Lately I've been having an issue about getting heat while the car is at any sort of speed. The heat works perfect at idle in the driveway, but when I increase the rpm's to anything over 1800, the heat gradually fades away to cool air. Sometimes the heat will return when at idle....sometimes not. I believe the ACC is working correctly as it cycles the vents and changes the temperature of air properly while at idle. The aux pump is also working correctly at idle ( I can feel it running with no vibration....nice steady hum.)

In checking the monovalve, I can hear and feel it clicking with selector in full cold. Conversely there is no audible click with full hot selected, although there is a very, very slight electrical vibration (barely detectable).

This spring, I had an issue with the monovalve. I was getting full hot all of the time. The rubber diaphragm was ripped. A repair kit solved that problem, but I did notice that there was some corrosion inside of the valve itself. I am wondering if this corrosion could be the cause of my current issues.

Any suggestions would be appreciated.

Jon

'85 300D 97k

Well, seems you could check your monovalve by idling with it either open or closed and detecting if the air got cold or hot.

My thinking is that the thermostat is not working properly. When the car gets moving, your radiator is working better and you end up with cooler coolant.

You might try putting a soft piece of cardboard in front of your radiator when it gets really cold and until you check the thermostat. Just keep an eye on the temp and don't let it get too high. That should keep you from freezing.

__________________
Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

IF the monovalve is opening and closing as it should, you are left with the symptoms of inadequate flow through the valve. Corrosion in the valve, as you mentioned, could be one problem. Improper coolant flow at higher rpm, possibly due to a thermostat malfunction, could be another problem.

Can you bypass the monovalve temporarily with an inline hose fitting and drive it to see if you get a lot of heat at speed? If so, the problem lies in the valve. If not, it could be a thermostat.

If I recall, the SD models used a small electric pump to provide constant flow to/through the monovalve to provide heat flow at all engine speeds.

Jon--Sorry this sounds so simple, but are you getting 85° on the engine temp gage after engine warmup?

The last thermostat I changed, I wouldn't get over 40°C, the radiator was dumping all the heat it could handle because the thermostat was wide open.

Also, I had a very similar problem after opening the the cooling system, i.e. allowing air into the upper areas of the heater hoses.

I solved the problem by "cracking" the uppermost heater hose connection and allowing the last of the trapped air to escape.

One other thought, I had an aux-pump that "felt" like it was running, but when I opened it up, the impeller was froze. The way that thing works, it uses magnets to spin the impeller, so the motor will run all day and not move any water. It was designed that way to to keep the cooling system tight and prevent water migration through the rotating seal of the motor. I'd look at this first along with the air issue.

A quick check would be with the key on, engine not running but warm, roll the temp wheel full hot. The monovalve will be unpowered at this point and the aux-pump will be running, rather than feeling the motor listen for fluid flow or "crack" a top fitting again, you should have flow, if not, your pump is spinning, but not turning the impeller.

Send me a PM with an email and I'll send you the climate manual troubleshooting chapters--they're over the 750KB max 1.4 and 3.1.

G

__________________
Mark
========
THE WHITE FLEET
2016 GLE300d 4-MATIC 38K BROWN!
2012 S350 Bluetec==94k WHITE

2007 ML320 CDI==166K WHITE (FOR SALE)

Under new management:
2005 E320 CDI--140K--WHITE
1995 E300-Diesel-133.5K--THE CAR IS BLUE
1986 300SL--97.5K (European) AND WHITE. Back in Europe!
1991 190E 2.3-73K California Perfect.--WHITE
1995 E320-Wagon-159K--WHITE (recently scrapped)
1987 300D Turbo-213K--WHITE
1987 190D 2.5 Turbo-288K--WHITE

The impeller housing is removable for inspection. 4 Phillips head screws, just be careful putting it back together, it is easy to get the o-ring inserted wrong.

Try hooking to 12 V and pumping water from a bucket, note that this pump has no lift capability to speak of so sticking the suction side hose in the top of a jug will not work. Put the hose in the water and have the pump slightly above the water level.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

You should be able to get good heat even without the aux pump. Did this problem start right after you replaced the rubber diaphram? Was the kit installed correctly?

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

Thanks for all of the input.

I just drove 360 miles today from Maine to NYC. The thermostat appears to be working perfectly. On these cool mornings I get a slow climb to 90 and it is rock steady in cruise. At idle, it may move to 85, and during the hottest summer day it might have gone to 95 on long uphill stretches.

With the engine idling, I can get all of the hot air I need and then some. With the engine rpms higher than 1600, the hot air gradually decreases to a 'cool' condition (slightly warmer than ambient).

The monovalve kit was installed in May, and the system worked perfectly until 2 weeks ago. The kit was an MTC kit. I just ordered another MTC kit from Rusty for about $35. (An OEM is $137 )

I'll also be replacing the thermostat as I have not done so since I got ther car and I am getting ready for a road trip of several thousand miles.

Thanks again,

Jon

if you still have the factory auxiliary pump, there will be a small piece of the spindle extruding from the pump pointing toward the passenger side headlight (roughly). when the aux pump is running properly, you'll feel fairly strong pressure on the spindle end, enough to push your finger off fairly quickly. mine went this past Feb (luckily it wasn't that cold this past winter in New England) and the wife (when she borrowed the coupe) complained of no heat at traffic lights. i decided to go bosch on it because I hate it when she complains. about $100 and 15 minutes of my time, it was easy! do it soon - supposed to be a cold one this winter! good luck. J

__________________
Cantabridgian

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1987 300TD Black/Palomino 120k on donor engine (thank you CS), 305k on everything else

MISSED 1985 300CD Blue/Palomino 250k lovely but had to part with it; now wife has Jetta TDI, you know how it goes

It was in fact the monovalve. Took it apart this afternoon, and at first inspection, everything looked normal. But when I extended the old valve I could see an ever so small tear in the rubber (1.5mm at most).

Installed new valve, and it works perfectly again.

This past week, we drove from Maine to Ohio, and on the first day, the symptoms returned as before???

On Friday, I finally got to a place where I could put a wrench to the car, it turns out that the aux pump was working intermittently, and when I touched the connector, it literally fell to pieces leaving the pins behind. I also checked the monovalve, and it had a small tear (1.5mm) in exactly the same place as the previous one.

Quick phone call to Rusty @ 3:30pm, and an aux pump and monovalve repair kit were on the way to my sister's house. They arrived the next morning @ 9:30, and system is working perfectly again!

Jon

'85 300D (just turned 100,000 this morning)

Jon,

My question would be what is cutting the monovalve rubber?

I would inspect the housing where the gasket seats or see if there is some foreign object in the valve body. I would be wiling to bet you will tear or cut the new one just like the other two.

Mark

It's the rubber equivalent of metal fatigue.

Since May??

Highly unlikely--Jon installed a brand new kit that failed in 3 hours. Something in the valvebody is damaging the seal.

I just went through this same issue and have solved it with a new monovalve insert. The one I bought a few months ago appears to have been bad.
1983 300td No heat at highway speed

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I could not own any MB without the help from you...fellow forum members.


83 300td 207,XXX

Typical for aftermarket parts that get here on long boat rides. It seems that the manufacturers do a good job of copying the appearance aspects but put little effort into the material side of things.

But, checking the monovalve for used razor blades won't hurt anything.