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-   -   No heat on highway, but lots at idle (http://www.peachparts.com/shopforum/diesel-discussion/307391-no-heat-highway-but-lots-idle.html)

Cruger 10-24-2011 11:10 PM

No heat on highway, but lots at idle
 
Alright guys, I'm having a bit of an issue with the Benz that I'm noticing now that the weather is getting colder. At idle I get loads of cabin heat, however when I'm on the highway, I get barely anything. :confused:

Coolant level is up, and the thermostat is only a few months old. Anyone have an idea on what could be the issue?

Car in question is the '85 300SD W126.

tangofox007 10-24-2011 11:15 PM

Torn monovalve diaphragm.


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Cruger 10-24-2011 11:44 PM

Quote:

Originally Posted by tangofox007 (Post 2815986)

Thanks for the info. Is there a "how to" out there showing how to replace it? I'm mostly familiar with VW diesels, and don't have a manual for the Benz. (yet)

vstech 10-25-2011 06:59 AM

take out the 4 screws in the top of the valve, remove core, replace with new core.
on your car, it's below the windshield inside the fake firewall about the center of the car.

toomany MBZ 10-25-2011 08:01 AM

I understand a torn diaphragm will result in heat all the time.

Sounds opposite from a malfunctioning recirculating pump tho.

Any leaks in the system letting outside air in? Such as a pod flap that stays open.

tangofox007 10-25-2011 08:05 AM

Quote:

Originally Posted by toomany MBZ (Post 2816065)
I understand a torn diaphragm will result in heat all the time.

That would reflect a deficiency in understanding. No heat at high RPM is the classic symptom of a torn diaphragm. There are plenty of reports on this forum that support that concept.

Mad_Matt 10-25-2011 08:49 AM

So what would cause heat all the time? That is another issue I have. Thanks

andrewjtx 10-25-2011 09:41 AM

I thought a bad t-stat would cause coolant to constantly pass through the radiator. At idle, no air moving over the radiator, so he would get heat. At road speed, it would be over-cooling the engine... right?

kerry 10-25-2011 09:51 AM

Quote:

Originally Posted by andrewjtx (Post 2816091)
I thought a bad t-stat would cause coolant to constantly pass through the radiator. At idle, no air moving over the radiator, so he would get heat. At road speed, it would be over-cooling the engine... right?

Depends on how it failed, but the symptoms as described are classic symptoms of a torn monovalve diaphragm.

vstech 10-25-2011 11:43 AM

Quote:

Originally Posted by Mad_Matt (Post 2816075)
So what would cause heat all the time? That is another issue I have. Thanks

heat all the time would be a stuck open diaphragm in the monovalve or it could be a defective CCU pushbutton unit. that symptom is classic for a siezed aux pump that has burned up the CCU board.

tangofox007 10-25-2011 02:43 PM

Quote:

Originally Posted by Mad_Matt (Post 2816075)
So what would cause heat all the time? That is another issue I have. Thanks

A poor electrical connection at the monovalve would be a common cause.

toomany MBZ 10-25-2011 05:37 PM

Quote:

Originally Posted by vstech (Post 2816194)
heat all the time would be a stuck open diaphragm in the monovalve or it could be a defective CCU pushbutton unit. that symptom is classic for a siezed aux pump that has burned up the CCU board.

Sorry, can't be the first issue, ask tango.

Cruger 10-25-2011 07:31 PM

Quote:

Originally Posted by andrewjtx (Post 2816091)
I thought a bad t-stat would cause coolant to constantly pass through the radiator. At idle, no air moving over the radiator, so he would get heat. At road speed, it would be over-cooling the engine... right?

Quote:

Originally Posted by kerry (Post 2816100)
Depends on how it failed, but the symptoms as described are classic symptoms of a torn monovalve diaphragm.

My thermostat is almost new. I bought the car last April with a bad thermostat that was stuck open. Replaced it in about March.

Thanks for the info guys. I'll order up that valve here later this week along with a new water pump since mine is leaking a tiny bit from the shaft seal. :(

whunter 10-25-2011 08:02 PM

Answer
 
Quote:

Originally Posted by Mad_Matt (Post 2816075)
So what would cause heat all the time? That is another issue I have. Thanks

Answer:
* Blown CCU fuse.
* Bad mono valve.
* Bad ground to the mono valve.
* Failed push button unit.



.

whunter 10-25-2011 08:04 PM

Agree
 
Quote:

Originally Posted by tangofox007 (Post 2815986)

Textbook symptom of this issue.


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