Engine coolant temperature climbs to 120_C, heater blowing cold air. seems as if coolant finally starts to flow across heater core because heater will start blowing hot and engine coolant temp gauge to ~100_C. Temperature up and down from ~100-120_C. Suspect ACC mono valve. Would like to hear how hard a job to replace on my 1990 300E.

buy the repair kit. Remove a few of screws and replace. It is next to the battery on my car. Parts shop or Fast Lane should have it.

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J.H.
'86 300E

Replace the thermostat.

The mono valve repair kit J.H. mentions isn't available for the '90 M103 engine, you'd have to replace the whole valve. I think it's a pretty straightforward job to replace because it's very accessible ... although I'm pretty sure you'd have to drain your coolant system.

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-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

The coolant temp rises to 120C! Forget the mono-valve!