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Experts stumped. Care for a guess?

1K views 7 replies 8 participants last post by  eric242340 
#1 ·
We recently took my wife's 2004 c320 in for a routine maintenance job. Soon afterwards the check engine light came on. We took it back, mechanic said it was a bad MAF sensor and replaced it. Check engine light came back on riding home. Mechanic said it may be a bad sensor and he'd order another one.

The next day her car stalled out in traffic but she managed to get it restarted after about 10 minutes. We let the mechanic know about this and he came out to our house with his diagnostics. Didn't see anything and said he was still waiting on the new MAF sensor.

Replaced the bad sensor, test drove it and gave it back to us. Within an hour check engine light was back on. The next day she was going out and didn't even make it out of the subdivision before it stalled. This time we couldn't get it restarted so had it towed in.

Mechanic said it was the crank position sensor and it actually restarted once it cooled down (I didn't really understand how much it could have heated up in less than .2 mile but whatever). Ordered a new sensor. Test drove it over 70 miles, said he was 201% sure it was fixed and had his driver bring it back to our house.

Within the day the check engine light was back on. Now he is calling on one of the MB high-tech gurus for some advice on how to proceed next, waiting on an answer.

This mechanic is no slouch. He's highly regarded and people from these forums bring their cars in from neighboring states just to have him work on them. Yet this is something even he has never seen before.

So now we're driving around with the check engine light on waiting to see what happens next. Anyone care to venture a guess on what could be causing the problem? Once we finally get it solved I will let you know what it was and we can see whose guess was the closest.
 
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#2 ·
Without knowing what the codes are, it's flying blind -- at night -- in a snowstorm. There are dozens of possibilities for your issue and scores of reasons that the CEL would be on, which is why codes exist...and there are hundreds and hundreds of codes. And at least to me, it seems in the realm of probability that something your mechanic did during the routine service (of course we don't know what that is) started this chain of events. He might have leaned on a vacuum line and cracked a "T", chafed some wiring, left an intake seal misaligned, who knows?

I'm guessing your mechanic will discuss his actual diagnostic results with "the guru", which hopefully will lead to a prompt solution. I appreciate your frustration, but it seems doubtful that -- with less than fifteen or twenty percent of the information the mechanic has -- anyone here will be able to tell you the cause (or the solution).

Good luck.
 
#5 ·
I'll take a guess - it's not the MAF or Crank Position Sensor, but the Camshaft Position Sensor. It is also a "hall effect" sensor like that on the crankshaft so it is susceptible ot heat, but it fails less often. As others have said, codes are the key!
 
#6 ·
Have him do this, then come let me know the result.

1. Check all fuses. Use a spark tester at one of the spark plugs or a noid light on one of the injectors to check for spark and/or injection pulse when the stalling/no start symptom is present. If there is no spark, or injector pulse, attach a scan tool and watch the data stream output of the scanner to see if it indicates engine RPM when cranking the engine on the starter. If there is no RPM indicated, or if there is no spark, or injector pulse, test the output of the Crankshaft Position (CKP) sensor.

2. Disconnect the connector on the CKP at the rear of the engine on the driver's side above the flywheel, and inspect the 2-pin connector for any corrosion on the terminals or the presence of any contaminating fluids, which could affect the signal.

3. To test the voltage output of the CKP, it should produce approximately 1.0 vac Key On Engine Cranking (KOEC).

And then this.

1. Test output of the CKP by reading data output on scan tool for engine RPM when vehicle is in a no-start condition. If no RPM is indicated, the Powertrain Control Module (PCM) is not able to read the signal from the CKP.

2. To test the CKP specifically, remove key from vehicle and disconnect connectors on PCM. Using a Digital Multimeter (DMM), test resistance between terminal 3/26 (+) Green wire and 3/13 (-) Green/White wire. Specification 600-1000 ohms. Switch meter scale to volts ac. Crank engine, cranking speed 250-300 RPM, voltage signal should be 1.0vac minimum.



My bet is the crank wire is burned thru somewhere.

OR?

Here is part of a repairtrac, I sent to Mercedes years ago.

The car would intermittantly shake and stall at stops. Never any codes. While driving car I found the right 02 shorting out. Pulling current from right side injectors replaced 02 the shaking stopped but still had no accel at times. Replaced maf meter to solve the rest of the problem.








Just goes to show, you never know what is actually causing the problem, until you catch it doing it.
 
#8 ·
I fully agree with the codes bit. But as you do trust this mechanic and yet he has to call in a guru, also given the cars year and model I would hazard a guess and this is an educated and experienced guess that the ECU has developed a problem.

Now he has changed the MAF sensor and the crankshaft sensor, I am dammed sure he will not want to tell you to change the computer without a second opinion. I have had three like this so far, but it is still only a guess, not enough info to go on and no codes.
 
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