I’m not sure if this has already been posted, but if so, I couldn’t find it. I have had virtually no problems with my 99 RX300. The last couple of years this has been an issue in the winter.

I will let my car idle until it gets to normal driving temp, but when I start driving my engine temperature, it goes all the way back down to C. It’s the cold air, but it shouldn’t do this, right? I assume it has something to do with the coolant going through when it doesn’t need to or something but I honestly have no idea.

currently I have 282,000 miles. I made sure that I change the oil regularly.
No, check engine, lights or warnings.

I have asked the local Toyota dealership(Lexus is an hour drive) and they look at me like I’m crazy…

any ideas would be greatly appreciated.

 

Lazy thermostat can cause this to an extent, but check the radiator coolant level and look for leaks.

 

I’ve checked for leaks already. my fluid levels stay pretty consistent. Never heard of “lazy thermostat” before. Any more information on that. I appreciate it.

 

I've never heard the term either. Maybe he means it stays open when it shouldn't. Have you ever changed it?

 

The thermostat is most likely not working properly and needs to be changed out i.e. it is being lazy and does not want to work anymore
If you change it, or have someone else do it, use OEM only!

 

Please do not idle to warm up. Read the owners manual. Start and secure yourself and occupants, scan the instrument cluster for warnings, then drive slowly till the gauge comes to normal operating conditions [keep rpms below 3k till the engine warms up].

As others have pointed out, you need to replace the thermostat. There is a DIY posted [check the DIY sticky thread].

Your mpg would be suffering with stuck open thermostat and also with idling to warm up.

Salim

 

got it, lol makes sense.

Another thing I forgot to mention.. when it does this I am unable to get the car to shift beyond second gear.

For example, if I do get the car warmed up and drive it, and as soon as it cools back down to C, on its own… if I have to slow down on the highway for traffic and go to speed back up, I then have to speed up very very slowly otherwise it won’t shift.

Even if I do go slowly sometimes it won’t shift in that situation. Does that make sense? If not I’ll try explaining it another way.

 

How cold of outside temps? Run a piece of cardboard in front of the radiator and see if that’s it. I do this with my truck when it’s cold. Just have to keep a close eye on temps and take it out if it gets hotter than normal.

 

like anytime it gets below 50(f). In the summer time the engine gets to normal running temp and stays there the entire time.

 

That's normal behavior for a cold RX300. Delayed upshift to help warm the engine faster

 

that I can understand, but the engine shouldn’t cool back down. Which it does which leaves me consistently in 2nd gear. Hard to maintain Highway speeds of 75mph (Texas speed limit) in second gear.

 

There are sequence of things that are happening due to stuck open thermostat. Engine block is getting coolant from the radiator [Tstat is stuck open]. As you drive the radiator gets cooled by the air racing through it [with or without electric fan]. This drops the temperature of the coolant.
Signal from the temperature gauge tells the CPU to engage choke and the cpu tells the transmission to lockout higher gear thus forcing higher rpms of the engine and the driver gives more gas to reach the driving speed.

Based on the problem description and miles, the logical suspect it the thermostat [although any of the items listed above can be malfunctioning]. Problem can be confirmed when you take out the thermostat [you will see it crusted open].

If you have another theory, please post. Read the DIY and if you think it is beyond your abilities/tools/facility take it to mechanic. There is hardly any Lexus/Toyota skill required to change the thermostat.

Salim

 

Your knowledge and detailed explanation on this is appreciated more than I can express!! Thank you so much. I’ll respond when I am able to provide the results of my findings!!🤓🙏🏻👍🏻

thanks again everyone and huge thanks to you Salim!