3rd Gen - Starting Issue & Backfire

[vanzep]

Just a quick note to mention the coolant drain screw can get blocked/gummed up - ive had issues bleeding coolant and had to take the bleed screw out a get a pin to clear out the bleed hole.

[Greg]

If it's starting to hunt as it warms up then this is the classic symptom of the idle control valve mounted under the intake plenum. When you open the valve on the thermostat housing you say nothing comes out? Does the radiator top hose get warm? Does it go solid like it's over pressurised? Have you checked the coolant for exhaust gasses or are there any signs of water/oil contamination in the engine. It seems odd that nothing is coming out of the bleed nipple does the heater blow hot? It's possible the water pump has failed but I'd expect other issues like a cold heater or overheating. If the head gasket fails and it's not obviously contaminating the oil or water you could try the dye test to see if there are combustion gases in the coolant, but I'd expect the top hose to be rock solid after a couple of minutes of it running if the exhaust gases were escaping into a water jacket. Also compression test would be all over the place. Like you say possibly a stuck thermostat, but I always thought the bleed nipple sat on the engine side of the thermostat but I can't be sure without checking. Remove the thermostat and see if the coolant flows normally and try bleeding it up etc. If your still not getting any circulation try removing the top hose and seeing if the coolant will flow obviously do this when the engine is still cold. If the coolant types have been mixed and they have reacted it can cause all sorts of blockages by removing the stat and the top hose from the radiator if it begins to flow then you might find it's the radiator that's blocked.

[wurlycorner]

Just a quick note to mention the coolant drain screw can get blocked/gummed up - ive had issues bleeding coolant and had to take the bleed screw out a get a pin to clear out the bleed hole.

Definitely that. Sometimes it won't clear with a pin, either.
Take the bleed screw completely out and see what happens then