I have No heat after a coolant change... - PeachParts Mercedes-Benz Forum

Two560Two · #1 01-28-2006, 11:11 PM

I just acquired a 91 560sel with 124k miles. It seems as if the PO just recently did a collant change to zerex? Well, I have no heat! The engine is at full operating temp but the heat doesnt come through. Only cool air blows through. The A/C works fine and was able to test that today since it was 60 deg. Now its a bout 39 deg and I get no heat. I ran the defrost on high thinking there was air in the system but still no luck. I did that for about 5 minutes though. I am thinking that i need a new heater core? There are no leaks or coolant anywhere. Please help

t walgamuth · #2 01-28-2006, 11:15 PM

chance your hvac controls are messed up.

tom w

Two560Two · #3 01-28-2006, 11:17 PM

well, all the buttons function as they should. the A/C works. blower works in all settings

Brian Carlton · #4 01-29-2006, 12:21 AM

Start with the basics.

Open up the monovalve and check the condition of the diaphragm. 90% of the problem with "no heat" is caused by a tear in the monovalve diaphragm.

If the diaphragm is torn, replace it and you're done.

If the diaphragm is good, put it back together and unplug the monovalve and drive it. See if you then have heat.

stevebfl · #5 01-29-2006, 10:30 AM

Quote:

Originally Posted by Brian Carlton

Start with the basics.

Open up the monovalve and check the condition of the diaphragm. 90% of the problem with "no heat" is caused by a tear in the monovalve diaphragm.

If the diaphragm is torn, replace it and you're done.

If the diaphragm is good, put it back together and unplug the monovalve and drive it. See if you then have heat.

Yea, that is exactly what I would do.

If that didn't solve it I would put a pressure tester on the radiator and open the return line from the core. I would plug the engine side and I would pump the system and see what came out (the valve should be open if the key is off). You might be able to do this without the pressure tester by just reving the motor with the engine side off and something to collect the flow. Doing it that way of course empties the system rather tahn bleeding as the pressure tool woul allow.

Brian Carlton · #6 01-29-2006, 10:38 AM

Quote:

Originally Posted by stevebfl

Yea, that is exactly what I would do.

If that didn't solve it I would put a pressure tester on the radiator and open the return line from the core. I would plug the engine side and I would pump the system and see what came out (the valve should be open if the key is off). You might be able to do this without the pressure tester by just reving the motor with the engine side off and something to collect the flow. Doing it that way of course empties the system rather tahn bleeding as the pressure tool woul allow.

Could the shadetree mechanic accomplish the same thing by putting a garden hose to the core supply side and watching the return line from the core to ensure a proper flow?

stevebfl · #7 01-29-2006, 10:48 AM

Maybe, I'm just not sure what the rated pressure of a heater core might be. I'm sure its designed to take the rated pressure of the radiator cap installed about 20psi max. The garden hose has the capacity to creat about 70psi, I think, could be wrong. Done cautiously with good flow control one might flush the core using that technique. I have seen pictures from GM cars with Dexcool mutations that looked like heavy mud. A good flushing might be the ticket, if none of the other issues are the answer.

Two560Two · #8 01-29-2006, 03:37 PM

I had my Mech check the car out and he told me that the Thermostat isnt \ working right, or if there is even one in there? The car came from california and he told me that mechanics there sometimes even remove the T-stat to keep the car from overheating. He explained that if the T-stat isnt working correctly, then the coolant will not flow completly through the system. He will open the water pump and check from there. Also, after the car has been on for about 30 minutes, the radiator is not as hot as it is suppose to be and the Temp guage reading is still below the 80 deg Celcius mark. The temp here is NY is pretty cool enough so the car wont overheat. Thanks Guys

stevebfl · #9 01-29-2006, 05:19 PM

He is more than right about removing thermostats. On some cars removing them can remove a problem and assure an open flow to the radiator. Most German t-stats including MBs are built so that as the valve opens the passage to the radiator a second plate moves over the by-pass hole. By removing the t-stat the water is free to flow where ever it choses. This reduces flow and can cause serious overheating as some of the coolant just circles the motor.

whunter · #10 01-29-2006, 05:31 PM

Quote:

Originally Posted by Two560Two

I just acquired a 91 560sel with 124k miles. It seems as if the PO just recently did a coolant change to zerex? Well, I have no heat! The engine is at full operating temp but the heat doesn't come through. Only cool air blows through. The A/C works fine and was able to test that today since it was 60 deg. Now its a bout 39 deg and I get no heat. I ran the defrost on high thinking there was air in the system but still no luck. I did that for about 5 minutes though. I am thinking that i need a new heater core? There are no leaks or coolant anywhere. Please help

Best guess:
#1. The system has air trapped.
#2. The bypass/bleed tube (block to head) is plugged.
#3. The thermostat bypass is plugged, and/or thermostat is junk.