Hi guys, have a 1997 w140 s600 that seems to lack power, acceleration isnt as sharp as it used to be. Idling is ok, no shaking or obvious roughness but subjectively is not as smooth as before.

First I had it scanned at an indy shop. Relevant engine faults were:

p0300 misfiring
po312 (?)
p0170 right cyl mixture adaptation reach limit value, current
p0173 left cyl mixture adaptation reach limit value
p1146 no fault text specified (found out this is left hot film air mass sensor)
p0410 secondary air injection malfunction chain (?)
P1444 no fault text specified (found out this is left cat converter temp sensor)

erased, drove it around, scan again, results are:

P1444 no fault text specified (found out this is left cat converter temp sensor)
p1146 no fault text specified (found out this is left hot film air mass sensor)
p0170 right cyl mixture adaptation reaches limit value, current

Indy shop lacks expertise to interpret and suggests I go to dealer. I searched the forums here before going to get ideas.

Had a short test done at dealer the next day, relevant engine fault codes are:

p1444 left catalytic converter temp sensor (b16/4)
p0170 B Self - adaptation idle speed at right limit value (n3/12)
p1146 left hot film air mass sensor (b2/6)

erase codes, drive around block, scan:

p1444 left catalytic converter temp sensor (b16/4)

I was ready to conclude AMMs but they didnt come back on the 2nd test at dealer.

in my forum search stevebfl mentioned that a failing air mass meter causing weak throttle response off idle so i took a look at the actual values.

self adaptation part load factor right - 1.22
self adaptation idle speed right - 0.6 ms

With this fault history and actual values would it be safe to assume failing left and right AMMs at this point? Anything else I should look at to confirm?

I guess what I am asking is that,

Is a 1.22 adaptation level and 0.5ms compensation normal?

would the air mass meter be a likely culprit for blunted mid to high end acceleration, adaptation at limit codes, and those adaptation and compensation levels above?

If I'm correct, this is a "lean" condition (1.22) and would give you the symptoms you describe.

Have never seen a "lean" condition failure, normally the failure in my experience, is "rich" so the reading is around 0.80.

__________________
Regards . . . . JimF
-------------------
'94 S500 Cpe

Visit my Mercedes Web Page

The AMMs on Motronics systems have to be the number one pattern failure out there. They always fail to under report (go lean) and the inaccuracy grows with the volume of airflow. In this case you are being compensated with an additional 22% fuel at the part load coditions the addaptation is done at. At full throttle it is probably considerably worse and thus not fully compensated, probably a cause of the misfire codes originally.

I would look again at your idle adaptation and verify whether it is plus or minus .5ms. The typical way an under reporting AMM gets adapted will have a large correction done multiplicatively to load (1.22 is large) and then at the much lower flow rate it will be more accurate and the universally applied 1.22 correction will leave it rich requiring a CTP (closed throttle position) correction subtracting fuel. If you in fact have a negative CTP value then the AMM is the likely candidate. It is less likely if you are straight line off.

Check archives for AMM and you will see that consistantly they are reading 1.25 and above in motronics cars that have been fixed with new units (of course they should be about 1.00 after repair)

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

It sounds to me like the entire left (#2) ignition controller is about to give up. This is a common issue with 120s - and almost always causes the other controller to fail with it. It can worst case take out the motor by firing early and bending stuff. I've seen at least 3-4 120 motors that have failed in this manner.

Is Bret from San Diego still hanging out nearby? He's one of the few people in the U.S. that can keep a 140/600 going - he'll know what this is right away.

And SteveB is right - they ALways report low values (it's analog). I guess it's the temp differential is wrong? Temp heat/resistor weak? But this is what I don't get - if they report lean (too much air in the mix) then it should compensate with more fuel. Right? To richen the mixture it adds more fuel at the injector.

Also, a lean mixture will overheat a motor in a hurry, and probably burn valves and o2 sensors.

JimF - Yes, I think you are right, as stevebfl kindly explained it is adding 22% more fuel. It seems from the archive search they can also fail the other direction. Nice site with lots of useful info btw.

stevebfl - Hi stevebfl, btw, thanks for that ac system diagnostic article you linked here to the forums, it helped me diagnose the faulty sensors the dealer was diagnosing as a busted evaporator!

I hope I am understanding you correctly...as I understand, 1.22 is high, but not high enough to trigger the fault. the upper limit is at 1.32 and exceeding this value is when an "adaptation value limit reached" fault will occur.

1.22 reading was at idle...so at higher rpm/load the adaptation value goes up to beyond 1.32 and thus triggering the adaptation limit fault? which would explain the misfires - lean mixture at high rpm/load as computer has reached the limit to compensate?

Additionally you are saying that at idle the 1.22 is too rich and thus explaining the -.6m/s compensation to lean out the mixture? I'll have to check if the .6 I got is a negative or positive value.

I don't get the last part though. It should be 1.00 with a new AMM as you say, unless you are saying these are the cases when a new AMM still causes enrichment because the fuel trim was not adjusted back to 1.00 either via the diagnostic computer or no self-adaptation has occured because the conditions for self - adaptation were not met?

ASaltyDog - crap, I hope not! What other codes or checks can I do to verify the ignition controller's condition?

Yes, from how I undersrtand it is adding fuel to the tune of 22% above the baseline. 1.00 is baseline and diagnostics read 1.22.

Guys?