How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Mike Your Own Question
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.
Type Your UK Car Question Here...
Mike is online now
A new question is answered every 9 seconds

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: 6 years ago via AskTheMechanic.
Category: UK Car
Expert:  Mike replied 6 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


  • >>>>>>>>>Please don't forget ....Please press the ACCEPT button if you are happy with my answer/advice given - as that's how I get paid for my time, knowledge and expertise given. Thank you.


Feedback and any Bonuses are always appreciated too. Thank you.

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.
Mike and 3 other UK Car Specialists are ready to help you
Customer: replied 6 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 6 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..

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
< Previous | Next >
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
  • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
  • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY
  • Thank you so much for taking your time and knowledge to support my concerns. Not only did you answer my questions, you even took it a step further with replying with more pertinent information I needed to know. Robin Elkton, Maryland
  • He answered my question promptly and gave me accurate, detailed information. If all of your experts are half as good, you have a great thing going here. Diane Dallas, TX

Meet The Experts:

  • Marty


    European Car Expert

    Satisfied Customers:

    ASE & Bosch Certified Master Tech. Over 30 years experience.
< Last | Next >
  • Marty's Avatar


    European Car Expert

    Satisfied Customers:

    ASE & Bosch Certified Master Tech. Over 30 years experience.
  • Greg's Avatar


    Shop Owner

    Satisfied Customers:

    17 years Shop Owner /Technician. 27 years experience,
  • Richard's Avatar


    UK Car Mechanic

    Satisfied Customers:

    12 years as a technician at a Ford and Jaguar dealer Ford and Jaguar factory certified
  • Matt's Avatar


    Mechanical Engineer

    Satisfied Customers:

    BEng hons Mech engineering, in auto industry 22 years
  • Trevor's Avatar


    UK Car Mechanic

    Satisfied Customers:

    NVQ 3, Modern Apprenticeship, MOT Tester, 15+ years experience diagnostic technician
  • Rob's Avatar


    UK Car Mechanic

    Satisfied Customers:

    city and guilds level 1 2and 3. mot tester , 28 years experience set up training college in afghanistan
  • martin jones's Avatar

    martin jones

    UK Car Mechanic

    Satisfied Customers:

    25 years as a mechanic and mot tester, owned own business for 10 years city and guilds trained