1997 Audi A4 Air and coolant system

1997 AUDI A4
120,000 MILES • 2.8L • V6 • AWD • MANUAL
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PJSTURRE
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Car 1997 Audi A4 Quattro Sedan (8D2) V6-2.8L (AFC)

Problem: Thermostat doesn't open. Suspect there is air behind it. How do you bleed the air out of the system? I've tried bleeding the heater core, back coolant pipe, but this engine doesn't have a front bleeder, it has all rubber hoses. I've changed, timing belt, water pump, thermostat, serpentine belt. Engine is still running too hot. Need help.
Feb 20, 2015 at 3:32 PM
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CARADIODOC
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If you suspect air under the thermostat, which WILL keep it from opening, look for a sensor or a threaded plug on or near the thermostat housing. Those can be removed to bleed the system as you fill it. If you remove a sensor, keep the ignition switch off while it's unplugged so you don't falsely set a diagnostic fault code. On a lot of engines you can also remove the radiator hose, then poke the spring-loaded center plate of the thermostat to let the air out.

If you continue to find air under the thermostat, have your mechanic perform a chemical test to check for a leaking cylinder head gasket. That involves drawing air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow. That air will also prevent the thermostat from opening. Thermostats have to be hit with hot liquid. Hot air won't do it.
Feb 20, 2015 at 4:22 PM