CLK320 Auto Gearbox query

[barney1]

Hi all,
I have a CLK320 Convertible and a week before Christmas the auto gearbox threw a fit and decided to self destruct To cut a long story short I bit the bullet and after a month searching the net found an identical box at a salvage yard. By February it was back on the road after my local garage performed a straight forward transplant and carried out a full service and MOT at the same time. So far so good and it was a relief to get my car mobile again. The 'new' box is perfect and changes gears a lot smoother than the old one. However, my fuel consumption has dramatically changed with the new box and was wondering if anyone can shed any light on the cause. For the past two years the car has returned 25 mpg at every fill up at the petrol station but since the new box was fitted the last 5 tank fulls have returned 19 mpg on average (the worst being 18.74mpg). My use of the car is exactly the same ie, same commute journey and normal use, the gearbox mode is in standard and not winter and I haven't turned into a boy racer overnight! The only slight difference I can tell is the box seems to holds the gears about a second longer than the old one. With all the electrical brains that are fitted to the cars nowadays is there anything the STAR system can diagnose and can any adjustments be made? Many thanks in advance, Steve

 

[mauno]

yes.star can adaptgearbox settings

 

[BlackC55]

Re-setting your adaptions won't give you any better fuel economy.

I suspect you have a faulty labda sensor or air mass. The live data on star needs to be checked.

 

[mauno]

I have a beer on your statement c55 ,it does change mpg

 

[mauno]

but on the other hand fuel eco is that bad then must be something else too.

 

[barney1]

Thanks for the quick reply guys. Interesting that you mention the lambda sensor C55 - the car flew through the MOT with no advisories (13 yrs old and 124,000 miles on the clock), however, the mechanic did mention he had trouble with the emission figures and it took a while for the engine to warm up before they dropped to an acceptable level. I am guessing if this is the case then it's just a pure coincidence the sensor has failed the same time as the gearbox! Is there a chance the sensor was disturbed when the old box was removed? I think the exhaust system (or at least part of it) was removed to facilitate the exchange. I have limited knowledge on mechanical stuff but a quick google suggests the sensor measures the gas in the exhaust but i would suspect this is nearer the manifold. Cheers, Steve

 

[BlackC55]

I have a beer on your statement c55 ,it does change mpg

Prove it! How can it?

 

[BlackC55]

A lambda sensor measures oxygen content in the exhaust gas. If they are s bit lazy then than can effect emissions and therefore fuel economy. Get it checked with s Star firstly. You have more than one lambda. Poss 4 depending on age of the car. Get either Bosch or genuine ONLY.

 

[mauno]

do I really have to?

 

[BlackC55]

do I really have to?

Yep!

 

[mauno]

ok, on the autoboxes I have reseted I had a feedback of better mpg. this was probably earlier sporty adaptation learning being reset to factory and then adapted to grandpa style.i dont think just alone the fluid change makes that much difference.also I have noticed the mbar value difference on the sequences which makes sense to me

 

[barney1]

A quick update and to say thanks to C55 :bannana:

Over the weekend I decided to get under the car and check out the lambda sensors if only to satisfy my curiosity and see what they look like and if it was possible to try to change them myself. I found them quite easy and it's obvious the exhaust has been removed to exchange the gearbox and the lambda's disconnected. Both sensors had a blue wire coming from them and clipped into an electrical socket up the back of the engine. At this point I realised the driver side connector was sticking out about 5mm more than the other one!! I pushed it in with my finger and heard a satisfying 'click' as it seated properly. So far on half a tank the MPG has improved by 3MPG so hoping the next tankful is back to normal. Just thinking out loud - if the sensor wasn't connected why didn't the electrical wizardry pick this up and send a fault code?

Many thanks again,

Steve