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  #1  
Old 01-10-2011, 04:53 PM
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Loss Of Heat at Highway Speeds

I have a 85 300D. Seems as though when the heater is on, the slower I drive the hotter the air gets. As I reach highway speeds, the air cools dramtically.
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  #2  
Old 01-10-2011, 05:00 PM
Craig
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What is the engine temperature doing?
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  #3  
Old 01-10-2011, 05:10 PM
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I think the problem is in your monovalve. The monovalve sits towards the back of the engine on the passanger side and it has a black plastic top which is an electrical connector. Heater hoses run through this thing also, so it is easy to find. I think if you pull that top off, clean up the electrical connector it might well solve your problem. I had to do that about once a year on my 85 300D.
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Last edited by junqueyardjim; 01-10-2011 at 05:11 PM. Reason: Change from driver to passenger side.
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  #4  
Old 01-10-2011, 05:12 PM
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Quote:
Originally Posted by Craig View Post
What is the engine temperature doing?

Nothing...............
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  #5  
Old 01-10-2011, 05:14 PM
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Monovalve

You may also find it is damaged/leaking internally. There is a rebuild kit available.

Quote:
Originally Posted by junqueyardjim View Post
I think the problem is in your monovalve. The monovalve sits towards the back of the engine on the passanger side and it has a black plastic top which is an electrical connector. Heater hoses run through this thing also, so it is easy to find. I think if you pull that top off, clean up the electrical connector it might well solve your problem. I had to do that about once a year on my 85 300D.
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  #6  
Old 01-10-2011, 05:22 PM
Craig
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Quote:
Originally Posted by diesel85 View Post
Nothing...............
If the temperature is normal, I agree that you should look at the monovalve.
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  #7  
Old 01-10-2011, 07:05 PM
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x3 on the monovalve. it's very easy to install the repair kit and you have the classic symptom of decent heat at low speed and no or low heat once the road speed gets higher.
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  #8  
Old 01-10-2011, 07:27 PM
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thats interesting...usually the monovalve breaks open. You know how when you stop holding something it tends to fall. The monovalve holds back the heat so when the diaphram rips it lets heat in. I suppose its possible to break closed but that would be a bit on the rarer side.



Pull that puppy and check the rubber diaphram. Then give it 12 volts and see if it moves.
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  #9  
Old 01-10-2011, 07:46 PM
Craig
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For whatever reason, they do have a failure mode where they work at low speed and not at high speed. Ask Brian, he's been through this.
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  #10  
Old 01-10-2011, 08:02 PM
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Once a monovalve is replaced, how long do they last usually?
Anyone have a newer one break yet?
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  #11  
Old 01-10-2011, 08:52 PM
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jeepers, if mine was that easy to access i would pull it out and put in a brass ball valve
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  #12  
Old 01-10-2011, 08:54 PM
Craig
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Quote:
Originally Posted by newtodiesel View Post
Once a monovalve is replaced, how long do they last usually?
Anyone have a newer one break yet?
I replaced mine once, years ago; they should last for a while.
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  #13  
Old 01-10-2011, 08:57 PM
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monovalves can fail open, closed, stuck in the middle... lots of failure modes.
another possible issue is the aux pump could be seized. if it's seized, the heat would work low, but not high speeds. if it's the pump, it can burn out your push button unit, so find out for sure.
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  #14  
Old 01-10-2011, 10:50 PM
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Quote:
Originally Posted by thayer View Post
thats interesting...usually the monovalve breaks open. You know how when you stop holding something it tends to fall. The monovalve holds back the heat so when the diaphram rips it lets heat in. I suppose its possible to break closed but that would be a bit on the rarer side.
That entire analysis is quite flawed. Hands down, the most common affect of a torn diaphragm is normal heat at low rpm and poor heat at high rpm. The diaphragm doesn't block the normal flow of coolant; it seals the solenoid side of the valve. A torn diaphragm allows pressure to build on what should be the dry side of the diaphragm. The pressure overcomes the force of the solenoid and causes the valve to close, resulting in poor heat when coolant flow is high, as in during high engine rpm conditions.

Quote:
Originally Posted by vstech View Post
another possible issue is the aux pump could be seized. if it's seized, the heat would work low, but not high speeds.
Actually, it's the other way around.
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  #15  
Old 01-10-2011, 11:31 PM
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I think I'm on my 6th monovalve so that's an average of about 4-5 years each. Some lasted longer than others.
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