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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22920
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have a Merc mlin U.K. I bought the car in 2013 the car was

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Hi there. I have a Merc ml(###) ###-####in U.K. I bought the car in 2013 the car was giving me approx 25mpg in town and 33mpg on motorway. Combined long term mpg was on 27mpg according to the computer. I have always been servicing the car twice a year oil, air filter and fuel filters but the fuel performance has been steadily going down. It is now giving me 17-19mpg urban and about 25-27mpg on motorway. Can someone help me by diagnosing the cause of this ?


I'm assuming that you don't have any check engine or other fault lights on so

This could be a number of things:

Incorrect fuelling due to a faulty water temp sensor. If this is the problem then the ECU will be leaving the car constantly ' on choke' and not reducing the fuelling as the engine warms - so unplug the sensor and clean the contacts on both sides with some switch / electrical cleaner (not WD40) and inspect the wiring for any signs of damage to the insulation especially where it runs over other components and through the bulkhead- if this doesn't help consider replacing the sensor as they're not expensive

Check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

One other thing is to ensure that you're running on the correct grade of engine and gearbox oil as if its too thick or overfilled this will really hurt the fuel economy

Binding brakes:

Check that all the wheels can be turned freely by hand, and free off any sticking calipers

Incorrect wheel alignment:

Check all the tyres for signs of uneven wear if it has any then get it checked out at a place that does 4 wheel alignment not just front tracking and ensure tyres are pumped up to their recommended maximum

Injectors are clean and not dribbling - add a bottle of injector cleaner to the tank

Use good quality fuel - from my own findings its worth paying the extra for BP or shell as typically you'll get another 10% fuel saving which outweighs the cost over supermarket fuel.

Customer: replied 1 year ago.
Hi Matt. Where can I find the water temp sensor on my car ? I also read that it could be the the MAF sensor but I couldn't clean it as the sensor was not visible in when taken out ?


the coolant sensor is located close to the top hose of the radiator

follow this back to the engine and look in and around that are where the hose joins the engine

and yes a dirty MAF sensor can give this issue

if your sensor looks like the one in the image (attached) then spray some contact cleaner into the little square port at the bottom of the unit

and its important to use proper switch / electrical cleaner not WD40 or other disperents

Customer: replied 1 year ago.
Hi Matt. Sorry but are we talking coolant sensor or water temp sensor or are these the same things ? Also didn't get your attachment...Also I have problem with electrics (main door not closing; car thinks that doors are open when not: tailgate bulbs fusing; and parking sensors working Intermittently due to low voltage in battery. I know I need to change the battery as I had the voltage checked and it was very low and not hanging enough even though voltage was feeding through.
Customer: replied 1 year ago.
could that be affecting various sensors not to work properly and dish out max fuel to the engine unnecessarily ?


coolant temp = water temp , the same thing, and the image is now hopefully attached

and you have it correct, a low battery voltage will offset all the sensors

so a faulty battery that can't maintain at least 12V will give you all sorts of issues

Customer: replied 1 year ago.
I think this is the MAF sensor. Can I just pull it out and clean ?
Customer: replied 1 year ago.
Is that the Coolant sensor ?

you've identified the MAF correctly but the other sensor is the header tank level switch

and looking at the MAF sensor you have I'd say its the same as the one I've sent the image of but its also the type thats held in with a spring clip so this metal clip needs popping off to allow you to withdraw the sensor element

Customer: replied 1 year ago.
Ok great. So once I take out the MAF sensor where do I spray cleaner spray ? Also still haven't located the coolant sensor....could you send a diagram or something thanks


if you look at the 1st image I sent once the MAF is withdrawn from the housing there's a square port where the air enters

this is where you spray the cleaner

and this engine image should help

item 85 is the coolant temp sensor

Customer: replied 1 year ago.
Thanks Matt. Do you know if the below battery will work ?Bosch S5 Car Battery Type 019


thats certainly powerful enough however I think you need a size spec 049 not a 019?

Still if you can get it to fit in the available space then there's no issue with fitting as big a battery as you can

Customer: replied 1 year ago.
HI Matt, i just thought ii just thought i would check the voltage before i go ahead and purchase the battery. I linked up the sinometer: positive to positive and the negative to the body of the car and the reading came up to pretty much exactly 12v. I did this when the If the voltage could there be anything else wrong with the battery that i could easily check ?


to test a battery properly you ideally need to put it under some load

unfortunatly just measuring voltage with a multimeter will only show how 'full' the battery is

whereas measuring current , measures the 'flow' out of the battery

to test it you need a load tester like this

most car electrical shops / specialists carry one of these and will usually test your battery for free - especially if you are in the market for a new battery

Matt and other Mercedes Specialists are ready to help you
Customer: replied 1 year ago.
Great..thanks Matt