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1975 W116 350SE + 2002 R170 SLK320 6-speed
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My car, a Euro 350SE, new to me in May, had a surging idle, very lumpy, with lots of smelly unburned fuel coming from the tailpipe. Mileage was grim.

Father-in-law who is a 35-year certified M-B tech remembered the US-spec 450SE/SEL's had wear issues with the Auxiliary Air Valve (sits on the front top of the engine and acts like a temperature-sensitive choke) and Throttle Positioning Sensor (located passenger side of throttle body). Because the AAV was frozen and did not move up or down as the engine warmed, these were the assumed culprits.

Part numbers identified but we soon discovered they were "NLA" (no longer available) through Mercedes. Fortunately our own Peach Parts has an LA-area vendor that rebuilt my Euro-spec AAV within a week. $340. Hurray, Phil Reinhart.

The TPS was another matter. This bizarre little antiquity has a gold foil circuit that transmits throttle postion (from the flap on the throttle body) to the primitive D-Jet brain. A post on w116.org mentioning TPS problems mentioned a guy on the Porsche 914 forum who rebuilds them. Very nice, clever guy is David Sprinkle. He agreed to copy the foil piece onto a new "circuit". Turns out the 350SE uses the exact same TPS as a 1.7 litre 914. Had mine rebuilt and returned within a week for $100.

We had to experiment with adjusting the TPS a bit with an ohm meter. Once installed, idle totally straightened out and the car runs as it was designed to do. Thank Heavens for the internet search engines and the ability to keep these cars running.
 
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