Low mpg return & poor auto gear change after exhaust replacement.

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I have a 2006 c180k Coupe (CL203), which was running like a Mercedes should. But recently it’s developed some sort of problem, causing bad mpg. It was 43mpg (ish) & now down to 35mpg (ish) which I’m finding hard to cure. It seems to start well & idle well. Although, the (auto) gear change is erratic as in it holds on to a gear longer than it did.

I’ve done loads of Googling, but all I find is conflicting advice. So I thought I’d post on here, just in case anyone else has had the same issue, or has some good advice.

Anyway, here’s the scenario. The car was running ok, but the exhaust was sounding a bit ‘fruity’. “Ahh, there’s a hole” I said to myself. This was confirmed by the MOT man when I got an ‘advisory’ for the back box blowing.

After much searching on the web, (Coupe exhausts are hard to come by!), I found a replacement back box. This was fitted in due course & everything was ‘normal’. However, after a couple of days the CEL came on & using my iCarsoft OBD II fault diagnosing tool, I got the fault code ‘P0171’ “System too lean bank 1”.

I know if anything is changed or replaced it can take a couple of journeys for the ECU to ‘re-learn’ & settle down, so I put it down to the fact the back box had been replaced & therefore didn’t have an air leak anymore. I cleared the fault. This fault has been re appearing near enough after every 30 miles or so & I’m flummoxed as to what it can be.

This is what I’ve done up to now:

I’ve cleaned the MAF sensor, (with the correct MAF cleaner). I’ve seen on some posts that the MAF on the 271 engine never goes wrong. Is this correct? Or might the cleaning I’ve done not worked? It didn’t look too bad anyway, to my amateur eye.

Whilst the air box was off, I had a check of the pipes/hoses I could see & didn’t see any obvious splits etc.

I’ve replaced both of the O2 sensors. Although the ‘downstream’ one was replaced before the exhaust leak. I’ve not taken it out & checked it, but the OBD II isn’t showing any O2 faults.

Is there anything else I should/could be looking at?

Shortly after I did the exhaust, I did let the fuel tank run down below a quarter. Not something I usually allow to happen & no ‘fuel light’ came on. I mention this as could some crud have blocked an injector & is causing the problem maybe? I’ve got the car booked in for a ‘Terraclean’ next week, so if there is anything in the system, I’m hoping that’ll clean it out.

So, after going around the houses, I’ve read that MAF sensors 1). never go wrong on the 271 engine. 2). MAF cleaner is good stuff. 3). MAF cleaner is a waste of time.

O2 sensors when changed, cure all ills.

Terraclean (other systems are available) is brilliant/only works for some cars/not worth it.

A late ‘update’. I went out to clear the P0171 code last evening. I decided to do an ‘analyse’ using the ‘Benz’ bit of the OBD II as well. It threw up a couple more codes that I didn’t know about. Namely: P201c-000 (part-load ventilation hose?) & P2029-000 (Fuel Fired Heater Disabled?). Could they be part of the overall problem? I’ve cleared them & will check again after a journey or two.

As I said above, any advice will be most welcome. Thanks in advance for your time.
 

LostKiwi

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The usual thing is to ask "what was the last thing I did before the problem" and work from there.
So...
Was the new back box genuine or after market?
What are the Long term Fuel trims looking like?
Have you tried resetting the fuel trims?
 
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The usual thing is to ask "what was the last thing I did before the problem" and work from there.
So...
Was the new back box genuine or after market?
What are the Long term Fuel trims looking like?
Have you tried resetting the fuel trims?


Hi.
Thanks for your reply.

The back box was an 'after market' one, namely from 'GERLACH'. I've read decent reviews on them, so thought I'd give it a go. Mainly because I can't get a genuine one from any of the usual suppliers. It's always "price on application" & "We'll ring when stock arrives (we don't know when that will be)". I can't understand why the Coupe (CL203) exhaust is so special!

I'm going to plead ignorance regarding the 'long term fuel trims'. Where would I find that info?

I've read today about resetting the fuel trims. Is this really just a simple case of disconnecting the battery for a while?

One thing I did do, which I didn't put in my original post, was 'reset' the throttle? I'd also seen this on a forum post, whereby the key is inserted into the ignition, pedal to the floor, ignition to position 2, count to 5, turn ignition off & wait at least 2 minutes. I've only done a bit of town driving today, so can't say if that did anything, or whether it's just a 'joke' fix.
 

LostKiwi

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LTFTs can be read and reset using a decent code reader.
A battery disconnect may or may not achieve anything.
 
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LTFTs can be read and reset using a decent code reader.
A battery disconnect may or may not achieve anything.

I've got an iCarsoft OBD II code reader. I'll have to make time to read the manual, lol!
Regarding the battery disconnect. As you say, it may or may not do anything. I'm on a decent drive tomorrow, so I'll see how I go. If it still feels the same (post throttle reset, etc.) I'll try the battery disconnect route.
 

LostKiwi

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An iCarsoft i980 won't do it. Not sure about the MB-II. I know Carsoft 7.4 can as can STAR.
 
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LostKiwi

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'93 500SL-32, '01 W210 Estate E240 (RIP), 02 R230 SL500, 04 Smart Roadster Coupe, 11 R350CDi
Just checked using our r230 and fuel trims don't appear to be resettable using MB-II but you can view them under OBD-II information.
 
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Update on the 'fuel trim problem'.

I took the car for a good run & then popped my MB II on. It returned readings of: STFT 18.8%. LTFT 22.7%. Pointing to a leak somewhere I guess. So, I'll have to strip out the airbox again & have a really good look round the hoses etc. incase I missed something when I cleaned the MAF sensor.
As I've got a long weekend (next week) I'll do it then.
In the meantime I'll buy the vacuum hose (A271 016 04 81) which seems to be the one that splits, as it's cheap, & fit it as a matter of course.

If there's any other suggestions, please feel free to comment.
 


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