I have a '00 Montana that has a bad intake manifold gasket and now don't have heat. Can this be caused by the bad intake manifold gasket?? Or now head gasket??

Intake Manifold Gasket: bad (leaking to the outside)

Thermostat: Has tendency to overheat when outdoor temp is hot but won't reach operating temp when outdoor temp is cold.

Is loosing oil and antifreeze (outside of engine) oil is clean but I have some slime on the radiator cap

Thanks

You have no heat either because of lack of coolant, or failure of your thermostat(stuck open) which causes a no-heat condition and over temping because the coolant is constantly flowing, but not cooling off. The thermostat needs to close so the coolant in the radiator has a chance to cool off. See what I mean?

Being a 2000, it'* probably way over due for a intake gasket job.

antifreeze is leaking but I've kept the level up where it should be. I'll replace the thermostat. The intake is bad, just haven't had the ambition yet to do it - guess I'll get all done at once

Thanks again

Yes, plan on a full upper gasket set, anti-freeze, oil change etc.

you dont want to wait, once it hydrolocks you turn a hundred dollar job to replacing the whole enigne

I agree with Justin. Don't wait on it.

Don't wait- the leak may also be internal as well as external. Many cases I have seen in the past that an intake leaking coolant leads to engine failure. It gets into the crank case and circulates after mixing with coolant. This can and will cause bearing failure. Head Gasket failures can reveal themselves in a few different ways. I.E a combustion leak- this will cause unwanted pressure to build in the cooling system and eventually over heating or something will rupture. Oil leaking into the coolant- or vise versa. If you are low on coolant- thermostats will not operate properly that is likely to be the problem with heat. Sometimes depending on the condition of your cooling system- the heater core could be restricted. I hope you get it figured out and fixed. best of luck to you!