my daughter has a 2008 mazda 3. she called me and said she had no heat the other day. i go look and there is no coolant in the bottle and a small puddle under the car. i can't find a leak so fill it back up, undo one of the heater hoses to try to let air out, seems to take about 3 litres. start it and run it, warms up, no leaks that i can find.

she finds a small puddle under it the next day. i go look again. seems that the lower rad hose is soft and never gets hot. what the heck, change the thermo just in case. refill best i can. can't get the bottom rad hose hot and the car only produces cabin heat when revved. hold it on 3.5k for a minute or 2 and finally the rad and lower hose gets hot as well as coolant retuning to the bottle. air bubbles for awhile but then just coolant. now the heat is hot all the time.
she drives it today and reports it has low power and says the heater is super hot.

even though the guage stays right in the centre, i think the thing is running hot because of the heater. the bottom hose is ice cold, small bypass hose is warm. i check the old thermo and it is fine so i don't suspect the thermo, yet it doesn't appear to be opening.

im thinking water pump but the coolant is pumping thru the heater system. is it possible to get an air pocket at the thermo that will not allow it to get hot enough to open? if it is truly hot why doesn't the guage say so? It runs crappy as though it's overheating, yet no engine light.
i'm stumped, any ideas?

 

Sounds like there is still air in the system, either do this cold or hot (careful with it hot but it works also) unhook the bypass hose right at the water pump and see if there is any coolant in there at all, if so well it looks to be the pump, especially if you had coolant under it before and find no leaky hoses it usually means the seals in the pump are toast and its leaking from the weaping hole. If there is no coolant in that hose you can try to "prime" the pump by pouring coolant down there, then start the car and see if coolant comes out then, if not run the car till its hot or until coolant comes out. Either way I bet its got a bad pump. Dunno about the gauge issue, if there is air by the sensor it shouldnt really read much of anything without something to sense (coolant), but if its "normal" there should be coolant by the sensor then.

 

turns out to be air in the system that is causing issues. there was a gasket leaking, transmissions side (sorry didn't ask exactly where). service guys said there was a bulletin about the gasket. he wondered if air would come back into the system thru the leak as it cooled so it always seemed like the issue would come back.

gasket fixed, air out, working better now

 

Great!! Gaskets are cheaper than pumps Hope it runs many more miles now without issues.