3 liter mazda mpv overheating

sounds like the radiator is blocked either coolant flow or ext air flow through the matrix the thermostat can get ruined when the system overheats so allways change the stat after a overheat problem GET THE RAD CHECKED FIRST if ok check the coolant pump

your easy, that is caused by AIR Pocket in the engine block
the gauge can not read, air temps so if air is below the engine
coolant thermostat the ECT reads hot then air (cold ) repeats
you lost coolant and the number one reason for that is trusting
the silly side tank, (they fail easy)
never do that, on the next cold morning , dead cold
you take off the RAD cap. look down the neck, is it full of green
AF? id bet you its not.
the cab heater will never work with no engine heat, ever. so ......
never do the above on any hot engine.
also the thermostat might be bad and the coolant overheated
and blew out, so get a new STAT.
the flush the system, and refill with 50% AF.
or worse. lots of ways to overheat.... ask.
the side bottle only works, (called the expansion tank)
if never overheated
and the tube in side is not dropped off
and you never let it go dry, ever.
those facts and others are why we check the RAD directly cold.

did you put new thermostat in the right way ? do you have a 50/50 coolant mixture in rad, 50 % water in rad is what gives you heat, aside from your thermostat, are your heat controls in the proper locations ?

Coolant system have bleed points. There is air in your system that needs to be purged out through the purge valves.. There maybe a few. You have to slightly open them when you first fill the system and let the coolant circulate till a little bit of coolant comes from bleed valves then you close them. Letting the system purge air by leaving the radiator cap open is not enough. Are you sure you put the correct temperature thermostat in , and facing the correct way?

Gasket probably will not seal now, and if your going to have to remove water pump to replace it, you might as well put a new pump on as well. The reason for heater cutting in and out, is probably just due to low coolant, and air got into system. When cool, remove rad cap,start engine, and turn heater on while letting it get up to operating temperature, and top up rad as needed, until you see increased flow in rad indicating thermostat has opened and coolant is circulating. Watch for that a couple of times, then put rad cap back on, and top up return bottle to proper level as well. That should get your heat back.

could be waterpump?
i was going to say rad fan,but if so heater would be hot.
lack of coolant will make heater core to blow cold,if engine heats up with full coolant,usually means poor or no circulation.
water pump?
run engine with full coolant,at operating temp.with rad cap off and look in rad cap hole and see if there is movement of water inside rad.
[rad fan will not come on if water is not circulating through rad water pump?]

Did you run engine from cold, with heater on when you refilled the system? Try again, rad cap off, with heater on, and let engine get up to operating temperature, make sure you have good flow through rad ,leave cap off, until rad fan comes on to cool returning coolant to rad at least twice. Give engine a little extra rev, for 5 seconds or so, every once in a while to make sure flow getting through heater core, and add coolant as necessary. See if that helps, as it may just be an airlock in system. If your cooling system has any bleed screws incorporated, crack them once in a while as well.

you've got a possible blockage in the head or block.

Start vehicle when cold. remove rad cap, start engine & put heater on (not defrost) and let it run for a while & top up rad as coolant goes down, to about an inch or 2 from top of rad. If you have electric rad fans, watch temperature gauge and around normal operating temperature or slightly above, they should come on for a minute or 2 then shut off. If not electric fans, just watch in top of rad for increased flow through rad, indicating thermostat has opened & coolant is circulating. Rev engine slightly every once in a while for 5 or 10 seconds just to help increase flow etc to help assist bleeding air out faster. Watch for increased flow in rad, 2 or 3 times at least, or listen for fan to go on & off 2 or 3 times before putting rad cap back on. As your doing this, keep topping up rad if it needs it. Make sure rad cap on all the way, check coolant return bottle level & add to mark if required, and you should be good to go. One of the things people forget to do the most, is to turn heater on while doing this, and that is usually what causes a lot of airlocks in the system, so don't forget that step.

Start it cold with rad cap off, and heater on high heat, and let it run until you see increased flow in radiator, indicating thermostat opened & engine reached operating temperature. Rev engine slightly every once in a while and hold it there for 10-20 seconds, just to help purge air better/faster. Watch rad flow again, and wait until you see increased flow 3 or 4 times. Then top up coolant in rad to an inch or 2 from top of rad, and put cap back on. Top up return bottle to appropriate level on bottle & then try it again. If it still heats up, get back to me. Does it overheat at all times? Rad fan is good i take it?