Having some odd issues with my ls400. Noticed the car temperature would rise a bar or two when at a stop. Temperature would drop back down when driving. Could this be an air bubble in the system? bad thermostat? i did a pressure test and didn't detect any leaks. also coolant level is still at full.

 

Original thermostat? Replace it in any event.
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Has coolant been serviced? (with OEM)

ECT Sensor original?

Fan clutch?

Radiator is original? Are condenser and radiator fins clear?

Does heater emit hot air?

Check water control valve (on firewall) for dried coolant.

Any aftermarket parts being used?

 

You could have a lot of debris built up between the ac condenser and the radiator fan. Does the temperature drop to normal when you engage the cabin heater? Also do you have access to an obd monitoring device that shows coolant temperatures? Useful information to have

 

You could have air in the system or your thermostat is on its way out.

 

please please replace your coolant temp sensor for the gauge first. I chased this problem for a few months and it turned out to be the temp sensor. I replaced it when I bought it but it still went bad lolll.

 

i suspect air in the system too as i've encountered your symptoms exactly. have you opened the system lately for any servicing? typically this happens after.

wouldn't hurt to change the temp sensor either but it's less likely to be the problem from my experience.

 

Clutch fan was on its way out. Replaced it and all good now.

 

Problems only when sitting still usually indicate fan system.