HELP! Rough idle, Rev bounce and poor fuel consumption

[A.Waffle]

Hi all!

I'm experiencing rough idle, rev bounce (even when coasting) and really poor consumption, 15-16l/100km kind of bad. (My brother's E34 is doing 11-13l/100km, both are M50B20)

So here are what I noticed about my engine's behavior;
-Whenever I rev slightly it seems to lose power at first and then jump up the revs
-When it starts to rev bounce while coasting, I'll need to blip the throttle then it'll stabilise
-Idle is between 500 and 1k rpm, could feel the engine pulsing but no obvious movements on the rev counter.
-When on the move, I will lose speed when I'm gentle on the throttle (OBC showing 8-10l/100km), while my brother's car will maintain or increase speed depending on gradient.
-Engine braking is very aggressive

What could possibly cause these issue?
Could MAF be adjusted in anyway to make the air-fuel mixture right?
How do I know if my engine is running rich or lean?

If there are any older threads with solutions already, be great help if you could link me up!

Thanks in advance!!

[moroza]

Start with the basics - check spark plugs, intake leaks, and engine sensors. MAFs can go bad but it's not common. It cannot be adjusted (well, there is a way but it's not for fixing problems on a stock engine). An engine running rich will smell more, possibly smoke (especially black smoke), and consume more fuel. An engine running lean will run poorly, possibly knocking or backfiring. None of these things will affect engine braking; if it really is more aggressive than it should be, you have too much mechanical drag somewhere - parking brakes, tires, main brakes...

[A.Waffle]

I did an engine oil, oil filter, spark plug change less than 3k km ago, it was running worse before tho.

There's a little screw like thing beside the maf plug, reading from some forums that could "reduce or increase" the tension of the spring in the maf which determines the airflow. I fondled with it recently and seem to make my car run more efficiently and a tad bit more responsive. (Kinda fix the consumption issue there)

Anyway thanks for the suggestions! i shall get my brakes checked as I just got my new set of wheels and tyres not long ago (235/40/R18)

[shogun]

I would not touch a MAF and there is no spring in a MAF = mass air flow meter , but in case your 520 has a M20 engine, it does not have a MAF, it has a AFM = Volume air flow sensor http://www.realoem.com/bmw/enUS/show...diagId=13_0292
but also that you should not touch.
Paul has explained that system http://paulaxford.com/html/bmw730i/index.htm
see Cleaning the Air Flow Meter (AFM) Potentiometer

[A.Waffle]

I would not touch a MAF and there is no spring in a MAF = mass air flow meter , but in case your 520 has a M20 engine, it does not have a MAF, it has a AFM = Volume air flow sensor http://www.realoem.com/bmw/enUS/show...diagId=13_0292
but also that you should not touch.
Paul has explained that system http://paulaxford.com/html/bmw730i/index.htm
see Cleaning the Air Flow Meter (AFM) Potentiometer

Hi Shogun! Thanks for the reply.
My engine is the M50 so it is using the MAF. And I've somehow adjusted it slightly and got my FC to be slightly better with my engine running smoother and with better response.
Since I got my car secondhand, the MAF might be disturbed.

The story goes like this, when I went to test drive the car, it was all good, smooth running engine so all good.
As the day came to collect my car, my dealer told me that the car couldn't start, so they sent it to a "reputable" workshop that handles "classic cars".
When I can finally collect my car, just as soon as I started it, the engine rev bounced like mad, going from 500rpm up to 1.5k rpm and keeps going on that way.
So I inspected and checked the air filter, to my surprised, the workshop didn't put it in there, and the receipt said they changed the pipe between MAF and throttle body.
Sent back to the workshop, 1 week later still the same and they told me it's the temperature sensor.

So I did some research and came to find out that it may be an air flow issue and some did adjust the MAF slightly and got better. So that's what I did and managed to get the engine running smoother. Rev bounce is still there but not very aggressive or often, fuel consumption got slightly better averaging mid 14l/100km (still not how it is supposed to be).
As for now, I'm thinking it might be O2 sensor that is causing my poor FC.

Anyway, I'll get my mechanic (BMW Specialist) to get it properly checked once I have sufficient funds.