W210 Mass Air Sensor & Check Engine Light

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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?
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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.
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Thanks Tiger,
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 think?
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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 evident. 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..
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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.

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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....
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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 CONTROL MODULE.
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 problem. ) 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
CSC
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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.

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The throttle angle can only be checked using proper scan tool!!!! THe best tool for MERC is of course the one we use everyday, the SD

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CSC is correct on throttle angle values .. I would take a peek at 02 values first as it is quite simple on HFM/SFI engines . If you have a DMM, you simply hook it to the sig output wire [ green'/under pass rug] 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..
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Hey - That is a neat trick and easy to do! Now just got to get at it!

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nice procedure AJ! try them and don't forget to report to us jack!
HAPPY THANKS GIVING
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Hi CSC/AJ,
So far not so good - I did the following:
- Reset codes (same as before P0170 & P0173
Vacuum leaks ----------------- - 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 anything?
I read somewhere that you could use propane as a leak check while monitoring the O2 sensor output - sounds dangerous! Any better way?
Connections --------------- - Unplugged-Plugged MAF connector plug and wiggled it a bit :) Contacts "look" OK.
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!

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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
CSC
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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 air sensing....
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I think that is w/o vac
My specs show w/vac 3.2-3.6 @ idle wo/vac 3.7- 4.2 @ idle
CSC.. did u get an email from me tnx A Dalton
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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 to normal.. 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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AJ! i received an email from you and will join the FORUM! now i'm busy with a damn old 300SD... we wil write about it later... ha!
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CSC - I also have an old 300D - turbo but with rebuilt '82 engine from a 300SD. Best car I ever had! And no check engine light!
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Jack send an email....
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