1999 pontiac montana 3.4 heat ing problums

sounds like you might have a problem with your cooling fans not working when you're driving on the expressway you have a constant flow of wind blowing through the radiator which keeps didn't include the Twin Cities dealer idle no airflow no fan blowing the car overheats

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likely leaky heater core. May try pellets that GM offers to seal small leaks. Contact dealer or good auto parts store. Rad pellets

Be sure that cooling system has been bled properly after . Start engine while cold with cap off, and heater on. Let engine get up to normal operating temperature and allow thermostat to open and close 2 or 3 times and rev engine slightly once in a while for 5 seconds or so to help improve flow. If you have rad where you can actually look in and see flow, that's better otherwise watch coolant return bottle and temperature gauge. Feel if any difference at heater as you bleed air out. Once you start to feel better /good heat, then it should be bled properly, then top up coolant return bottle and re-install cap properly. Let me know if that helps.

There's a blockage some were as when ur driving the rpm is high so pressure is pushing hot water through when idling very low pressure u need to use strong flushing .agent and flush out your system also revirse flush if still doing it after this it will b ur main rad needs re coring not heater

If the gas gauge does not work correctly and the temp gauge reads hot, you could have a problem with the instrument panel.
Both gauges get power from the same circuit, but each gauge is grounded by its own sending unit.
If the temp gauge is reading correctly, you could have air in the cooling system and it is not actually full of liquid coolant.

This is usually caused by air in the cooling system. While the engine is cool, check your fluid level in the coolant tank. Then check the radiator to make sure it is full. If both of them are full, the system has to be bled. To bleed the system, take the radiator cap off, start the engine. If there is air in the system it will bubble out and the fluid level will drop, add coolant until this stops.

I will normally remove rad cap when cool. Start engine, set heater in car to heat & turn on blower, if you use defrost setting, the rad fan &/or AC fan will come on, and you don't want that. Then while running with cap off, make sure coolant in rad is an inch or 2 from top of rad, and let it run. When engine gets to normal operating temperature on the gauge, you'll start to see more flow in radiator, and rad fan should come on shortly after, to cool the incoming hot liquid in the rad, that tells you the thermostat opened as it should, and the process starts again, engine coolant gets to temp, thermostat opens, hot goes to top of rad for cooling, and cooler liquid goes out the bottom to engine, etc etc. If your heater is not on while doing this, the coolant doesn't get to heater core in most cases, thus creates an airlock, or will cause level in rad to go down when it is turned on, and thus reduces amount of coolant in rad, plus airlocks in the entire system. So, let it run to the point that the rad fan comes on & off on it's own, about 3 times, while you watch the gauge, to make sure it doesn't overheat, if it does, shut it off and let it cool down & try again. if all appears good, top rad up if needed to an inch or so from top, put your cap on properly, add coolant to proper level in return bottle & try it and see what happens. Remember though, I am assuming thermostat was put in correctly, rad in good shape, fan is working properly as well as water pump doing it's job. If this method doesn't work, get back to me with results.

Yes that is cool if it's correct. 195 F is approx.as low as you should have. I'd change it. Be sure to run with cap off, and heater on (not defrost) while refilling system. Wait for rad fan to come on 2 or 3 times before you put cap back on, then add coolant to return bottle as needed. Make sure rad fan only comes on when up to normal running temperature. If it runs all the time, it may be causing low coolant temperature too.

You need to check coolant level in rad, when cooled down, and top up your return bottle to marked level.
Start by having coolant (antifreeze) ready, remove your rad cap, and top up the rad level, then start the vehicle, and turn on your heater (not defrost) . Let it run with cap off for a while, and keep an eye on level of the rad & top up as necessary, but leave an inch or so from top.Let it run & observe your guage while doing so, and when gauge gets to normal operating temperature, your rad fan should come on, run for a while then shut off for a while, then back on again after little while, and off again. Let it do this about 3 times, top off the rad as described above, put cap back on tight, and top up your coolant return bottle to mark on side. If your rad fan doesn't come on around normal operating temperature, and gauge heads too much towards hot reading, shut vehicle off, that's an indication that your rad fan fuse has blown, or relay, or fan itself is defective. If all works well & you now have heat, great, but keep an eye on your coolant return bottle level for next while to see if it keeps going down, and add as needed if it is. If the level in bottle continues to go down, you've got a leak in the system somewhere & need to address it a.s.a. p. to hopefully avoid bigger problems, but we'll get to that only if we need to. Do what's up above and let us know the results.

How cold is it there and does your engine cool down any while going slow? If it does it may mean you need a hotter thermostat installed, My first thought would be to check the coolant level in the radiator and the coolant reservoir! Hope this helps and Merry Christmas