In a 1989 420 SEL, if I do not turn on the heater, the engine temp gauge shows 80, which is the proper temp. after a highway run. However, if the heater is turned on, the engine temp only reaches 65 or 70. Also, with the heater on max. heat and high fan, the inside temp. is not really hot.

The temperature outside is -20c. Is it a bad thermostat?

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1989 420 SEL

I would think so.

Had the exact problem in my wife's 560SL we had bought a month before.

Bought a new thermostat assuming the old one was stuck open. Previous owner had totally removed the old one!

From what I heard, MB thermostats fail by sticking open rather than closed like other cars. The result is exactly what you're experiencing.

I'd get a new thermostat in there ASAP. By the way, don't buy it at MB Express in Etobicoke.. you don't want to know what those things cost at dealer prices /how much I paid for mine at MB Express...

Neal

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)

Order it from fastlane at the top of the page. Over time, the thermostats generally start to open at cooler temps as they age.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record