Lots of heat when under weigh.
Bad thermostat?
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What kind of car?
07 xterra
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Air in the cooling system.
coolant could be low
Ok ill check the coolant before I start throwing parts at it. Could probably use a change anyway. Original from the factory and has 57k on it now.
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My money is on thermostat stuck open.
Only if the cooling system has been opened and exposed to the atmosphere. ie: Removal of the cap or hose… or a leak somewhere. Perform a cooling system pressure test. The cooling system should be completely sealed and air tight. (except for the radiator cap in the event of a blown headgasket or engine overheat) Radiator caps are intended to relieve pressure to prevent the radiator or a hose from exploding from excess pressure.
I’m leaning towards this if everything above checks out normal.
Last time I had this problem in a Nissan (it was a frontier) the culprit was the Blower Motor Resistor
You sure about that one? If the blower motor resistor goes then you only have hi speed fan operation, it doesn’t matter what speed the car is traveling
Funny you mention that. Mine is definitely on the way out as fan speed 3 doesn’t work, but 1 2 and 4 do.
But when that dies isn’t it usually just that the HVAC stops blowing? Mine blows normal (on 1 2 and 4) but I get no heat at idle.
I’m guessing Jay’s right. It would make sense that it’s stuck open so the coolant is moving through too fast to heat up at idle and only gets hot when the engine is loaded.
I bought this thing new so I can pretty much guarantee that not so much as even the overflow tank has been opened on the coolant system so if there’s air in the system I’ve got a big problem.
T-stat’s like $20 bucks. I’ll pick one up tomorrow and throw it in.
Fyi, I was able to resolder the resistor on mine when it went bad…yeah I’m that cheap
Doubt that its the thermostat. Symptoms of that include overheating and your temp gauge being in the norm or slightly bellow followed by it dropping fast to the “nut freezing cold” zone as soon as you start moving. My money is on air in the system. Im a little biast though as my allroad just had that issue. Air was trapped in the heatbercore/hc lines. Some pressure bleeding got the problem solved. Now I no longer have to wear a winter survuival suit just to go to the store. But in all seriousnes I had the exact same issue. A reasonable amount of heat while driving on the highway but next to no heat below 2000rpm. If it was your thermostat you would have heat at idle but little to non while moving.
I had the problems when my heads/lim were leaking on my last car… Probably from air getting in the lines…
If you pull your tstat you can check it in a boiling pot of water…
If the Tstat was stuck open not causing heat at idle… wouldn’t the same be true when moving? Since higher rpm = faster coolant flow? negating the extra heat from the engine?
Unless an Xterra is different, I don’t know shit about cars honestly.
My money is on air in the cooling system.
Level is good. A quick google search points directly at Don being right. Surprise. 
This! Just changed mine last night as a matter of fact
So. How do you bleed a cooling system?
If the thermostat was stuck open in this weather this engine would never heat up, ever.
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Run it with the cap off, top off once the thermostat has opened. You will know when this happens because the engine fan will turn on the level will drop and both upper and lower hose will be warm.
OK cool. Rad cap or overflow tank cap?