Hello all MB experts- I had the fuel trim malfunction code on my 95 C280 and I purchased a new MAF (from ******** in AZ
-$262) and installed it and cleared the codes by taking the battery cable off. The light stayed off for one whole day and came on the second day. I had it checked at AZ and I got the dreaded fuel trim malfunction- bank 1 message. The car had a lot more zip after I put the new MAF sensor on. Is it possible for the new MAF to go bad in one day ? I have read a lot about gas cap leaks- could this be the problem? I have no other codes showing. Should I reset it again by taking the cable off or should I have AZ reset it with their reader?
Thanks for your insight.

Larry

When you cleared the memory , you put the ECU back to normal. You assumed the MAA caused the adaptation numbers to trip the code.
Now , with a new AMM, the adaptation has gotten out of limit again.
Did you get a bad AMM, I doubt it.
More probable that your original condition was caused by a vac leak or a bad fuel pressure regulator.
That is why it is always recommended to check those first , before jumping to the AMM.
The reason Techs go with the AMM test , is b/c they have one to use as a test tool , saving time without spending $$ on a part. They can leave the bad adaptation memory , do a quick AMM swap, drive around the block a couple of times and watch for the adaptation to get back in line. [ confirming bad AMM]
But for DIYers, I always recommend [and so does AllData ] looking for vac and doing a fuel pressure test.
If you go back a couple of post that you posted on, read the one by Steve B. and MB Stars... they explain it to the T...
The reason your car ran better is b/c you cleared the memory back to normal. It was not the results of the new meter, but rather the proper/ short lived ECU mapping. Which has now gotten back out of limits.
As I said , you may have gotten a BAD meter, but I doubt it......

Thanks Arthur- I don't know what a "test AMM" is or where to get one. How does a DIYer get this done. I already paid hundreds of $ for the initial analysis that I needed a new MAF! Anything I could look for without the diagnostic tools mentioned ? Thanks!
Larry

L
I quess I was not clear on the "Test AMM"

What I mean is that one replaces the suspect AMM with a known GOOD one.
That now becomes a "TEST AMM TOOL "
It is just an easy way for quick diagnosis. Like changing a tail light bulb with a known good one to save the time of checking for voltage and ohmmeter-checking the bulb .
There is no AMM TEST TOOL. It is just a Tech trick of swapping the part with a known good one for testing purposes...
Techs usually have access to these, DIYers don't..
Unfortuately for diyers , AMM and MAF sensors are very hard to diagnose
without proper equipment..
But regulator test and looking around for vac leaks is easy and sometimes are the problem..

Thanks Arthur- I guess I already did the "test" when I installed the new MAF. How do I do a regulator test? Is there a chance the gas cap is still a possibility ?

Larry

I can't tell you what is wrong without another code retrieve, but I doubt it to be the gas cap.
You may want to take the vac. hose off the fuel pressure regulator and see if any evidence of gas/fuel in that line or at that port. That would be a diy test for bad diaphragm on the regualtor and that would cause your fuel trim code.
A better test would be a fuel pressure test. That requires a gage hook up and some specs.
Look everywhere for vac leaks...specially under the front engine cover at the SOVs..the line that feeds them is common fault as it is plastic and cracks from heat fatique..
Another one is the purge valve sticks open and causes a vac leak when not called for .. this one is hard to find b/c it is an internal leak and not seen or heard. If one suspecs that, it is usually coupled up with another purge code ... but not always
Diy trick here is to plug that line and run the car for a few miles to see if the trim codes clears itself.. it will bring up another code for purge, but you are expecting that code .
Scanner is pretty much a needed tool on all these 0BD2s, so you are not guessing..

Hi Arthur,

I took my C280 back to the garage to find out why I got the CEL light and fuel trim message right away after installing the new MAF and the mechanic did several checks including the fuel regulator (checked with pressure gauge), vac leaks (didn't take the valve cover off) and several other checks. After an hour of diagnosis, he said he thinks the problem might be in the "chips or network". I got scared and asked him if he thought I would hurt the engine running it like it is and he said he didn't think so. He said he thought it would take another hour or two to diagnose the problem. ??????

Thanks for your help!

Biglar

These can be a real pain.

Does your car fall into the age range where there are serious wiring harness problems? If so, does it have a new harness? The insulation falls off the wires, causing all sorts of strange behavior.

If this is not the problem, you must chech ALL the vacuum systems (including the climate control) for leaks -- idle control system, brake booster, tranny modulator lines, climate control, EGR if you have one (too new of a car for me to know off my head on that one), etc. Anything larger than a very small leak will cause over-correction from too much unmetered air getting in.

Once you pass the ten or twelve year mark, the rubber parts of the vac system in the engine compartment start to get rock hard and leak, causing trouble.

Also, check the seal on the oil filler cap -- they get hard and leak oil one way and air the other -- new cap is a couple bucks.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks PSFRED, Are there any vacuum lines under the cover on the very top of the engine where the spark plugs are located. Neither I or the mechanic took this cover off to look for vacuum leaks.

BTW- my car had a new wiring harness installed early in it's life courtesy of MB.

Biglar

There are no lines next to the spark plugs. "Chips and Networks" dont often set such convenient codes. There are a couple of common air leaks. There is a hose that connects to the aluminum part your intake manifold directly in front of your oil filter. Other times the canister purge valve- the one that ticks real loud when driving located behind the left headlight- can stick open or close and act as an air leak. A third is a little tricky. There is a line that runs to the pressure modulator on the right side of the transmission, the modulator itself can leak-the car may shift fine but still be getting unmetered air. good luck

I've seen a bad seal on the plastic part that caused a similar problem. It may look fine externally, but will have dust traces across the seal on one end when you remove it. Cannot be fixed, you must replace the whole thing, of course.

I'm not familiar with anything that new, really, but the tranny modulator, power brake booster line, and ACC vac supply (if that model still uses vacuum) are all posibilities.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Something that you can try that is not too expensive (of course other than chasing for vacuum leaks) is to change your fuel filter.

After removing the fuel filter, I usually take a white (or light colored) ceramic bowl and tap the filter towards the center of the bowl (ie... like you are trying to empty it out in the opposite direction of the normal flow). Be prepared for some residual gas to go into the bowl. Upon doing this you can get an idea of how much crap was in the system. You'll be amazed sometimes. Anyway, if it looks like there's a lot of crap in the system ... run some "high quality" fuel injection cleaner through it. Red Line makes a pretty decent fuel injector cleaner. This has helped clear up the "Fuel Trim" error in at least 3 cars that I've worked on. If it does help, then I usually suggest you take it in to have the fuel injectors professionally cleaned (on car cleaning is usually around $130 US). Best way is to remove them yourself and find a place that you can trust and have them reverse flush/clean them properly.

Hope this helps.

Thanks SL600-Cruzin!

I hate to admit this, but where is the fuel filter located?

Thanks!

BigLar