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  #1  
Old 02-11-2021, 04:51 PM
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85 300d Heater troubleshooting help

Hello everyone, I have an 85 300d that doesn't like to blow hot air anymore especially at high speeds. I am wondering how I should go about troubleshooting this problem. I think It could be a clogged up heater core or a valve problem. Should I try to bypass the valves before I try to flush?
thanks


Last edited by Alexreed300d; 02-11-2021 at 05:03 PM.
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Old 02-11-2021, 05:56 PM
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No heat at high rpm means a failed monovalve
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Old 02-11-2021, 06:00 PM
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I'll try to bypass that first then
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Old 02-11-2021, 06:11 PM
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Originally Posted by Alexreed300d View Post
I'll try to bypass that first then
You will get constant high heat which may be a good idea at this time of the year
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Old 02-11-2021, 07:01 PM
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Originally Posted by Alexreed300d View Post
I'll try to bypass that first then
May as well just replace you monovalve insert. No heat at highway speeds is a sure sign of a torn diaphragm.
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Old 02-11-2021, 09:05 PM
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Sounds like I will either replace or bypass that mono valve ill let you all know what happens after I do it.
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Old 02-13-2021, 08:43 AM
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Originally Posted by Alexreed300d View Post
Sounds like I will either replace or bypass that mono valve ill let you all know what happens after I do it.
I can let you know what is going to happen if you bypass the monovalve; you are going to have more heat than you can stand.
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Old 02-16-2021, 01:21 PM
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On my W123 with a similar issue I had to flush the heater core. First I replaced the monovalve, which may or may not have had a torn diaphragm. That didn't help so I bypassed it. That also didn't help, so I pulled all three hoses from the heater core, flushed and backflushed several times with a garden hose, and then hooked it back up. Has put out incredible heat since then.
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Old 02-18-2021, 10:47 PM
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Quote:
Originally Posted by Alexreed300d View Post
Hello everyone, I have an 85 300d that doesn't like to blow hot air anymore especially at high speeds. I am wondering how I should go about troubleshooting this problem. I think It could be a clogged up heater core or a valve problem. Should I try to bypass the valves before I try to flush?
thanks
I would not run coolant flush through the heater core unless you absolutely have to.

Since 2007 it has only happens to 2 members but after the flush their heater core leaked inside of the Car. The same happens slightly more often with the Core/Freeze Plugs.

With either one you could say it was eventually going to leak any way but it is a lot of work to remove the heater core.
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Old 02-22-2021, 01:31 PM
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Originally Posted by tangofox007 View Post
I can let you know what is going to happen if you bypass the monovalve; you are going to have more heat than you can stand.
One appealing feature of my BMW E30 was a simple, straightforward heater. A control on the valve admitting hot coolant into the core, and three valves to control where the hot air was sent. Mercedes apparently thought that work too taxing.

My '84 300D has more heat than I want. No dialing it down apart from turning it on and off. I'm still working on it. First need to get vacuum correct.
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Old 02-22-2021, 01:50 PM
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The 240D and 230 in the us market had manual climate control with a normal heater valve and linkages etc... .
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Old 02-22-2021, 02:39 PM
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Oh well, it is what it is. I had plenty of oppoetunity not to buy my current Benzes and I knew about climate control but I bought them anyway. My E30s had their own headaches.
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Old 02-22-2021, 03:11 PM
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Originally Posted by cmac2012 View Post
One appealing feature of my BMW E30 was a simple, straightforward heater. A control on the valve admitting hot coolant into the core, and three valves to control where the hot air was sent. Mercedes apparently thought that work too taxing.

My '84 300D has more heat than I want. No dialing it down apart from turning it on and off. I'm still working on it. First need to get vacuum correct.
Interesting. On my 240 with manual climate controls, it doesn’t matter if the heat dial is turned completely or just a hair. Your choice is “hot” or “not.” No vacuum. Sounds ridiculous but I always figured this was just an old design quirk.

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