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Bit of a mystery this: last month it began to dawn on me that mpg was persistently going worse, and have also noticed that if i apply any hard acceleration - especially if I drop down to third (mine is the manual version 2005 slk350) to overtake or go up some of the ever-so many hills around here, then all becomes quite sluggish. There was no CEL, no poor idling or any other noticeable symptom at the time. So I suspected the MAF may be dirty and tried a run with it disconnected - then lo-and-behold the mpg was the best I've ever seen (27 mpg in 26 miles of mostly urban with a lot of stop-start, and 34 mpg on a 20 mile stretch of motorway - actually think that could have been higher if I stayed on the motorway). Also the pull in the lowers gears were restored, or in some situations best I've experienced since purchase - it was definitly a changed beast.



Well I cleaned the MAF and put it back expecting even better performance, but there was no change over or under the increased performance seen from when it was disconnected. So it seemed a dirty MAF was at least one reason for the poor performance, although it doesn't explain why the disconnection led to the best mpg I've seen - unless the car was rubbish to start off with, but an average of 23-25mpg urban and 30 to 32 mpg on long motorway stretches always seemed to me pretty respectable for a 350.



Anyway, I thought of checking the air filters and they seemed ok. So after a few taps to relieve them of any loose debris (there wasn't much) they were replaced. As I was doing that I remembered how I meant to inspect the front of the intake manifold for a possible broken lever plate, as this I found out is a common fault. The plate was indeed broken with the swirl flaps set in the 'open' position, yet the levers still moved freely.



This ony adds further to the mystery of best ever mpg and increased perkiness when all of what I read on this topic says I should be experiencing poor idle, lower mpg, as well as poorer performance?? Yet it's running better than ever and now wondering is it worth not bothering with the lever plate. But an MOT is due soon and wondering if anyone who has experienced this and continued to run their slk in this state can tell me:


1. Did they pass an emissions test?

2. Where there any detrimental consequences in the long run?

3. Have they too experienced better performance in this state?

Apologies for the copious text, but will be grateful for any thoughts on this.
 

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Your report is most intriguing. Question for you: how long did you drive your car with the MAF disconnected? I've never disconnected one for more than a few minutes on the driveway - I've certainly never driven highway miles with it disconnected.


It's odd that neither your disconnected MAF nor your broken lever isn't triggering a CEL. Are you sure the light is working? (It should come on briefly when you first start the car).


The swirl flap does provide better performance under higher rpm, the fact it's stuck in "low rpm" position probably explains what you're seeing. I would fix the lever first, then perhaps replace the MAF if you suspect it's still defective.
 

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Hi, this is posted in the 170 section, I think it should be in the 171 area?
Anyway, I can report that my 350 has a broken swirl flap, I’m not sure which position it’s stuck in but the fuel economy is about 5mpg down on what I think I should be getting.
As for power, I can’t really tell but will have a better idea once it’s been repaired. I have the parts and am just waiting for some time to do the job.
 

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Oh Da Horror!

Consider:

A mass (air) flow sensor (MAF) is a sensor used to determine the mass flow rate of air entering a fuel-injected internal combustion engine. The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine.

If this sensor is disconnected, the ECU doesn't receive the information needed to determine how much fuel the engine requires for combustion. So what happens? I suspect the ECU has a default value for the amount of fuel probably on the lean side. That would explain the MPG experienced. This probably isn't good for the long-term health of the engine.

I'm not sure what position the tumbler flaps are in, but they need to be fully open during normal operation.

The thing to do is to stop guessing and screwing around and fix these issues:

Replace the intake manifold and MAF with new parts.
 

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Hi, this is posted in the 170 section, I think it should be in the 171 area?
Anyway, I can report that my 350 has a broken swirl flap, I’m not sure which position it’s stuck in but the fuel economy is about 5mpg down on what I think I should be getting.
As for power, I can’t really tell but will have a better idea once it’s been repaired. I have the parts and am just waiting for some time to do the job.
Mike, I suspect you are correct. Although his profile reads 2004, his data card read 2005.

@Odysseus would you please advise us and correct your profile if necessary. Until I hear from you , this is being moved to R171 because of what your data card says :tu:

From your d.c.:

 

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Discussion Starter #6
Your report is most intriguing. Question for you: how long did you drive your car with the MAF disconnected? I've never disconnected one for more than a few minutes on the driveway - I've certainly never driven highway miles with it disconnected.


It's odd that neither your disconnected MAF nor your broken lever isn't triggering a CEL. Are you sure the light is working? (It should come on briefly when you first start the car).


The swirl flap does provide better performance under higher rpm, the fact it's stuck in "low rpm" position probably explains what you're seeing. I would fix the lever first, then perhaps replace the MAF if you suspect it's still defective.

Was disconnected for two days with probably around 65 to 70 miles in that state, and the CEL definitley does work as it always appears briefly on ignition. I may be wrong, but isn't stuck in low rpm mean the flaps are closed? In my situation they are stuck open.
 

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Discussion Starter #7
Mike, I suspect you are correct. Although his profile reads 2004, his data card read 2005.

@Odysseus would you please advise us and correct your profile if necessary. Until I hear from you , this is being moved to R171 because of what your data card says :tu:

From your d.c.:

Corrected, thanks.
 

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Was disconnected for two days with probably around 65 to 70 miles in that state, and the CEL definitley does work as it always appears briefly on ignition. I may be wrong, but isn't stuck in low rpm mean the flaps are closed? In my situation they are stuck open.

Good question about open vs. closed - I'll have to check on my next drive (some time next week probably).
 

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Discussion Starter #9
Hi, this is posted in the 170 section, I think it should be in the 171 area?
Anyway, I can report that my 350 has a broken swirl flap, I’m not sure which position it’s stuck in but the fuel economy is about 5mpg down on what I think I should be getting.
As for power, I can’t really tell but will have a better idea once it’s been repaired. I have the parts and am just waiting for some time to do the job.

I would say they are in the closed position as you are experiencing poorer fuel economy, yet you can move the levers to the open position for normal fuel consumption after 3900 rpm [found that out from a section about the intake manifold in a document introducing the m272 engine] - at least you'll be able to go on dual carriageways and motorways without too much penalty [in my case with extra benefit]. But you can only move the levers provided the swirl flaps themselves are not gunked-up or warped.


I'm yet to be convinced whether I should replace the broken lever plate as my fuel consumption and performance is the best I've experienced thus far in the time I've owned this car [which is not from new], but there doesn't seem to be any further input to this thread to go on. But if I do become convinced that I need to replace the lever plate, then I'll go for replacing it with an aluminium one.
 

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Wow, you should never run the car without the MAF on purpose, that just isn't smart. The MAF is the most vital information the car needs to calculate the correct amount of fuel going in to the engine. Taking that information away means it is just guessing. You could seriously damage your engine doing this. Your MPG going up is not accurate, so don't think your MPG actually went up.
 

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The car widout maf will go on a safe map. You will not damage your engine.
"Safe maps" aren't always that safe. They are obviously not meant to be doing full throttle or extended periods. The ECU is missing a lot of vital information that it needs to function properly. They are meant to limp you to a service shop, not be driving on normally.
 

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Discussion Starter #13
Wow, you should never run the car without the MAF on purpose, that just isn't smart. The MAF is the most vital information the car needs to calculate the correct amount of fuel going in to the engine. Taking that information away means it is just guessing. You could seriously damage your engine doing this. Your MPG going up is not accurate, so don't think your MPG actually went up.


Thank you for the advice, but the car is running with the MAF connected, and I gave it a clean after I saw a significant improvement when it was briefly disconnected for a short run. The only thing disconnected or broken is the intake manifold lever plate, and the mpg is still signifcantly higher along with improved acceleration - this must mean the engine has suddenly become more efficient in this state, which the fuel gauge confirms as it doesn't seem to go down so quickly. So I would say the mpg reading is as accurate as it has always been.
 

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Bit of a mystery this: last month it began to dawn on me that mpg was persistently going worse, and have also noticed that if i apply any hard acceleration - especially if I drop down to third (mine is the manual version 2005 slk350) to overtake or go up some of the ever-so many hills around here, then all becomes quite sluggish. There was no CEL, no poor idling or any other noticeable symptom at the time. So I suspected the MAF may be dirty and tried a run with it disconnected - then lo-and-behold the mpg was the best I've ever seen (27 mpg in 26 miles of mostly urban with a lot of stop-start, and 34 mpg on a 20 mile stretch of motorway - actually think that could have been higher if I stayed on the motorway). Also the pull in the lowers gears were restored, or in some situations best I've experienced since purchase - it was definitly a changed beast.



Well I cleaned the MAF and put it back expecting even better performance, but there was no change over or under the increased performance seen from when it was disconnected. So it seemed a dirty MAF was at least one reason for the poor performance, although it doesn't explain why the disconnection led to the best mpg I've seen - unless the car was rubbish to start off with, but an average of 23-25mpg urban and 30 to 32 mpg on long motorway stretches always seemed to me pretty respectable for a 350.



Anyway, I thought of checking the air filters and they seemed ok. So after a few taps to relieve them of any loose debris (there wasn't much) they were replaced. As I was doing that I remembered how I meant to inspect the front of the intake manifold for a possible broken lever plate, as this I found out is a common fault. The plate was indeed broken with the swirl flaps set in the 'open' position, yet the levers still moved freely.



This ony adds further to the mystery of best ever mpg and increased perkiness when all of what I read on this topic says I should be experiencing poor idle, lower mpg, as well as poorer performance?? Yet it's running better than ever and now wondering is it worth not bothering with the lever plate. But an MOT is due soon and wondering if anyone who has experienced this and continued to run their slk in this state can tell me:


1. Did they pass an emissions test?

2. Where there any detrimental consequences in the long run?

3. Have they too experienced better performance in this state?

Apologies for the copious text, but will be grateful for any thoughts on this.

My 55 is very 'picky' about air filters. First time I changed them I found that idling in drive had significantly more creep, especially noticeable in 'C' mode, and acceleration off the line was brisker. Also, bearing in mind I always wipe the carbon from the exhaust tips when I fill up, there were fewer deposits produced with new filters. Fuel consumption decreased slightly. At that time, the old filters produced a little grit when I tapped them out, but by most standards, they wouldn't be described as dirty. Subsequently, I pay little attention to miles covered, but replace the filters when the creep feels less and the exhaust tips appear to be contaminating faster. The old filters look clean when replacing. but the engine always improves with new filters. If you think about it, the highest volume of particulates is likely to be microscopic, and embed themselves deep in the matrix of the fibres of the paper element. Effectively, the dirtier they get, the more particles are trapped, but air flow must be compromised at the same time. New OEM elements from ECP or CP4L on sale are chump change - so I imagine they pay for themselves via improved fuel consumption.
 

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Discussion Starter #15
My 55 is very 'picky' about air filters. First time I changed them I found that idling in drive had significantly more creep, especially noticeable in 'C' mode, and acceleration off the line was brisker. Also, bearing in mind I always wipe the carbon from the exhaust tips when I fill up, there were fewer deposits produced with new filters. Fuel consumption decreased slightly. At that time, the old filters produced a little grit when I tapped them out, but by most standards, they wouldn't be described as dirty. Subsequently, I pay little attention to miles covered, but replace the filters when the creep feels less and the exhaust tips appear to be contaminating faster. The old filters look clean when replacing. but the engine always improves with new filters. If you think about it, the highest volume of particulates is likely to be microscopic, and embed themselves deep in the matrix of the fibres of the paper element. Effectively, the dirtier they get, the more particles are trapped, but air flow must be compromised at the same time. New OEM elements from ECP or CP4L on sale are chump change - so I imagine they pay for themselves via improved fuel consumption.

Thanks, that does make a lot of sense - this should mean I can look forward to even better mpg on top of the improvement I'm experiencing with the broken plate if I renew the filters. More importantly I have an MOT next month and have read somewhere that a broken lever plate with the flaps stuck to open may not necessarily fail the emmisions part of the test as some - so to help with the likelyhood of passing, I'll definitely be replacing the filters very soon.
 

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Your problem is not the mpg...
If you runer lever is broken because of the flaps being stuck inside you can have pieces of plastic going in admission then yo can have a disaster by bendding valves.
If your lever is broken because of age ( can use vacuum and das and confirm that the flaps are moving ) then you are ok. I change mine as preventive maintenance.
 

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Discussion Starter #17
Your problem is not the mpg...
If you runer lever is broken because of the flaps being stuck inside you can have pieces of plastic going in admission then yo can have a disaster by bendding valves.
If your lever is broken because of age ( can use vacuum and das and confirm that the flaps are moving ) then you are ok. I change mine as preventive maintenance.

The levers move freely from one end to the other when I move the plate, so presumably the flaps are not gunked-up or warped inside as they are free - or do you think this a false positive?
 

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You can check in das via actuation or remove the intake manifold and look inside. I will not risk it.
If all ok the new aluminium lever is £29, 2new gaskets £7.
While are you in there you can change the knocking sensors and water temp sensor, 2 gaskets on the maf
New manifold £400
 

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You can check in das via actuation or remove the intake manifold and look inside. I will not risk it.
If all ok the new aluminium lever is £29, 2new gaskets £7.
While are you in there you can change the knocking sensors and water temp sensor, 2 gaskets on the maf
New manifold £400

Thanks for the advice. I don't have access to DAS but I do have an Icarsoft scanner that tells me that if the levers - which move freely - are pushed to the right I get an p2004 code; and if pushed to the left I get a p2006 code. This must mean the flaps are not warped or gunked-up if the sensors sense the movement either way, so things should be quite clean in there. But am going to order an aluminium replacement plate and gaskets then see if I can have a go myself at replacing it myself. Seems a daunting task, but more worried about introducing crud, etc into the valves when the manifold is lifted away - maybe there'saway to pre-clean the area before lifting the manifold away - any relevant advice would be greatly appreciated.



Will have a look inside of course, then if necessary order a new manifold and replace its plastic plate with the aluminium one I'm going to order [see lots on Ebay plates made in China - are they any good as they seem the cheapest?]. But why would I need to change the knocking sensors and water temp sensor if my only fault code is p2004?
 
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