No heat when driving down road

wow 1,double posted, why do that.
wow 2, contradicts the first.
here is your other post
"Have heat when sitting still no heat driving down road replaced water pump an thermos state"
wow3, did this happen before removing parts?
or did this precipitate all actions.
or just doing tuneups and now fails.?
context is everything, and history.
wow 4, no engine of the 4 stated.
you left out HAVE HEAT, this time. why?

why not add to the first post
better yet, answer the questions?

here i ask 1 more time
1: have heat where, (cab heat, or is it engine heat missing or BOTH?)
2: what does the below meter do.? under all conditions, moving car and not. hot engine.????

see the red arrow for good heat,?
the gauge must start low, (at ambient air temps)
the rise to the (purchased) thermostat rating (unstated)
the hold there, parked and moving at any speed.
that is NORMAL>

x2 levels in actual rad.
x2 not burping all air out of system,
wrong stat
installed backwards or missing seal.
see the seal on my 3.7l (1 of 4 engines)
if missing or wrong sized or old and ratty,
can leak.even inside leaks.

Could be the radiator...What year is it and engine? Does it overheat sitting idle or do you have to drive it down the road to get it to heat up?

Assuming the thermostat was properly installed, you are low on coolant (dont open cap if engine is hot) or your coolant temp sensor is bad.

It appears that you have a viscous fan hub driving the fan or should I say failing to drive the fan. The fix is to replace it . If you have electric fans then there is a problem in the temp sensor system and the fans are not cutting in on temperature The problem you state is indicated by the fact that when driving there is sufficient air through the radiator to remove the heat from the radiator ..but when driving slow there is no air flow because the fan is not drawing air through the radiator and the temperature climbs until over heat is reached . Typical indicators of fan speed problem is overheating at stop sign/lights, slow heavy traffic driving, slow up hill driving, and rapid cool down as road speed is increased , the faster the speed the cooler the motor

i think this is the first sign of the water pump impellor coming loose on teh drive shaft ,these modern vehicles use a plastic impeller on a metal shaft and they can and often do start slip on teh metal drive shaft ,common problem but as a local mechanic to have a drive and possibly confirm for you .

Ihave a 2000 Dodge Durango i have put a new radiator and the gaskets came back good um i replaced the coolen fan also the thermostate and tested it it it came back good when i run it its fine but when i am in line its hot when i drive it cools down........

sounds like air pocket in system. try running it with radiator cap off until it hats up and thermo stat opens . might burp it out. Don't drive it , just let it heat up sitting in driveway.

Check your radiator, any coolant left?

i would go for the water pump, often people change the head gasket but remember,,, something usually makes this blow ie thermostat, dodgy water pump, blocked radiator ect,,,, but water pump would be my diagnostic

the thermostat is stuck open. Youv'e got a little rust in the coolant, right?