXX XXXXX have helped you - Please ask me if you need more help...

Kind Rgds, MIKE


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Customer: replied 7 years ago.

Thanks for the reply. I had already asked the garage to fit a new thermostat and it seems the ECT sensor is integral with that.

The result has been an increase in the running engine temperature to around 95 Deg C and an improvement in the low speed consumption, say speed not exceeding 50 mph, but the consumption over this speed remains conspicuously high.


I am beginning to believe that the car is incapable of decent mpg and were it not for the fact that I own an older model with virtually the same engine would simply sell.


Do you know what other sensors feed back to the ECU?

Is it possible to have the ECU re-programmed, that is the same program uploaded and re-written?


Many thanks

Expert:  Mike replied 7 years ago.
Hello again.. My apologies - I have only just had your reply as above...

Regarding what other sensors feed back to the Ecu - They all do essentially either directly or indirectly... so that aspect wouldn't really be helpful...

It is possible to have the Ecu reprogrammed on most yes depending on the exact system.... you would need to speak to your local Benz dealer about that its done with their diagnostic equipment...

The other thing that may be worth considering is the likes of a plug - in "tuning box"... you can get these from many places these days - but have a look at this company HERE...

Speak to them, tell them you are after more economy rather than extra power as there are different systems.... Im sure they will be able to help you - its worth speaking / contacting them anyway..

I hope that helps you further...

Best Rgds, MIKE..