White exhaust smoke and engine shuddering?

Hello NILS,

Have you been losing coolant at all? If not, then the P0125 - Insufficient Coolant Temperature for Closed Loop Fuel Control code is the clue. It is telling us the engine is not warming up completely which will cause excessive condensation inside the exhaust system which will cause the white smoke. This typically means the engine thermostat is stuck open. Here is the location of the thermostat and a guide to help you change it out step by step instructions in the images below for your car.

https://www.2carpros.com/articles/replace-thermostat

This guide may help as well:

https://www.2carpros.com/articles/white-smoke-or-steam-coming-from-the-exhaust-pipe

Check out the images (below). Let us know what happens and please upload pictures or videos of the problem.

Thanks Ken,

I am not losing coolant at all. I've been carefully monitoring that. However, the engine oil was quite dark despite only a couple thousand miles. I'm also now showing codes P0171 and P0174 for running too lean on both banks.

The engine does reach operating temperature within 5 minutes. Doesn't seem to take too long to reach normal temps. Would a stuck thermostat cause an engine to take much longer to warm up? I spoke to someone who suspects it may be a bad exhaust valve seal. Can you share any knowledge on those for this generation of Highlander?

Yes, a stuck open thermostat can casque the engine to run cold therefore holding moisture and causing white smoke, here is the thermostat location so you can change it out to help fix the problem. This guide can help walk you through the steps as well:
https://www.2carpros.com/articles/replace-thermostat

Please start a new question for the codes P0171 and P0174. Don't forget to install the bleed hole of the thermostat on top. Check out the images (below). Please let us know if you need anything else to get the problem fixed.