So, this is going to sound pretty crazy. I originally had an OE Nissan radiator cap that's been in there for over 200,000 miles (outlived the original radiator). But, I am almost certain it was causing me to run low on coolant in the reservoir. I couldn't find a leak anywhere, but my coolant was often running low. I actually thought my waterpump was leaking, since I've never changed it. Not sure if the previous owners did though.

So, I changed the cap out twice in a month: 1 with a stant and now I have a toughone branded cap from advanced auto. My coolant doesn't run low or requires top ups anymore. However, after trying 2 radiator caps, it tries to overheat when I'm on the highway climbing mountain grades. This ONLY HAPPENS AT NIGHT, and on my last trip...I've realized it only occurs with headlights on. I end up having to turn the heat on full blast to keep it from overheating.

Earlier this month, I drove with the A/C on going on a steep uphill for several miles (if anyone knows about the nevada/california border going south on i-15). No overheating issues. But, with the lights on after car has been running a bit, car starts to overheat (if anyone knows about the i-80 going from california into nevada in the sierras) my car kept running hot and I couldn't finish the trip and ended up pulling into a rest stop for the night.

Now...I didn't have this problem with the Nissan radiator cap. But I was losing coolant with that one (it was an old one though). So, I'm wondering is it maybe something with the fans or the radiator cap? BTW, this never happens when driving around town. Only going up hills.

edit: I'm also not using any kind of high powered headlights either. Just regular bulbs.

 

For all those symptoms I would:

Replace the thermostat with OEM. Aftermarket thermostats really do suck for this car.
Check the alternator, or simply unplug and replug all its connectors to clean up the grounds.

Maybe Also clean up all the grounds in the engine bay by un-screwing and re-screwing in any wire that terminates to a body panel or the motor.

 

Re-reading your post I would suggest an oem rad cap first...then do the others if needed.

Actually, I would do the grounds anyway......18 year old car....can't hurt. You might even notice your headlights working better too. Halogen Headlights respond to minute changes in voltage.

 

I don't know how the headlights would cause overheating, but maybe the electrical load of the headlight is too much for a poor ground condition. Very curious.

In addition, I wonder if the radiator could be clogging up.

 

Overheating after going up a hill may also point to your lower radiator hose going soft. As the water pump works harder, the suction can cause the hose to collapse. Don't know why the headlights would affect that though.

Are you sure the engine was actually overheating? Maybe there's a grounding problem that causes the temp gauge to read wrong when you turn on the lights?

 

In answering everything at once: I have replaced the thermostat couple years ago with a beck arnley. Didn't had any issues after doing that. The Nissan therm wasn't broken, just preventative maintenance.

The engine bay is pretty clean, but I did have some success cleaning the connections around the oil pan the other week. Engine Light stayed off for short after, then came back on after restarting.

I'm pretty sure the engine is trying to overheat. When it gets to the 10 o'clock position, I turn the heat on full blast and I've nearly scalded my hands when feeling the heat coming out the vent, which is higher than normal. If I don't turn the heat on, it starts going into 11 o'clock position, and I don't want to end up with a warped manifold or blown headgasket.

 

The engine has 2 temperature sensors, one for the ECU and the other for the temperature gauge. The one for the temperature gauge plays no part in how the car runs.

Have you checked to see if the cooling fans work? It's easy enough, just turn on the a/c and the cooling fans should also turn on - both fans.

Do you have or could you borrow an OBC code reader? The better readers will display and keep updating various things, one of them being engine coolant temperature. You should see the temperature keep moving up and down between approx 180º and 200º F.

 

The aftermarket therm was significantly different in size compared to OEM, right? I would go OEM.

Also, you have green coolant, diluted correctly 50/50 etc?