Introducing myself, sort of

Celeste1999

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Jan 27, 2019
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I have years of experience in the automotive industry. My reason for joining is I have run out if ideas to try and resolve a heater/cooling system issue. Maybe someone else has run into this before?

'99 V6 5-speed. The heater stopped working suddenly. That was months ago. Thus far, I have changed the radiator cap, the coolant bypass tube, put a restrictor in the heater hose outlet at the heater core, changed the HVAC head unit out. A year or so ago I put a new thermostat in it as well. I can burp the system and the heater works correctly when the system is full. As the temp climbs and the pressure climbs the overflow tank fills. All like it should. But the coolant is sucked back into the radiator as the temp and pressure drop. What should I try next?

This has stumped a couple of Ford Master Technicians as well.

Thanks in advance for the assistance.

Regards,

Celeste
 
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But the coolant is sucked back into the radiator as the temp and pressure drop. What should I try next?
As the coolant gets hot it expands. As it cools it will contract. It should be expected for excessive coolant in the over flow will be drawn back into the motor during cool down.

Over multiple heat/cool cycles does the motor cooling jacket remain air free?

Did you follow the V6 coolant refilling procedures?

V6 coolant refilling procedure:
https://www.allfordmustangs.com/for...stang-irregular-rpms-showing-hot.html#2604913

Consider testing the coolant for combustion byproducts.