My 2004 3400 closed system ,runs hot in town unless heater is on. driving up a mountain pass it gets very hot.on a flat highway runs normal. new t state,resivior cap, both fans work no leaks or smell

May be a bad fan or thermal switch not working. The a/c condenser sits in front of the radiator. It may be that the load on the engine going up the mountain is too much for just the air blowing across it to keep it cool, and the car shuts the a/c off to try and keep engine temp down. Cruising along on level ground is enough to keep things cool. Going up the mountain means more engine heat and probably reduced speed, which reduces air flow, especially if a fan isn't working.
Let car cool off, start engine and let it warm up without driving. Monitor temp and fans. When temp gets about normal, look for fan to kick on. Then when turning on a/c, look for the other fan to kick on. Don't let engine overheat. If one or both fans don't come on, start there. Check fan motor not locked up, relay working, fuse/wiring good.

Usually it's a leaky thermostat allowing to much coolant to pass through it when it's closed.To check this, clamp off top rad hose completely(acting like a closed thermostat) If you have hot air from the heater now ,you do need a replacement thermostat.

I would check the vanes on the water pump and the inside of the radiator. Both can cause this.

the radiator should be a uniform temp if there are cool spots, you have blockage. start looking there.

Your heater control valve will only cause you problems if it leaks or fails to switch to "off" in AC mode. It will not cause the engine to run hot. (unless it leaks coolant). You did put the hoses back where they belong?
I'd replace the thermostat as they can get stuck partially open. On a 92 I'd also suspect the radiator as having excessive rust plugging part of the core. Most times flushing won't get the rust out unless it's done by a radiator shop. Normal temp should be near 210 (f). Use a 195 thermostat.

The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.
If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.
The hose goes in to the engine to a crossover and comes out the other side close to the thermostat housing,the blockage may be there.
Also note that the cooling fans don't come on until the coolant temperature hits around 225 degrees.
During normal highway driving the fans will very rarely come on(if at all) due to constant air flow through the radiator.They will only usually come on in heavy traffic conditions on a hot day.
But......if you have the Air Con switched on, the fans will run all of the time.

It's probably not the thermostat @ all !!!!! what it sounds like to me is a faulty temp. sensor.

It actually sounds to me like you're low on coolant. My Jeep leaks coolant and whenever it gets low the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

Hot water runs through the heater core regardless of whether or not your thermostat is open or closed. That's why your heater works in the winter even before the engine is at normal operating temperature. The fact that the heater stops working is a good sign that the thermostat is probably not the culprit.

As for the water pump. If the water pump was bad, your temp gauge would go into the red and stay there. Water pumps generally don't work intermittently. Either it's good and ot works, or it's bad and it doesn't.

However, if you are in fact low on water, as I suspect, it means you probably have a leak somewhere. The leak could be in the water pump housing gasket, so depending on where you take it for repairs, they may try to sell you a new water pump anyhow. So just beware of that.

Its overheating when you go up long hills because the engine is not getting adaquete air in the system. The radiator is angled to get optimal air a certain way. It may overheat but you shouldn't start seeing smoke.

seems like air in system problem, park the car such that the radiator is the highest point of the car, can also use jacks, put the heater in the car on, run the car with radiator cap open til theres hot air coming throung the heater,make sure radiator is full.