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Discussion Starter #1
I am wondering if someone can help me out in this one.

My Engine Check light came on (ML320 1998) about a few months ago. Using a code scanner at the professional Body Shop I was able to determine that Gas Cap is leaking. Through the dealer I have purchased a new Gas Cap and installed it right the way. After what the code was cleared and the light went off. About a week or two ago the light came back on and the scanner reads the same code. BTW, the light does go off between tank fills. Do I have a bad Gas Cap again? Or maybe there is a bigger more issue, like major vacuum leak some where.
 

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Are you getting a code for a major leak or a minor leak? (I don't remember the actual code number) Many times a common evap leak on an ML would be a bad vent valve above the spare tire. I'd have someone leak test the system and try to find where the leak is.
 

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Discussion Starter #4
Thank you all for replies. The Gas Cap is MB OEM cap. I will check the code again, but as far as I remember the code reader was pointing at the Gas Cap.
 

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There's no way for the control module to tell if it's a gas cap. If it's saying it's a gas cap, you most likey have a code for a major evap leak. There's a number of things that can cause this code to appear. Best thing to do is leak check the system.
 

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Discussion Starter #6
I have finally got around checking up the code using Actron CP9135 OBDII Scanner.
The scanner gave me the following readouts:
P0170 Fuel Trim Malfunction Bank 1
P0173 Fuel Trim Malfunction Bank 2

Does anyone know what it is and its location?
How hard it is to replace it if required?
Please advice guys.

Many thanks.
 

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These mean that your engine is running either too rich or too lean on both cylinder banks and it has reached the limits of being able to compensate for it. About 75% of the times it's caused by a bad mass air flow sensor on a 112 or 113 engine.
 

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Discussion Starter #8
Thanks Zewerr for reply.
It kind of bites to spend ~$280 on the Mass Air Sensor and discover that it wasn't it. You mentioned 75%. What else could it be if not the Mass Air Sensor? I just want to be prepared then feel sorry. :wink:
Anything special I should be aware while replacing the Mass Air Sensor? I do know that element inside is very sensitive and fragile.

Thanks.
 

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Other things that I've seen cause this include, vacuum leaks, clogged air filter, clogged fuel filter, rat nests in the intake system, bad throttle actuator, bad fuel pump, air leak in intake tube between MAF sensor and the throttle actuator. So there are a number of things that can cause this. It's just that the MAF sensor is the most common.

Replacement of the MAF sensor is pretty straight forward, but the mixture adaptions need to be reset in order for it to be done correctly and completely. Many people have had luck doing this by simply disconnecting the battery for a few minutes. But the proper way to do it is with a proper scan tool, as an MB shop would have.
 

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Discussion Starter #10
Thanks again for all your support.
I have been able to reset the "Check Engine" light and will keep my eyes open if it comes back.
BTW, how would I know if mixture adaption was done right (amature way with the battery)? What would be the symptoms if it doesn't work?

Thanks.
 

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Check Engine light

It has been my experience on ML's that the Charcoal Canister Shut-Off Valve is very common cause for fault codes related to "Major EVAP system leak". The valve is located in a really bad spot. On earlier ML's it's right above the spare tire and on later 163 chassis ML's it's mounted to the frame rail right behind the right rear tire. Both these locations expose this valve to considerable amounts of road debris which can enter the valve and prevent the diaphragm inside from sealing properly causing your major evap leak. Further testing is required to isolate this component as the failure.

Will Stahnke
BenzTech1
 

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Discussion Starter #12
Thanks guys.
All ML issues have been curred for now. The leak was the Gas Cap itself. The Fuel Trim for both tanks was the MAF. After replacing the MAF I have disconnected the battery for about 10 minutes to reset the mixture adaption. Everything worked great. It has been 3 weeks and no Engine Light is on.
 
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