I did not have a check engine light and the check engine light bulb does work.

I was clearing a code on another car and I thought I'll check the 1999 ML 320. The code is fuel trim malfunction bank 2. And like I said I have no CEL. I cleared the code without any problem.

What might be going on. The vehicle runs perfectly.

Edited:

My guess is MAS going bad, however, I don't want to spring for $310. if I don't really need one.

The way I understand it, the conditions have not being met to trigger the CEL. There is what they call "pending codes". I had that once, then they may completely go away, or eventually trigger the CEL.

I had once the P0173 which turned the CEL on, and P0170 pending. I cleaned the MAS, new air filter, and used silicon oil on the MAS O ring, cleared the CEL and it has't come back in 22K miles. Not even as pending codes with no CEL

Rene

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'99 ML320, '00 C280

I took the MAS out and it looked like new, not even any stain or discoloration on the wires. I blew it out with air anyway and reassembled. I'll just check for codes every week or so for a while to see if I get any.

I found in a little more detail what the code actually represents.

Attached Images

codes.bmp (35.4 KB, 334 views)

Ron,

Mine was very clean too. A friend told me to clean with electronic cleaner and alcohol specially at the wire and a small passage in there. It worked so far.

Rene

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'99 ML320, '00 C280

Check in light came on 525 miles in to my 600 mile trip. When I arrived at my destination I pulled the code, it was P0173. That seem to be the MAS. Vehicle is running fine, I cleared the code, however it will problably reappear on my trip home next week.

Looks like I'll have to go ahead and get a new MAS.

Only $202.50 + shipping here http://www.trademotion.com/partlocator/index.cfm?action=getLocator&siteid=213951&chapter=EPI346&appSectionid=3&groupid=10046&subgroupid=61073&make=67&model=Ml320&year=1999&catalogid=1

Below are the explantions for the two codes I'm getting now. Nothing is mentioned about the MAS. I wonder what the problem is?

Ron, what is your multiplicative (long-term) fuel trim? If it's more than +/- 25% your MAF needs replaced. The ecu can only adjust to 25% rich or lean. All Bosch MAFs need to be replaced sooner or later.

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Lenny

There are only three sports: bullfighting, motor racing and mountaineering; all the rest are merely games. --Ernest Hemingway

'10 GL550/'04 BMW 545/'99 BMW 323/'98 ML320/'87 VW GTI (race)

Lenny,

I just have a generic code reader an Actron CP9125 so I can not get the information you discussed. I did however, get a new MAF sensor, brand new Bosch unit off ebay for $200. I installed it last week and so far no codes or pending codes. I'll check every week or so for pending codes, but it seem that the MAF was the problem. The ML has 113K miles on it.

Couple of questions re P0170 and P0173:

1) I have faulty catalytic converters and am on the list for replacement. CEL Light is on. Codes are P0170 and P0173. Could the faulty cat's possibly cause fault codes P0170 and P0173?

2) Forgive my ignorance, but, where exactly is the MAS (or MAF)? Can someone post a picture or schematic? I have 150K miles on my 99 ML 320 and am still on the original MAS. I'm nearly positive it needs replaced, but I have been waiting to eliminate the possibility the cat's are causing the codes.

See http://forums.mbworld.org/forums/showthread.php?postid=400780#post400780. for location MAFS

Some of the laptop based generic code readers will display fuel trim. Multiplicative fuel trim faults are usually the MAF, so I'll bet your new MAF solved the problem. I replaced mine at 129K, after the MIL lit up.

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Lenny

There are only three sports: bullfighting, motor racing and mountaineering; all the rest are merely games. --Ernest Hemingway

'10 GL550/'04 BMW 545/'99 BMW 323/'98 ML320/'87 VW GTI (race)

I thought I might paste my reply from technical as it applies here too:

Would one believe that the sensors in MAFs could be similar and interchangable?

The answer to all such analysis is to monitor the adaptation values. The P017x codes are all for exceeding the adaptive capability of the two separate adaptation systems; partial load and CTP (closed throttle). By watching the values the type of problem can be surmized and the solution proven. A code that says the CTP adaptation is at the limit for correcting a lean condition (additive value at 1ms or above) is likely to be a vacuum leak.

A part load value of a multiplicative correction of 1.32 is usually a MAF. Substitution of any piece and a short drive after reseting adaptation to zero will verify the solution as adaptation of ME systems happens in a few miles.

For those with real diagnostic capability one can do it scientifically. Go to a search engine and find a Volumetric efficiency calculator (they were first devised for deciding which size Holley double pumper to put on your small block). Run the engine full throttle (under load) while monitoring MAF, rpm, and intake air temp (this is best accomplished on a dyno or with a graphing scanner see: http://www.autoenginuity.com/ ) The calculator will need rpm, maf (in proper units), cid (in proper units) and intake air temp (in proper units). The answer should be above 70% for most engines and above 80% for most 4 valve engines and as high as 100% or more for turbo or supercharged engines.

As to the question about adaptive codes and catalysts. An interesting thing happens with a restricted cat. The MAF meter reading is an average of the intake of all cylinders. If one side of the motor is restricted, those three or four cylinders are using less air. Since the MAF only reads total it has an averaging effect. The amount of fuel that is calculated is too small for the free breathing side and too much for the restrcted side. The readings for adaptation will then be plus for one side and minus for the other. If the MAF is already weak then the lean side (the free flowing side) will blow the limit. In this case a MAF will fix the problem for a period but adaptations will continue to be significantly different side to side.

One other bit of info in this thread that is wrong for MBs is the total multiplicative adaptation. Twenty five % is common with many cars but MB goes to 32% on ME cars. That is probably due to all the problems they had with 18% in early LH and HFM cars.

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

How can you tell if Catalytic Converter is the problem or MAF sensor?

Is what problem? If you have P017x codes for adaptation limits broken, the problem is not likely only a cat. Before one would have a variation in mixture significant enough to hit the limit due only to a cat it is likely one will already have P030x misfire codes. The answer in general is that one needs to observe the data. A fault code is a warning to observe the data.

An OBDII scanner is a crude but sometimes effective way to observe the data, otherwise one needs a diagnostic crystal ball. I can get a link to a company selling them: http://www.auto-video.com/product_detail.php?id=26

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