I topped off from engine top ( throttle body port), then expansion tank, drove with heat on and ac on, will have to keep an eye on it and do system pressure test.

 

Reporting back.
I changed the cap, pressure tested system, pressure doesn’t drop - no leaks.
Visual inspection of the old cap suggests it was the culprit.
It was the original 265k miles cap. Check your cap and replace it if it’s old!

 

I had a problem with coolant overflow on my LS last July ( read above!) changed the expansion tank cap and everything was fine... until today.
Again, in traffic, idling and AC on ( my AC doesn’t blow very cold air, I was told my compressor is failing but I live in Colorado Rockies and don’t need AC much).
I noticed that the temp gauge is high, shut off AC and rolled down windows, it helped from overheating too much.
I got on the interstate and noticed that that temp goes down as I drive at high speed, even with AC on ( not full blast).
Drove for over 100 miles keeping an eye on the temp gauge, everything was fine, then got off highway and temp started climbing again, I turned on heat, thinking that it will draw heat from engine and help cool it ( was I wrong?) This time temperature kept climbing even when I was going faster, not stop and go traffic. I ended up calling Uber and leaving car about 25 miles from home. Going to pick it up and tow if necessary home tomorrow.
Going to fill with coolant from top port ( throttle body port) first and see if it overheats again.
Where else would you start? I am ordering a new thermostat, the old one us only 3 years old or so, Not original. Radiator IS original and the car is 20 years old, is it time ?
Thank you all!!!

 

I'm not terribly experienced with this, but having had my thermostat fail recently, I would say they usually fail toward cold, rather than hot. Mine was having trouble keeping the engine hot enough, and that's how I knew. It was also intermittent, so it took more than a month for it to fail completely and I just replaced it with a Lexus thermostat the other day (no problems since then). However, I heard that sometimes they can fail toward hot, which would be your case... but even then, they should first fail to close which would give you cold engine.
In your case, my money is on the pump. Is it OEM? Who installed it and when? If it is aftermarket, it could be the culprit.

 

Last timing belt was replaced at 200k by a shop specializing on Toyota and Lexus, I am not sure if they replaced the pump at the time.

 

You might contact them and ask if they can pull the invoice to see what parts were replaced. That's prob the 2nd TB service so I doubt you're still running with the original water pump and thermostat. If the radiator is original, however, there's a very good chance it's the culprit. 20+ years of heat cycling will build up coolant sludge inside the core impeding the flow and not allowing the radiator to do its job. Luckily they are fairly cheap and easy to replace.

 

In any event radiator is old and they are cheap.Start there.Only problem is ordering online they come damaged....a lot! get it from the parts store is what I would do if cost isnt crazy.

That doesnt work then I'd look at pump.... if hoses,stat,and caps,fluid level are all good.

Overheat is bad news for these cars.My take,rebuild/replace entire system,its not a big job or expense.

 

The test this guy does in his video "When This Happens, Your Car is Screwed" seems easy enough. Not so sure about the solution ....

 

I also have a CEL , read the code, it's the Engine Coolant Sensor. Last time I changed it was 63K miles ago.
I filled about a GALLON of coolant at top of engine, didn't knwo I was that low, don't know where it disappeared...
Anyway, after filling it up temperature is normal after a short drive, will take it for a longer drive...

 

my coolant sensor plug lock is broken and I have been using it as us for years.
where can I get a new plug ?