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Old 11-23-2016, 06:15 PM
Join Date: Nov 2016
Location: Bedford, NH
Posts: 2
1998 500SL Diagnostic Code P0442

I have been chasing this diagnostic code on & off for a few months. The code indicates an "Evaporative Emission Control System Leak Detected (Small Leak)". Once I clear the code from the OBD 2 memory it usually returns within a 100 miles of operation. I hit on a YouTube video where a guy identified a cracked rubber coupling hose at the Purge Valve (#0004701593) as the source of the leak.
Today I changed both hose couplings at the valve as a precaution but really found them in good condition. I traced the plastic tubing up to the motor and inspected all of the other hose couplings and attach points that are connected to the intake system. I found one clamp somewhat loose at the left intake manifold attach port nipple. I applied tywraps at all of the push-on coupling hoses as a precaution as well.

Is there anyone out there that has diagnosed this problem and found a source/location of a vacuum leak that triggered the diagnostic code?

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Old 11-24-2016, 09:20 AM
Diseasel300's Avatar
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Join Date: May 2016
Posts: 6,043
Another often-overlooked source of vacuum leaks in the evap system is the fuel cap seal. When the purge valve operates it places the whole tank under a slight vacuum, if the cap seal leaks, the vacuum won't happen and the sensor won't register a "purge".
Current stable:
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Old 11-25-2016, 01:11 PM
Join Date: Nov 2016
Location: Bedford, NH
Posts: 2
Purge Valve Operation?

Thanks for your response. The fuel cap was replaced with a new MB OEM Fuel Cap just as a precaution. The small leak Code came back after about 75 miles.

I did note that when listening to the Purge Control Valve (MB # 0004701593) operate with the audible clicking sound that it stopped momentarily and then started clicking again.
Do you know if this is normal operation for the valve? Or should it be clicking all of the time while the engine is operating?
I cleaned the internal parts of the Valve today with carb/choke cleaner to see if that helps.
The Valve is the original one from 1997 so I'm ready to replace it as another likely fix if the Code comes back again.

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