Low heat output in cabin

I have a 1993 190E 2.3L and the cabin does not get very hot even with the dial set all the way to full heat. The auxiliary water pump, monovalve and thernostat were all changed and the problem is still
there. I have forward/reverse flushed the heater box and the problem went away only to resurface again.
Any ideas on what the problem is?
Do I need to change the heater core?
Chris
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" have a 1993 190E 2.3L and the cabin does not get very hot even with the dial set all the way to full heat. The auxiliary water pump, monovalve and thernostat were all changed and the problem is still there. I have forward/reverse flushed the heater box and the problem went away only to resurface again. Any ideas on what the problem is? Do I need to change the heater core? Chris "
I assume the blower is working correctly, the temp gauge on the car shows normal engine temp, but the air isn't hot? Have you checked for any kinks in the hoses? If you can flush water throught the heater core and it flows freely, then that can't be the problem. Have you verified the aux pump is running? Try hooking 12 volts up to it with the car shut off so you can hear it run. And verify that 12 volts is appearing at the pump with the car running. I also assume you verified that the coolant level is full?
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What's the engine's operating temperature? If below 80 degrees C. the thermostat ought to be replaced.
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The engine operating temperature is sometimes below 80C on cold days

CP
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Sometimes below 80 degrees C.?
IMHO the engine, being a gas engine, should be at 80 degrees C. after 5 to 10 minutes of driving.
If not, then the thermostat is the problem. It can't be any different.
Happy, hopefully warmer, Holidays!
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Can a thermostat fail this way (Lazy thermostat)? I think it's rated to open at 87C. When I'm driving at highway speeds on cold days (<30F) the engine operating temperature goes below 80C.
Could you also tell me what your opinion of the "Low idle and oil pressure when engine warms up" topic I created in this newsgroup (alt.autos.mercedes)?
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Definitely sounds like a thermostat with regards to the heating issue. The low idle thing is kind of mysterious, but if when you manually rev back up to 700, the oil pressure looks ok, then that isn't a problem.
Marty
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Replace the thermostat. Period.
Then, if the idle remains abnormal after the engine temperature is correct - 80 or 87 degrees C. , whatever - then focus on the idle. The oil pressure is OK at 1 BAR when idling, not a problem.
What is M-B's idle speed specification for this engine? Let's not mess about trying to achieve something that's not specification! I once owned one of these and vaguely recall it idled quietly and slowly - or it seemed that way after owning a diesel!
CP, always, always first look at the simple things for they're not only cheaper to fix but usually THE problem's cause.
Finally, change the oil, do the required maintenance and don't look for trouble. These cars are designed to run for long periods.
Happy Holidays.
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I think I have as similar problem with my 81 300SD. As soon as the temp gage goes to 80, I turn on the heater and the gage will drop to 60 within a minute and there will be hot air inside the cabin. Hot air will only be coming out for another 5 minutes and afterwards only cold air will be coming out. Someone told me it's the monovalve and/or monovalve insert. If it is the monovalve, do I I buy both monovalve and monovalve insert as two separate items?
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Buy the insert first.
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If you are saying that the temperature falls to like 40 degree C on highway, it is definitely thermostat that is broken.
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It goes down to 70C when at highway speeds on cold days...
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Interesting that you have an idle issue (based on your other thread) and a low temp (possibly thermostat) issue. There are sensors that detect air and coolant temperature that affect the air/fuel ratio (namely the air temperature sensor and water temperature sensor). I wonder if one or both of these are bad.
The good news is that all of these sensors are relatively inexpensive. I'd start with the thermostat since it already appears that the engine is running cooler than it should.
Josh

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That is normal. Nothing wrong with thermostat. You said you replaced the aux. water pump? Are your coolant level normal? If coolant level is low, you will lose heater.
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