Check Engine Light seems to be a common problem on MBs and other cars.
Advance, Autozone etc all provide free check of codes and resetting.
On our MB, I get told that problem is likely the MAS or O2 Sensor or wiring
harness. However, there does not seem to be an easy way to just test a
sensor and find out if it is good or bad. Dealers just want to replace parts
whether needed or not. These things are not cheap!
Does anyone know whether or not Bosch or MB offer a service to actually test
the MAS or O2 sensor? It should not be difficult or expensive to set up a
test bed that checks the signal.
How about the wiring harness - Is it just the plugs that would be faulty? If
the wires themselves were bad or broken, you would not think the car would
continue to run properly.
Another thing that puzzles me, is that after the CE light comes on, the car
still runs perfectly. Surely the MAS must still be working if the engine
runs well, does not misfire and gas consumption remains normal?
MAS will adversely affect the engine performance if bad. Yes, there are way
to test those items without ripping it out... simply using OBD-II scanner
will reveal the the readings.
I suggest you seek another dealer or service rep as they seems incompetent.
O2 is fairly cheap to replace if you buy them yourself online.
I hope that MAS is OK, because car runs fine and I have driven it 1000+
miles with no problem.
I have an OBD II scanner, but all it tells me is that there is that CE light
is caused by Fuel Trim Malfunction on Banks 1 and 2 (P0170/3).
Question is, what is causing this? If MAS is suspected, then how do we check
it? If it is harness, how do we check it? If it is harness connections, how
best to clean them?
I am sure that MB dealer would eventually correct problem - but I am
concerned that they may change MAS ($400) , Harness ($2500) , O2 sensors
($??) and labour ($??) at a cost equivalent to ~20-25% of car's value :(
I think I could handle that but that would produce a different code, I
Fuel trim out of adaptation limit are caused by overly rich/lean condition..
The ECU can not correct above/below 32 adaptation limits [ +/-].
The first check is for vacuum leaks
[ lean] and the second is fuel pressure regulator [rich]
The reg has a vac hose attached .. take that off and see if fuel drip/odor
On 210, common vac leak is cross-over vac hose in front of engine [ plastic
hose -they crack from heat exposure]
If the MAF will not correct for adaptation, it is the fault, but do not change
it til you cheack the above first two suspects..
Maf testing requires adaptation numbers
with real time scan..
Thanks for advice - I will check hoses - hopefully I will find the correct
one. We did have a fuel odour on our last trip.
Wish they had shop manuals for these beasts!
Would you suspect the MAF if the car otherwise runs well - no excessive
vapour from exhaust or rough idling.?
Last time this happened I cleaned connections, removed and cleaned MAF
internals with contact cleaner, made sure air ducts were sealed - CE light
did not come back on for 3000 miles.
Make sure there is no air able to get to intake between the MAF and throttle
actuator , as this would be un-metered intake of air..] False Air ]
Many times guys do not tighten the cross hose clamps of forget to snap in the
Intake Air Sensor . The MAF can not read air volume entering the engine after
the MAF...so it can only report back to the HFM module what goes through it....
I strongly agree with AJDalton7.
actually if you really have a good scan tool, you can read out the O2 sensor
voltage and it will tell you something, but for the Air flow sensor,
sometimes the AIR FLOW SENSOR faulty will not generated a fault in the
I face lots of W210 problems and mostly is the AIR FLOW SENSOR GIVING BAD
SIGNAL! i will do the following
1. Check the throttle angle, if the throttle angle is too large of too
small, it would tell you something wrong with either AIR FLOW SENSOR or O2
sensor! the throttle value will be 1.8 degree to 2.2 degree ! ( the HFM will
auto adjust the throttle actuator if HFM senser BAD air or mixture
2. Check if there is any "SELF ADAPTATION" value out of range!
3. Swap these sensor with ANOTHER SIMILAR CAR to feel the problem
HAVE A NICE day and HAPPY THANKS GIVING
Thanks CSC and AJDalton for your valuable input - Wish I could bring the car
to you for checking :)
How do I check the throttle angle ?
I have been thinking about getting a better computer based scan tool. This
may let me see what causes the problem.
CSC is correct on throttle angle values ..
I would take a peek at 02 values first as it is quite simple on HFM/SFI
If you have a DMM, you simply hook it to the sig output wire [ green'/under
Unplug this wire so the ECU does not get the feedback signal [ so it can not
correct a/f mix].
Leave the other 02 wires connected , as they are heater wires and you want the
02 hot for output testing..
Run engine til warm and then pull the vac hose off the fuel pressure regulator
[ plug the vac line for this test as you do not want a vac leak]..your DMM
should shot right up to .9 -1.0 volts..
Rehook and now pull a vac line off intake
[ I use the one going across to SOVs that I mentioned in earlier post], but
any will do].. now the DMM should go lean --low volts , .0 -.2v .
Now , put your thumb on/off the vac hose a watch how fast the 02 jumps up and
down with your open/close of vac.
1/2 sec is an OK time frame
If these do not happen, change the 02 sens
All is this test does is use the engine as a rich/lean a/f mix generator and
the DMM reports if the sensor responds to your r/l inputs.
Good trick for DIYer
I use a scope ... but I am not a DIYer..
So far not so good - I did the following:
- Reset codes (same as before P0170 & P0173
- Checked that hose clamp on MAF was tight - OK
- Checked that 45 deg rubber elbow was not leaking - added a film of sealant
(what is the black chamber this elbow goes to?)
- Could not see any other places there could be a vacuum leak - am I missing
I read somewhere that you could use propane as a leak check while monitoring
the O2 sensor output - sounds dangerous! Any better way?
- Unplugged-Plugged MAF connector plug and wiggled it a bit :) Contacts
Fuel Pressure Regulator
- Checked all hoses across front of engine - not sure which one AJ was
referring to - there are some small tubes (~1/4") and one large rubber hose
(~1" diam) across top front of engine. Took some of these off and no gas
smell. Not sure I am in right place - what exactly does the regulator vac
hose look like and where is fuel pressure regulator? In any event, all hoses
looked OK - no cracking or brittleness.
Went for run in car and CE light came back on after just a few miles.
I don't have proper scanner (Just the code reader/resetter), so some test
will have to wait until I get one.
Plan is to check O2 sensor per AJ,s procedure. I also found some resistance
values to check on the MAF.
Last time I had this problem, I removed and cleaned MAF and connector with
contact cleaner - This lasted 3 months, so I will try that again too. Maybe
it is not self-cleaning properly?
Thanks for your interest and help!
CODE 0170 = fuel mixture problem on right cylinder bank
CODE 0173 = fuel mixture problem on right cylinder bank ( of course!)
I would do the following carefully
1.Check for intake air leaks!
2.AIR FLOW SENSOR measurements!
3.FUEL PRESSURE REGULATOR TESTING!
fuel pressure at idle should be 3.7 to 4.2 bar with vacuum!
CHECK MORE AND TELL ME WHAT HAPPEN
if possible give me VIN
Based on my experience, lot of W210 is of the MAF sensor! these stupid
sensors will not generate fault code some times although they are faulty! So
the air cleaner of these cars should be changed intervally so that the
internal heating element of the sensor is not covered with dirt and give bad
The propane test is to create a rich condition that can be picked up as a high
volt reading at the 02 sensor..
This test is OK on reg engine , but you have Electronic FI, so the easy way to
richen the a/f mix is the procedure I mentioned of taking the hose off the fuel
pressur regulator..what this does is it eliminates the reg keeping the pressure
down where they want it.. so , the pressure goes UP and causes more fuel to
flow through the injectors [ rich condition]
The 02 sensor has to see this enrichened mix and respond to it.
..the advanage to this is that not only are you testing the 02 sensor, but you
are also testing the regulator. If the 02 responds and then you put the vac
hose back on the reg , the 02 will come back down cuz the pressure has returned
The other way requires a pressure test at the Fuel Rail w/pressure gage.
That is the preferred way.
The reason I ask if fuel is coming out the vac line at reg is they have a
common fault of a leaking diaphragm. This not only lets the pressure get too
high , but the engine sucks the fuel into the engine through the vac line and
makes for a rich condition that the system can not correct for [ Out of limit
code -fuel trim]
The reg is that little silver round device w/vac hose attached , next to/foward
of the oil dip stick.
As CSC says , very likely the MAF could be bad and they are quite common fault
on 210, but these others simple vac leaks and fuel pressure will do the same
codes...and should be checked first.
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