How to check if a thermostat is stuck open

[daverclasper]

Hi. Any checks without removing the stat?

I guess when starting from cold, then no flow of coolent into the top of rad, visible with the cap removed?

Thanks

[Josef]

Well you’ll be able to (carefully) feel the top of the rad near the top hose fitting. If it warms with the engine, or remains cold till the engine is hot and then starts to warm as the stat opens, then you have your answer! An IR thermometer would also do the job. I guess you could do it visually too as you say.

[DanMi]

I would say watch the temperature gauge. If it rises fairly quickly and then stays at the same place under different driving conditions, then it is working fine.

[ahebron]

On my VW Amarok I diagnosed an open thermostat because when I went down hill the temperature gauge dropped and when I went uphill it went to normal temp.
This was late summer driving.
The replacement was 9 hours I will never get back but the re-replacement due to a pinched o-ring on install only took 6 hours.