Misfire/loss Of Power

[Pocky]

Hi, I'm new to BMW & still finding my feet a little.

I have a 2003 (E46) 316i. The car has a very rough idle and when I drive it the mph does not read as it should for what the revs read. If I put my foot down, it runs rough, like it's misfiring for a second or 2 then it kicks in & sounds as it should, all cylinders firing, although the power is just not there and like I said before the rph do not match what the mph should read i.e. 4,000 revs accelerating but speedo creeping up from 30 to 40mph.......Unplug the Mass air sensor and she runs near on perfect!!! I know it's not the mass air sensor though as I have already replaced this...result remains the same...Help!!

Oh, one other thing...the power train light is on when the mass air sensor is disconnected, it goes out when connected.

[Guest Mr.Beemer]

hello kevin, welcome to the forum -

has the car been serviced & have you had a diagnostic scan done ?

[Pocky]

Hi Mr Beemer...Recently serviced & new plugs installed...Diagnostics did show up an ingnition coil fault, but that's now been dealt with. Like I said, without the Mass Air sensor connected she runs well.

[karlos2010uk]

Hi Kevin, welcome... there is your problem, if you have unplugged the MAF and the car runs fine then its a duff MAF, sometimes they can be cleaned with some electronic cleaning solution in a maff bath but I have failed each time I have tried resulted in braking the sensor..

Euro Car parts are showing one for £90 for your model, but I know £90 isn't a snip, give them a ring and quote you reg plate they tend to be cheaper over the phone, or if there is a local scrappy near by you could see if there is one on a scrap car, try it on yours and if runs fine you know its that..be sure to get a matching part, part no's are normally stamped on the MAF...

[Pocky]

But Karl...I have already replaced the Maf, as I, like you, thought this was the problem....but nothings changed :(

After changing the MAF & connected it I only drove the car a mile or 2, noticed the problem was still there, so disconnected again...is it possible that the car needs a few miles to 'sort itself out' with the new part??

[karlos2010uk]

I wouldn't of thought so, although with some cars you can do an ECU reset by disconnecting the battery for 45mins (that was how long it took in my Bravo HGT not sure about the BMW) then reconnect, start the car but but do not rev at all let it idle for 10-15mins and then drive like a sunday driver the ECU should learn this style of driving.. or plant your foot from the go and it'll want to go all time...

You may have an issue with a Lambda Sensor (usually on the exhaust down pipe looks like a spark plug screwed into the downpipe with a wire coming off it.)... a faulty Lambda will cause rough IDLE where the idle speed bounces up and down feels like the car is going to stall then comes to life... I think the only way to tell if there faulty is via an emissions test but don't quote me on that..hope that helps

[Guest Mister Dabsy]

Good answers Karl... A faulty lambda gives a dash light tho...

I would thoroughly clean the Throttle-body and "soak" all the intake AND MAF in a solvent (do not physically clean the Maf, let it soak), this is always the best place to start ;)

[Pocky]

Thanks for your replies Guys...this really helps and will give me something to work on...I'll report back when I know more.