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Pete, Mercedes Mechanic
Category: Mercedes
Satisfied Customers: 28258
Experience:  Over 15 years experience as a Mercedes dealer technician
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Marty or pearl I was looking at this heater problem from a

Customer Question

Marty or pearl I was looking at this heater problem from a year ago and I am having the exact same problem. no heat in any control position with engine running, shut the engine off and turn key on and it has full heat until the engine cools down heat. All of the same items have been checked. blockage. proper electrical operation, aux pump runs. Thermostat, replaced, everything is exactly the same as this article. It doesn't look like it was ever solved I also saw another case at this site
JA: Is the car in "Park" or "Neutral?" Do you know if your car has a "wheel lock" feature?
Customer: I'm sorry Pearl this is a 1987 560 SL what in the world is a wheel lock feature
JA: Are you fixing your SL yourself? What have you tried so far?
Customer: Everything in this posting a year ago
JA: Anything else you want the mechanic to know before I connect you?
Customer: I'll be a minute Sorry
Submitted: 8 months ago.
Category: Mercedes
Expert:  Pete replied 8 months ago.

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

Did you flush the heater core both ways until good flow was achieved?

Let me know so we can continue,thanks Pete.

Customer: replied 8 months ago.
it will flow full capacity of a garden hose
Expert:  Pete replied 8 months ago.

Is one heater hose hot to the touch and the other much cooler with the engine warmed up and running with heater on?These would be the hose at the heater core one after the mono valve and one inserted near that hose at the firewall.

Customer: replied 8 months ago.
no they are all cold unless you turn the engine off and key on then they are all hot the mono valve is open and the aux pump runs
Expert:  Pete replied 8 months ago.

To be sure this was the inlet and outlet hoses that are both cold am I correct?

Is the engine running at the correct temp or is it running cooler then normal?

Customer: replied 8 months ago.
you are correct, inlet left side and outlet, mono valve side or suction side are both cold. the engine runs at just over 80C and I just put in a new thermostat in case the bypass wasn't closing properly
Expert:  Pete replied 8 months ago.

O.k.,it sounds like low flow to the mono valve/heater core.You should have heat one one hose and then when the heater is working heat on both hoses.With cold hoses when running the pressure build up when running is restricting flow in the coolant system for some reason and allowing to flow when engine is off.I have seen leaking head gaskets cause this internal leaks with them but usually there is also an over heat issue.

If you have not had an engine coolant flush done this needs done.I would have it done paying attention to making sure there is good flow out of the heater core hoses that leads from the engine.I have seen small blockages in the engine cause poor flow to the heater core hose causing this fault.This would be my next step here.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

Customer: replied 8 months ago.
I did a flush while the thermostat was out and removed the heater hose at the mono valve and flushed back through the heater core and through the left connection until water came out of thermostat opening in water pump cooling system is super clean.
Expert:  Pete replied 8 months ago.

Did you flush the opposite heater hose as well in case there is a blockage in it or its port into the engine?

Customer: replied 8 months ago.
Hey Pete you are probably gone but I found the problem after speaking to my friend. It's the cheesey aftermarket Mono valve. The spring pressure to open the valve is too light and the slight water pressure of the engine running combined with the suction of the auxiliary water pump forces the valve shut and stops flow and it will not open back up until the engine is shut off and then just the key is turned on. I proved this with a piece of clear plastic hose from Lowes installed between the Mono valve and the heater hose about 2.5 inches long. I could see coolant flow until the engine was started and immediately flow stopped. I installed a Mercedes Mono valve for a low low price of 250.00 instead of 25.00 for aftermarket, and the heater works perfect