How to get rid of Dexcool sludge buildup

A friend of mine has the vehicle listed above. Last week the engine started overheating, with the temperature gauge jumping up and down constantly. I noticed there was plenty of sludge buildup, likely from previous owner mixing coolant.

I drained out most of the coolant, ran a garden out through the system and heater core many times. I then used Prestone cooling system flush with water. Drove it about 40 km, emptied the system out, and ran water through it again several times, and then mixed the Dexcool concentrate.


Since then it has been a little better but it has overheated twice. There is still a bit of sludge in the reservoir tank. I opened the bleeders and there was a lot of air coming out. It stopped a little then I noticed more and more air coming out the bleeder on the thermostat housing.



I also bought a new thermostat and cap. I tried to get the thermostat out but gave up because of the exhaust in the way after I pulled out the filter, air box and throttle body.


Should I bother flushing the system again, or is it just air?