The heater on my 91 300 SE with 80k blows cool air intermittently with engine fully warmed up. Fan blows fine, but air is cool sometimes. Engine temp gauge reaches just under 80C when fully warm. The monovalve was replaced 35k ago by the former owner. Could the thermostat be sticking intermittently?

Is the engine temp cold intermittantly?

Do You get heat if you turn the temp wheel to "max"?

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve: yes, I get heat if I turn the temp wheel to "max", but te engine always heats up to just under 80C. This morning, before engine was fully warmed up (after 5 minutes), heater was working fine (blowing hot air) but 10 minutes later, by which time engine was at just under 80C, the air became tepid. hmmm...

Is air getting sucked in through the temp sensor?

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

not sure. How do I find this out?

I had the same intermitent heat problems with my 1991 300SE. 145Kmiles.
I replaced the monovalue diaphram (Old one was torn) with a new monovalve rebuild kit ($45 approx). Heat is now great. I do not know how long those valve diaphrams last, it looks like rubber from a tire innertube. The job only took about one half hour.

thanks for the tip. Is the monovalve easy to get to and take apart to put in the rebuild kit? Does this procedure drain coolant or can it be done without draining the coolant?

I used ebay to buy a repair kit,(41.00)searech under mono valave mercedes the repair is easy and I did not have to dissconnect the hoses just removed the 4 screws holding the valve to the assy.

Theres a tiny grille on the dash or ceiling with a sensor behind it. There should be a small amount of suction that you can test with a piece of tissue paper. If theres no suction you need to check the sampler fan behind the dash.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

I would just open the coolant cap to let some of the pressure out. You will loose just a little bit of coolant. Be carefull with with four screws, it is easy to drop one. Be sure to carefully examine the old one to see exactly how the new one goes in and the washer placement.

The monovalve in your car is against the firewall, right next to the battery. I did NOT take the battery out because I did not want to mess the the radio codes, but there would be more room if it were removed. There is not alot of room back there. The part is available from any mail order foreign car place.
(Mercedes-benz-parts-direct.com or alloemautoparts.com or many others.)

There is just one power plug-in on the top. Sometimes pushing the little side clamps can be difficult in such tight quarters.

There is a temp sensor on that car in the sunroof control panel. It has a tiny, square grill opening about a half inch square.

Good luck

Good Luck

When you say intermittantly is there any pattern to it? If it gets hot on a low fan setting, then when you turn the fan up it cools down I sugest you should suspect the heater matrix is blocked. If this is the case add a bottle of fernox central heating cleaner to your cooland and run it until the fan kicks in. Then allow it to cool and drain the coolant. Now disconnect the heater in and out pipes and flush the matrix through with a hose pipe on full power.

Finally reconnect the heater and refil and bleed the cooling system.

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if it seems too good to be true, it probably is.

many thanks, folks. I looked at the photos on the MBZ CDs for R n Ring the monovalve and it said to first remove the ABS control unit to get at the monovalve container. Is this truly necessary? By the way, does it make sense to anybody that a torn diaphragm in the monovalve would produce such intermittent symptoms?

I had heat sometimes but never when it was really cold outside. I did NOT remove anything to change the monovalve, Other the top on the valve covering that holds the monovalve.