My 97 E420 operating temperature is running low around 60 C degrees. The heat is not that hot even on HI. I top off my coolant level about a 1/3 gal which improve the heat a tad. So are these symptoms indicative of a duovalve problem or a thermostat problem?

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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
97 E420 Midnight Blue 197K sold
98 BMW 328i Vert White 100K, sold
95 BMW 525i White 125K, sold
93 BMW 525iT Red 193K, sold
95 E320 Green Wagon 125K, sold
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85 300SD 156K Grey (Annie), sold
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99 ML 320 Black (lease), 1998 C230 White (lease)
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T-stat

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

Just had the exact same problem on a '97 S500. Temp was very slow coming up and rarely got above 60 degrees centigrade. A new thermostat fixed it. Now it warms up faster and the gauge goes up to a hair width above 80 degrees centigrade and stays there. The climate control is much better as well, especially on days like today when it's 8 degrees Fahrenheit!

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

Sounds Like T-stat to me too. My duo/mono valve was spotty...symptoms were low heat from the vents at idle/low speeds and heats up at highway speeds.