P1485

P1485 on a Mercedes is an EVAP Purge Valve Stuck Open fault — the purge valve is a normally-closed solenoid between the EVAP charcoal canister and the intake manifold, and the ECU pulses it open during cruise to draw stored fuel vapor into the combustion chambers. When the valve sticks open, fuel vapor leaks into the intake continuously, the engine pulls a vacuum on the EVAP system at all times, and fuel trims swing rich at idle (LTFT runs -8 to -20%) because unmetered vapor is being added to the air charge. The ECU detects this either by running an EVAP system pressure/vacuum test and finding the system can't hold pressure with the purge valve commanded closed, or by correlating long-term fuel trim drift with EVAP commanded duty. Diagnostic order: (1) Disconnect the purge valve's vacuum line at the intake manifold with the engine running — if idle quality changes or fuel trim shifts, the valve was leaking. (2) Apply vacuum to the valve port with a hand pump while the valve is unpowered (healthy = holds vacuum indefinitely; failed = vacuum bleeds off in seconds). (3) Ohm the solenoid coil — typical resistance is 22-30 ohms; open or shorted means the valve electronics are dead and it may be stuck in last-commanded position. The purge valve is a $35-80 part and a 15-minute job on most Mercedes. Expensive-misdiagnosis caveat: don't replace the purge valve before inspecting the entire EVAP charcoal canister and its vent valve — on W203/W204 C-Class cars, the canister itself saturates with raw fuel from years of overfilling at the gas pump, and a saturated canister will overwhelm a new purge valve within weeks, leading to a repeat P1485 and a complete canister replacement at $250-400.

Vehicle-specific patterns: 2005-2011 Mercedes C-Class/E-Class/S-Class with the M272 3.0L/3.5L V6 or M273 4.6L/5.5L V8 (W203/W204/W211/W212/W221) is the canonical P1485 platform — the purge valve sits on top of the intake manifold and is exposed to engine-bay heat that cracks the internal diaphragm around 80-100K miles. 2003-2014 Mercedes C-Class/SLK with the M271 1.8L Kompressor (C230/C250/SLK200/SLK250) sees P1485 from the same purge valve plus a known plastic intake manifold tumble-flap failure that compounds rich-running symptoms. 2007-2014 Mercedes ML/GL/R-Class with the M273 V8 throws P1485 more aggressively in hot-climate markets because the valve sits directly above the exhaust manifold heat shield. The OM642 diesel platforms don't throw P1485 because diesels don't use spark-ignited EVAP purge — that's a gasoline-engine code only. Estimated repair: $80 (purge valve plus 15-minute install on a C300) to $550 (purge valve plus full charcoal canister replacement on an S550 with a saturated canister).