e36 M50(TU) 320 Rough Idle

Hi all,

I've been reviewing the posts regarding lumpy/rough/etatic idle with the M50 / M52 engines.  I have a 1991 e36 320 with the M50(TU) engine which exhibits rough idle when cold, warm and hot.  The idle does not hunt (revolutions are static at around 800RPM) but feels rough as though the engine is missfiring.  There is also a relatively strong smell of fuel on idle.

The car has a LongLife stainless exhaust from the downpipe back.  I also have a K&N induction filter fitted.

I have replaced the following components:

• Coil packs (second hand BOSCH coil packs)
• Lambda sensor (new four wire sensor)
• Air filter for K&N induction filter
• Exhaust for straight through stainless steel from downpipe back
• Spark plugs (NGK)
• Complete oil change

When I first attempted to change the lambda sensor I had the wrong type (Titania type four wire type with black, yellow, red and white wires) which did not match with the wiring under the car.  I had this changed for the correct type (Zirconia type four wire with black, grey and two white wires).  I beleive that the type of lambda sensor used changed in 1992; 1990 to 1992 a Zirconia type was used, 1992 onwards a Titania type was in use. 

However after replacing the lambda sensor the idle appears to have become slightly rougher.  However I am not sure if this is because the lambda is incorrect (the wiring matches and my local motor factors who are usually excellent tell me this is the correct lambda sensor for the car) or if now the lambda is functioning correctly it is amplifying a problem elsewhere.

The coil packs have all (6) been replaced with second hand BOSCH components supplied by Quarry Motorsports.  The coil packs all appear to be visually ok.

Spark plugs have been replaced with the correct NGK replacements.

I am at a loss as to what is causing the rough/lumpy idle.  So my questions are:

• Could it be the ICV?  I would be supprised as this wouldn't cause a "missfire" like symptom.  If so where is it and how do you clean it?
• I have read that the oil used on K&N filters can cause the MAF hotwire to become coated in film that can cause this issue. Is this the case?  If so how do you clean the MAF?
• Could the Crank Case Vent Valve cause rough idle?  If so how do you clean it out and where is it located?
• Could the lambda be incorrect?  If so how?
• Any issues surrounding the crank sensor?
• Any issues surrounding the ECU? (I'm thinking about having this remapped/chipped anyway)
• Any other ideas?

I am supprised to be having this issue on a BMW.  I used to own RX7s and was used to all sorts of idling and air/fuel ratio issues.  The symptoms seem to indicate either overfueling or poor spark.  However I am at a loss as to what could be causing this having replaced the obvious components.

Thanks for all you help!

Hi Jayson,

I'll check the ECU today, pretty sure its a BOSCH (pre 1995).  Have been looking into your Viper2 chip, very interested in getting one of these.  Do you have any technical data for the torque/bhp gains?  Any comparison of torque curves?  I'm not really looking for bhp increases but a definite smoothing of the torque curve would help.

Are the idle issues known to be linked to the ECU/map?  The car seems to run fine the rest of the time.  Smoothly accelerates, is ok on light throtle and WOT.  Occasionally I have had the "heal and toe" incidents where the revs seem to dip when coming to a slow.  I believe this is linked to the ICV so will try to get this out to clean.  Do you think that having a remap/chip (such as the Viper2) will sort out the idle issues?  I've been putting off getting one until I was happy with how the motor was running.

Cheers!

I have 1990 with a similar problem, lumpy idle with high emissions. When I bought the car I put the lumpy down to the stainless steel exhaust fitted, but I did note that there was a strong smell of fuel off the exhaust at idle, and when I got a gas analysis done it has very high HCs, so seems like some sort of misfire. Like you, I got the plugs and plug leads replaced as part of a full service. While this has improved the running of the car, it's still failing emissions at idle due to the high HCs and idle is still lumpy. Another symptom I have is the car kangarooing slightly when at low revs, such as in slow moving traffic.

I'm hoping to get a detailed analysis of the car soon, which hopefully will track dowm the root cause. I'll post whatever information I get.

I've tried a lot of things to eliminate the problem.  At the end of the day there are only so many things that can cause fuel mixture to be too rich.  I've changed the following:

• Full service including new NGK spark plugs
• BOSCH coil packs
• TGK zirconia four wire lambda sensor

I'll put an orriginal air box back in to see if that makes any difference.  Even if it doesn't I'd rather have the car sucking cold air from the front ducts than hot air from the engine bay.  I can definitely see that the air induction filter could well be causing an issue.

Is there a possibility that the stainless exhaust could be causing any issues?

I'll pull the plugs out sometime this week and see what condition they're in.

Is it possible the ICV or Crank Case Vent could be causing the overfueling?  I can't see how as the idle is absolutely steady at 800ish RPM.  If it were either of these the idle should hunt all over the place (and worse case with the CCV make the engine go kaboom).

Jayson- does the ECU need to be reset after changing the lambda?  If so can this be done without a trip to the dealers?

I'm all the way down in Wiltshire- where are you?

How much information can you get from the OBD?

Looks like I'll be popping up some weekend soon.  It'l be a couple due to the car being in the workshop next weekend and Silverston F1 the following weekend!  What's your address.  If the car checkes out ok how much to stick a chip on (moderators I have asked and Jayson has not attempted to sell me anything via the forums, if this is a breach of the rules I appologise and ask that Jayson send me the details to philsaxby@hotmail.co.uk)

Thanks Jayson, will PM you.

Very interested to see what codes are thrown up.

Ok so dummie question about the air sensor!  Found it after having a fiddle around the inlet manifold.

Does the OBC read this sensor to give the external air temp?  Or does it have its own?

My OBC thinks the external air temp is -37 degrees C so if its reading the same sensor I think i've found the problem!!!

Anyone know ???

When you say the car has a stainless steel exhaust from the down pipe back, have you removed the catalytic converter?

I get a similar thing intermittantly on my car when idling, although when it does it it's not continuous, more like a little shudder every couple of seconds.

I have been told that the crankcase ventillation pressure regulation valve can cause this problem. I don't know if your car has the same arrangement as mine since I have a later engine, but the part for my car is only about £40. Not got round to changing it yet since the problme is only intermittant on mune and it doesn't really bother me that much.

 

Edited by Peter Fenwick