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!

take a look at your ecu, see if it is bosche or siemans.

this idle problem is common on this engine (well car anyway)

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!

Hello again,

Been trying to resolve the idle issues today, still getting nowhere.  My local parts factor says that the ECU may need resetting after replacing the lambda sensor.  Any truth in this?  If so can it be done DIY?  Does replacing the chip reset the ECU anyway?  The ECU is a BOSCH serial number reads 0 261 200 403.

I've come to the conclusion that some of the lumpy idle (its more lumpy than rough like it's overfueling) may be due to the induction filter sucking in hot air (especially when stationary).  As far as I am aware the M50TU engine has no air temprature sensor so the ECU would not be able to compensate for the hotter air being sucked in by the induction filter when stationary.  The lumpiness seems to take a few seconds to come back when coming to a standstill, which makes sense regarding the air temperature suggestion as it would take a second or two for the air in the engine bay to reach its "hottest" when standing stationary.

Any feedback regarding this?  I'm thinking of getting a standard air box and using a decent panel filter as the air box seems designed to suck cold air from the brake ducts in the front bumper.  Anyone think this may help resolve the lumpy idle issues?

Jayson- would putting a standard air box/panel filter back and going with a Viper2 chip likely sort out the lumpy idle?  (it really does feel like its overfueling on idle).  What sort of price are the chips?

the 403 ecu is not the one that normally causes the problems your talking about.

the 402 does but not the 403.

the chip will make it run smoother for sure but if there is an nderlying problem thisneeds to be sorted but you may have it right wit the filter idea as bmw engine bays run very very hot and the cone filters are a nightmare unless sealed off.

i would fit the standard unit back on and see if it helps.

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?

where are you lopcated ?

if your local to me you can pop over for a free diagnostics on the car.

this way you will have an indipendant report done in front of you so you know whats wrong and what isnt.

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

How much information can you get from the OBD?

everything.

i think it best to keep the sales off the forum so if you could pm me or email me ill give you all the info you need but its the cheapest price in the UK.

as for the car, on average they give around 30 faults over the car so theres always something need resetting.

Thanks Jayson, will PM you.

Very interested to see what codes are thrown up.

Anyone have any feedback regarding common faults with the following:

• Lambda sensor
• MAF (hotwire, especially in combination with K&N filter oil)
• Crank Sensor
• Idle Control Valve
• Crankcase Breather Valve
• Throttle Position Sensor
• Coil packs
• BOSCH ECU

Does the M50TU have a air temp sensor?  I find it hard to believe that it doesn't being that it has no Manifold Pressure Sensor!!!

Any other common faults known to lead to lumpy idle?

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.

Hi Peter,

I didn't have a CAT.  My car was an H reg and had hence pre CAT laws.  Was quite supprised that it didn't being that similar cars of that age usually had CATs.

How does the crank case vent valve cause overfueling?  I haven't checked mine so it's a possibility.

My damned brake pad wear light's just come on too (the circle with blocks at the edge) but my pad's look ok!!!

Does removal of the CAT cause a lumpy idle?  I can't see how this is possible if the lambda is still in place. 

As an aside, as the car is pre CAT legislation the emmisions test shouldn't be too much of an issue.

finite wrote: Does removal of the CAT cause a lumpy idle?

Finite, sorry but I can't answer that.  Needs someone with a better technical knowledge of things mechanical to chip in here.  Chippeduk maybe?  I'd imagine that any effects would likely have an impact back to the ecu.

Ok been doing some research.

Now from what I can make out this is what happens with the CAT/lambda (can anyone confirm if this sequence is correct- ie is the lambda before the CAT?)

air in --> fuel in --> spark --> combustion --> exhaust --> lambda detects level of oxygen and sends voltage reading to ECU --> exhaust gasses go through CAT where some chemical reactions occur to reduce emissions --> exhaust gasses out of tailpipe.

Ok so if the O2 level is too high the ECU ups the injector duty cycle and hence more fuel is used in combustion and vica versa.

From this sequence we can deduce that the lambda/ECU feedback routine would work the same regardless of whethere there is a CAT or not.  Now what it doesn't tell us is how efficient the CAT is and how much it reduced emissions (and hence the smell of fuel out the exhaust).

I've had a look at the Haynes manual which also talks about the crank case emmision control (CC Valve) but goes into no detail about how to replace it/clean it.  It also talks about the charcoal canister and purge valve- both of which could lead to overfueling if they are working incorrectly. Anyone have any experience with these?

Cars! They'd be so simple if it wasn't for government legislation!

The cat removal will yield more power but it wont cause masses of extra emmisions but enough to failt the mot of course.

it shouldnt cause lumpy idle at all.

Have to find a friendly MOT centre I guess.

Any idea how much the emissions go up by?

too true!  or one who will pass a rotten tub bucket for the right money.

shame in many ways.

finite wrote: How does the crank case vent valve cause overfueling?  I haven't checked mine so it's a possibility.

Sorry, no idea. It was just suggested to me as a potential cause of a lumpy idle

finite wrote: My damned brake pad wear light's just come on too (the circle with blocks at the edge) but my pad's look ok!!!

Could be a problem with the sensor.

chippeduk wrote: The cat removal will yield more power but it wont cause masses of extra emmisions but enough to failt the mot of course.

I thought it was OK to remove the cat if the car was made pre August 1992 (J reg or older) ?

Hi Peter,

As far as I am aware all cars pre J plate (and some J plates) did not have to have a CAT.

However in order to pass the emissions test the emissions have to be to the manufacturers specification for the car +/- x% (usually 5%) I believe.  If this is the case then it is possible that a decatted e36 may output higher emissions than a non-CAT e36 as the combined emissions systems are different.

Jayson- unfortunately I live in the countryside and we have way too many "£100 MOT" centres locally.

Hi,

Just a thought, why not take the oil cap off whilst it's running and see if it gets worse or the same.

I have a 328 and that engine is smooth, one day I forgot to tighten the cap in any way and it ran very lumpy at idle until I worked it out.  It smoothed out when revved, then after settling for a few seconds it came back.  I'm not saying it's the cap obviously but if there is a lack of pressure/vacuum then this could be the same.

Also i've had a couple of 6 cyl BMW's all of late 24v variety and they all had a strong petrol smell on startup and they have all been excellent runners without any faults.

Ok so I've fitted the standard air box back after aquiring one from a breakers.  It came with a K&N pannel filter which was a bit of a result.  All fitted minus the plastic cowling that fits the front of the box to the scoop in the inner wing/chassis leg (I assume there's some plastic that goes here or the box will still be sucking some hot air?).  I'll try and get the cowling this w/e.  It's definitely made some difference, but the idle is still lumpy.  Looks like I'll have to strip down the vacuum/inlet system to look for leaks/clogged valves.

Anyone got any advise on this process?