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Old 04-08-2002, 08:21 PM
Posts: n/a
Low engine temp no heat

Hi All
My 1989 300 se has a problem with heat/defrost at 60-70 mph.The fan blows cold air.Engine temperature was 60 c and would not get any higher.At idle the temp would reach 80 c and i would get heat and defrost.
Today i replaced t/stat with a 87 c stat .Bled system topped off radiator, and the heater drove you out of the car.

Super!! I took the car for a drive engine temp at 87c as we rolled onto the hiway temp started to drop like the A/C was on.The temp got colder as we went along.
Returned to the GUY-RAGE ,played with all different settings on the heater control panel and the heat started to return until it drove you out of the car again.

The aux fan did kick in at idle but am not sure what temp. it kicks in at.

Usually low engine temp no heat means a bad T/stat but this is a Benz and i need your help.


The first one to solve this gets a 5 pack of the new Gillette Turbo Mach 3s
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Old 04-08-2002, 08:55 PM
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596

I would believe you have a defective second thermostat or you installed it backwards. The MB cooling system uses the thermostat to mix cold water from the radiator, which in cold weather can be really cold, with hot water from the engine before it enters the waterpump and gets pumped intp the head. This avoids thermal shock when the thermostat opens. It also adds a function to the thermostat making it a more complex device. Thus, new ones sometimes do not work correctly, and more often than you might expect.

I had a similar problem with my 1986 190E 2.3-16 (now my son's car - still have a hard time with that). The thermostat is the only thing that controls engine coolant temperature and if the engine coolant is not reaching design temperature, the coolant flow is not being regulated correctly. Check the action of the old thermostat and then compare it to the new one. You should see both sections of the thermostat move by the time the water boils. Add some cool water and you should see the part that controls flow from the radiator close up some. On my 190E thermostat the part that controls water from the radiator would open all the way, then hang up and stay fully open. This prevented the car from staying fully warmed up.

I have heard there is a small pump for the cabin heater water flow on some models, and that when this unit fails you do not get much heat. I am not sure if the failure mode is the same as yours, and I am fairly certain if your engine coolant is not staying at the normal temperature as you drive, you have a thermostat issue. If it is installed backwards I think it may not seat correctly, and that could lead to the same symptoms. Good Luck, and I hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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Old 04-08-2002, 09:16 PM
Posts: n/a
heat problem

Hi Bribenz

Oh where Oh where is the mono valve located on this car??

You explained the problem to a T.

1989 300 se
Where do you want the blades sent to
Also do you have a Mach 3 handle
Please email me
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Old 04-09-2002, 05:26 PM
Posts: n/a
No Heat


Thank You
I will order a replacement kit from Phil today

Please email me
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Old 04-09-2002, 05:59 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 18,745

This post might be of some help. It describes how I replaced the monovalve in my 91 SE.

cold air from defroster vent

91 300SE
87 300SDL
81 300SD (on ebay)
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