Hi guys.

I had to change the MAF about 3 weeks ago as my old one failed, I bought a cheaper non genuine MAF first of all which to begin with was fine and the car ran great but I noticed the MPG was lower then I used to get. Unfortunately this MAF failed after 3 weeks and so I took it back to the store for a refund and this time paid the extra and got a genuine BOSCH MAF.

However my MPG with both the MAF's was lower compared to how the car was before the orginal MAF failed on me.
I used to get on the highway anything up to 38 MPG and in town around 23-25 MPG. Now however the highway MPG is at best 30MPG with the town being around the same as old at 23-25MPG.

Can anyone suggest why my highway MPG has reduced? Does the ECU take time to learn the new MAF as its interesting how both the new MAF's were lower MPG then before?

Thoughts:
I got a diagnostic done to prove the original MAF was bad and when I replaced the new MAF I got all the codes cleared too.
Im not thinking its the fuel filter as its only been changed in the last year .
The spark plugs are denso irridium and i've had them for a about 3.5 years? The air filter is K&N and I have recently cleaned this out and purposly not oiled it.
Could a bad oxygen sensor give lower MPG?, but its a bit of a coincidence how on changing the MAF instantly the MPG has changed? so doubtfull.
Was getting 38MPG on my old MAF strangley high but perhaps happened because it was old and failing?

Any advise appreciated.

Thanks

 

You need to ask why the original failed.

Start by clearing the adaptations stored in your engine controller. Then look in your wiring harness for signs of oil contamination.

You can't just change a MAF.

 

Sorry but Im confused at what your getting at here?! Im not sure why you said " You can't just change a MAF" YES you can as the nature of a MAF is that its a a hot film sensor and measure the change in current that is altered by the varying airflow that passes over the plate that is heated slightly, hence a Mass Air Flow sensor. The device is simple in terms of its function and the cars ECU will then calculate the fuel and air mix respectively from the change in current. MAF's are not designed to outlast the car and as the original was 10+ years I think it did good. The reason it failed was due to a blocked breather pipe in my engine, which I have fixed the source of the problem but sadly from cleaning the MAF too many times it died.

I had taken the car for a diagnostic to get a print out of the bad MAF to take to the store to prove it was faulty. When I fitted the new MAF I also had all the codes cleared.

As for the wiring harness having signs of oil contamination, this is really odd suggestion as it would have no impact on MPG at all ? Wether the wiring harness was good or bad would not give MPG issues and why from just changing the MAF to the new would the wiring harness now be a cause of concern as it was fine before hmmm!?

Does anyone have clearer good suggestions.

 

Since I know nothing I will stop helping.

 

WHAT?

 

It will many cycles for your adaptations to get updated after changing this part.

How many miles have put on the car since the new mas?

Perhaps adaptations are not complete?


Another possibility is the old mas was bad making your car super lean causing unusually high mpg? Gl

 

This thread regarding throttle calibration and MPG might be of interest.
https://mbworld.org/forums/c-class-w...libration.html

 

Thanks, I have done the throttle reset before but only waited 2 mins, it was interesting reading that if you wait 3 mins people has said they here a click at the key, so perhaps I was not waiting long enough? I will try this again and see if it helps. It also said you need to give the car about 500 mile to learn the new throttle reset,

Thanks

 

I now have my mpg back to its normal amounts thanks to your advise on replacing the oxygen sensor thats before the cat . I bought a Denso sensor as they had great reviews and I have been using their irridium spark plugs for over 3 years now with great results .
The sensor was easy to replace and I found that the best access was from underneath the car that way I did not have to dismantle anything. One 22mm spanner and it came off really easy thanks to the copper ease grease, re-grease the new Denso sensor with the supplied copper grease and connect the plug at the top of the engine , done !
MPG is back into the 34-37 on highway and running around town its 22-26 mpg.

 

Agree w/ dlbehrns, WHAT ?

Yes, you can "just" change the MAF.
It's not a belabored job, but first would try cleaning it. I have original MAF (12Yrs.). Clean it periodically. Likely, that your original one throwing bad code could have been cleaned and plugged back in. Next time try cleaning it before shelling out the dough for parts.
Here's an instruction I put together about 50 years ago. It's for a 4.3L but you should get the idea. Of course, a 4.3 comes in at 22mpg, so, I envy your 30+ mpg., but I sure like my horses:

http://www.benzworld.org/forums/w208...ir-filter.html

 

Good instructions, but dont forget mt engine only has 4 cylinders and is about half the power of yours ! so 22mpg for you is ok especially as you can enjoy the POWER you have, plus if I had as cheap fuel as you US guys I would drive the biggest engine I could !!!

 

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You can't just change a MAF.[/QUOTE]