(I think I'm in the right forum now)

I've read many of the threads here dealing with the mass air sensor (same as air mass meter?) and I'm not sure if I should have it replaced, replace it myself (how hard is it?) or just live with the check engine message. It's a '97 E320, all functions work, drives fine, mpg's same as new; but a week ago "CHECK ENGINE ENGINE ELECTRONICS" popped up on digital display on dash. My dealer mechanic said probably air mass sensor needs replace, at $600.00 cost, but he reset computer to eliminate message and said wait and see if message comes back. It came back yesterday. So, should I try to replace it, should I let dealer replace it, or should I do nothing until car starts running erratically? What kind of erratic behavior will I likely see if I don't replace it?
Thanks to MBDOC for his quick reply. I checked fastlane; the part is $275. I think I'll try replacing it myself.....what does the MAS look like and where is it?

I'm guessing for your car since it's a 97...

Basically, the MAS (mass air sensor, or air flow meter) reads the intake air volume and density and sends information to adjust fuel mixture to accomodate the changes. In the old days, your carburetor choke did this during warmups...needless to say, this unit came about with fuel injected components and computer management systems.

The MAS is housed in a black plastic cylindrical chamber (about the size of a small coffee can), and resides between the air cleaner housing and the intake manifold...it is usually directly behind the air cleaner. There should be a wire harness attached to the side of the chamber where the sensors electrical posts are attached.

Removal and installation should be fairly simple...it may require removal of the air cleaner assembly for easier access, but the operation can be done without tools. Clips hold everything in place, so maybe a screwdriver is needed to aid in popping the clips open.

Again, I've done mine, but I have a 93, so your configuration is probably different...

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Thanks G-Benz, sounds simple enough. I'll take a look at it tonight. I already ordered the part from fastlane.

These do go bad but usually you will have a starting problem ( no negative injection pulse ) . Why do you want to replace it . I would have the code checked again and make sure the diagnosis is correct. 275 bucks is alot of money to say ( i think). Just my 2 cents. good luck

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euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck

uh oh, are you saying that if the MAS is bad some symptom will be evident? I have no symptoms of a problem other than the check engine, engine electronics message.

I had the same problem on my '97. The mech told me to ignore the check engine lite but it blocks out the all the other readings in the dash. I figured I would have to replace it sooner or later, so I did it sooner. Ed

Ed, you didn't have to get it done sooner. There's a reset button that clears the big CHECK ENGINE ENGINE ELECTRONICS message that covers the digital display items......it's the "R" button under the "H" & "M" buttons that you use to set the time. I just push that after I start the car and no more big message.... (there's still a little window box on the dash that holds the check engine display, but it doesn't bother me).

mtodd51
Thanks for that info on the reset. I guess I did not read my manual close enough. The mech had reset the computer twice and I decided to fix the problem, but the reset would have been handy. Ed

When the MAF sensor died on my 104 engine the symptoms were pretty obvious. Engine hunting between 400 and 1200 rpm in drive and in idle as well.
Wait for the symptoms to appear, why spend the money needlessly. However if you already ordered the part as might as well keep it sooner or later it probably will go. Changing it is probably one of the easiest maintenance procedure, right up there with changing the airfilter, at least on the 104 engine. Good luck.

Alex

My dealer has told me that not having the gas filler cap tightened can cause this warning.

Thought I'd fill you in on the DYI option I took on the MAS. It is very easy to do.....but it doesn't make the Check Engine message go away. I took my E320 to the shop, thinking I had just wasted $300.00 putting in the MAS myself, but it turns out, after you replace it you have to reset the computer to take the error codes out. It was confirmed that the MAS was bad based on the 170 code stored, and no current code showing.

They said the check engine message would have probaly gone away by itself after a period of time after the computer "learns" the new readings. I drove about 200 miles, and started the car at least a dozen times before taking it to the shop.....how long does it take before the car figures out the problem is fixed?

the p0170 code can be of 3 things, MAS (99%) of the time, vacuum leak or fuel pressure regulator i think.

I change my MAS recently and took 3 days for the CE light to go out.

Everything is fine cept the engine still hunts around.

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I read in other forums that after replacement of mass air sensor and resetting the code, the fuel trim adaptation will have to be reset by the dealer/indep. tech, does any of the experts on this forum know if this is true?

thanks

Forty key cycles to reset the fault codes I think (maybe it was 80???).

If one replaces an AMM with an adaptation of 1.32 (normal limits for late HFM and all ME cars) with one that needs no fuel correction, the fuel trim will be 32 percent too rich. The proper job would have one clear the adaptation back to 1.00 on the lower and upper partial and 0.00ms on idle.

The ML I did this morning I drove with the light on to see performance. As with most warm addapted engines it idled fine and moved off fine. Above 40% throttle the car started to go slower. This is a classis case and I know the answer already but I do it properly. I read the fault codes P0170 and P0173 (naturally) and I go to actual values to view the adaptation values = 1.32. So with the symptoms my evaluation is confirmed. I then clear the adaptation back to 1.00 and drive it about 3 miles. It has then partially readapted back to 1.11.

At this point I replaced the AMM and left the adaptation at the 1.11. I drove the car 10 miles and again read the adaptation of 0.99. All the driving symptoms are gone full power to the redline first and second gear.


This is final proof of the repair. Notice I also didn't clear the adaptation values from the 1.11 number of the short old AMM test. if I had left the numbers at 1.32 they also would have readapted back to .99 in 8-10 miles. The problem is that initially the mixture could be so rich it could set misfire codes or others before it did its own readaptation.

For HFM cars the readaptation could take 100 miles of driving so a controller stuck with at limit adaptation values can easily have other problems before it finds its way home.

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

Stevebfl,

Thank you for the insightful info. I had to read it 10 times before I fully grasp (maybe 90%) of what you are saying. If your shop is in So. Cal, I would definitely bring my car to you and have you reset the adaptation values. It's just that most of the shops I dealt with here are either incompetent or are sharks.

I will drive my car and see if my car will readapt to the proper values. If not, I will take it to a shop and have it reset.

Thanks