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Implications of unplugging the maf

M

Mike6

Senior member
There have been a number of posts including from me about engine management lights where the fault could lie with the maf. If you buy one from BMW you are faced with hundreds of pounds so others plus me have asked how you can test whether the maf might be faulty.
Suggestions have included unplugging the maf and if eml lights do not reappear then maf is likely culprit.
I tried this today and at first didn't notice any difference but as engine got hotter I noticed that at tickover engine started hunting and then discovered that touching the brake pedal resulted in an increase in revs but performance wasn't really affected .
Not sure why revs should increase when touching the brake pedal but others might find this affect interesting. Eml lights hasn't come on again but this isn't conclusive as light only came on periodically.
 
What problem are you chasing?
If you unplug the MAF sensor the car will run on a baseline map setting. So predetermined air fuel mixture etc.
so if it is faulty the car will run better.
 
j3nks79 said:
What problem are you chasing?
If you unplug the MAF sensor the car will run on a baseline map setting. So predetermined air fuel mixture etc.
so if it is faulty the car will run better.
Click to expand...
Therefore, if unplugging it leaves you with the same issue then it is something else.
As the brake servo uses vacuum from the engine intake side then potentially start looking for air leaks, including the pipe to the servo. Part of this is rubber and is known to collapse.
 
I think enuff_zed is probably right thinking its an air leak particularly because of the change in revs when brake applied as that will use what vacuum is in the system. If you have a split or holes vacuum pipe or intake boot then that would explain the symptoms you have. If you cant find any air leaks then a smoke test should pinpoint where they are. Favourite places are the intake boot, all vacuum pipes and failed inlet gaskets but often missed are the blanking rubber caps on the rear underside of the intake manifold, these perish split and fall off. The O ring in the dipstick is another often overlooked source of an air leak. Air leaks will eventually cause the EML to light up the reason being is that to correct what the exhaust sensors see as a weak mixture because of the unmetered air getting in and not being measured by the Maf. As a consequence the DME will demand more fuel at the injectors to richen what its seeing as a weak mixture. It will continue adding fuel until it reaches 20% more then finally realise its not correcting the issue so sets the EML with a code for exceeding the fuel trim levels.
Apart from doing the disconnect Maf trick to see if it improves things and unless the Maf sets a specific code for itself it can lead you astray from what is the correct cause of the issue. In my experience they work or they don't and don't always set codes if indeed they are at fault.
If you have to replace the Maf only go for Bosch or Seimens oem brands do not buy pattern parts they rarely work or last long, been there paid twice.
 
Thanks COLB my journey started with eml for failing heater element in a post cat oxygen sensor. Couldnt access myself so got my local garage to replace both with Bosch ones. They had to burn the old ones out as impossible to get out conventionally. Thereafter I was getting codes for 2882 and 2883 ie DMD mixture preparation on both banks. Took car back to garage to incase air was getting in around new sensors or there was an air leak on the exhaust joints. They said all was fine (i trust them) and I should focus on inlet side. I replaced spark plugs as precautionary measure and stripped inlet side including DISA to check for air leaks where you suggested. I could find nothing so then thought about dodgy MAF and disconnected it. Now with car idling a touch on the brake peddle results in engine revs increasing.
I replaced that pipe to the servo a few years ago but is there a valve near the servo itself which could be letting air back into the engine.
I am completely stumped by this and dont want to take car back to garage again unless I have eliminated everything else first.
 
Smoke test it to be sure the system is fine do it with the throttle fully open so the smoke can fill in easier. Take the Maf off and pump the smoke in the inlet boot using a rubber glove stretched over the boot to obtain a seal. Pay attention. To all the vacuum pipes and gasket seals across the system. Have traced as few hard to find leaks using smoke, intake manifold gaskets on a Z3m and a sucking jet valve on a 2.2 Z3 both couldn't be found just with a visual inspection
 
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