coolant is low or the water pump is not working
Or you have symptoms from either...Blown HG for example. Check your plugs or your oil on dipstick. Compression test while you are there. However this is not always accurate to diagnose a blown head gasket. If it is a coolant issue then it is either leaking to the outside and you'll have coolant on the ground or leaking into the cylinder where you will have rough idle, possibly a white tipped spark plug, coolant in your engine oil (looks like chocolate milk on the dip stick)
Bottomline, no coolant through your heater core (low level-leaking out through HG or through any other cooling system component, stuck thermostat, bad pump)or the coolant isnt hot (not warmed up(thermostat has been removed)). If youve replaced your thermostat and checked your coolant level and made sure the heater core isnt leaking then I would chase 2 things
1. Water pump. does the system circulate or build pressure? If so then your pump is probably good. Plus you will heat up pretty quickly on drives
2. You have the ever so common blown HG...From your latest post, this seems like the culprit...
There arent many components to trouble shoot in cooling systems...radiator, thermostat, water pump, heater core, block channels. It is a circle just chase the most common problems first...Unfortunately a blown HG is almost as common as a stuck thermostat.
Let us know how you make out
