Lean Miss
 

On my 1992 500SL, I recently had the 60K
mile maintenance performed. I also had
the fuel injectors replaced to fix a
cold start/warm start idle "rumble" problem.
The engine would rumble for about 2 sec then
smooth out.

The idle "rumble" is fixed. The car starts fine
and idles very smoothly except for an occasional
lean miss when in drive and at a stop light.
The idle is at 600rpm and very smooth except for the occasional lean miss. In park, the lean miss is more frequent.


Interestingly, when the O2 sensor is disconnected,
and with the car in neutral, 700 rpm, the lean miss is gone. I had the O2 sensor replaced but the lean miss problem is still the same.

Any ideas?

Thanks

Andy

I can't recall the production break, but I think your car would be CIS fuel injection. If it is, I would try just adding a little more fuel in with the adjustment.

Leam Miss SOLVED!
 

Thanks!!! I adjusted the air/fuel mix by
turning the screw 1/2 turn CW. I did this
in small adjustments. In park I could
"hear" the miss and when adjusting, I could
"hear" when it stopped!

The engine is smoooootthhh!!!

I'm now adjusting a little more for
performance. I'm looking for the sweet spot.
I've made another 1/2 turn CW and am driving
to test.

Do I have much room to play with it? When will
I know if I've gone to far?

Thanks again!!!!

Well you have just screwed the pooch.

The mixture is only adjustable after you have over-riden all the electronic and lambda controls a good hiway drive and you won't have any catalyst left (maybe no floor).

The reason you have corrected the misfire with mixture is undoubtably due to a secondary ignition short circuit (usually plug wire ends). As soon as the mixtures drive required voltage above the shorted path voltage, you don't fire on that cylinder anymore. Leaner mixtures require higher firing voltages. The overall warm mixture is not adjustable! Only the base point is adjustable.

The normal correction of mixture even after component replacement is usually in the realm of 5 degrees of rotation not a half turn (180 degrees). Just add nitrous and you be running top fuel eliminator (bg).

Thanks Steve,

I assume so much.

Steve is right. When we check lambda on vehicles, if all is in proper operation, we only turn the adjustment screw a very little at a time with the lambda meter giving us feedback.

Benzmac,Steve:
Thanks for the input and cautions. I had
my mechanic check and adjust the fuel mixture last
monday so that is in spec. It is now back to "normal", no damage done, but I now have my original lean miss problem.

So, since the "lean miss" problem "seems" to go away or masked with either the 02 sensor removed or with the 02 sensor connected and providing VERY rich fuel mixture, what could be at fault here?

my mechanic seems to think the fuel distributor
is the next likely cause. does this seem
reasonable?

thanks again for all the help, comments, and
input!!!

OK Guys, I followed your advise but still no sucess..

Ignition wires and plugs have all been replaced
and "lean miss" still exists.

any other recomendations?

THANKS!

lean miss source
 

I had 2 MBZ with the CIS. When a 'lean miss' developed at idle, usually caused by hardened seals around the injectors where they fit in the head. Renewing those seals always improved the idle. It also could be dirty injector nozzels, or just plain wear and funky pattern. I always took the set of eight to the nearby Porsche/Bosch shop and had them presure clean and test. Sometimes a single injector will start showing dribble or poor pattern at very low flow.

Thanks for the reply. Injectors have been replaced and seals checked-all OK. again, if
lambda is disconnected or overridden, engine
is smooth as silk.