I have a 99 GS300 that I bought Brand New. Currenlty has 76K miles on it. It has been a flawless car. Yesterday, the temperature outside was over 90's and I was doing a lot of stop and go driving, with the A/C running. I noticed the Temperature gauge was all the way up to high.

I immediately shut off the A/C and got the car going up to 55 mph. The temperature gauge started dropping right away. I drove home about 30 miles and as long as the car was going 40-50 mph, it stayed cool, even with the A/C on (which worked fine). As soon as I got into stop and go traffic, the car would start heating up again.

This morning, when the car was cool, I added a little bit of antifreeze and could not get the car to over heat when it was about 80 degrees outside. But this afternoon when it got in the 90's I experienced the same problem in stop and go traffic. Temp gauge all the way up. Cooling down when A/C shut off and car moving along at 50mph.

With the car running and very hot, I checked under the hood and all fans were running and belts were intact. No steam from the engine compartment at all.

Does this sound like a problem thermostat? Anyone have a similar problem or know what might be causing this.

Thanks for comments.

 

When was the last time the coolant was flushed?

Make sure the air is bled from the coolant system, otherwise air would decrease the efficiency of the coolant system. Try this before looking into a thermostat.

__________________

1999 Lexus GS400
-Stock

MUAHAH

 

my vote is the thermostat...good luck.

 

did you check if the cooler fans are turning on?

 

bingo- sounds like your electric fan is not coming on.

it is full of coolant right? you've changed the coolant a couple of times over the last 76k miles, right?

edit: I see you said the fans are running. with the cap off, can you see the water circulating? if so probably not the T-stat. maybe waterpump is bad.

 

He mentioned running fans

I'm pretty sure it is thermostat

 

Check for movement of coolant and air in the system...same thing happened to my computer

 

Have your radiator flow tested. This is symptomatic of a partically clogged radiator. Chances are your running the original coolant.

 

No Way! - I had the coolant replaced at 30K and 60K, about 16K miles ago.

 

Was distilled water used on your coolant changes...?

No matter, the indication of a clog still exists. Have the flow checked to be sure. And consider replacing the thermostat.

 

I suggest trying the easiest thing first. Bleed your system. Open the radiator cap, and start the car, wait till all of the air bubles come out, they see what happens. Change the radiator cap while you are at it.

 

Most likely you have a problem with your thermostat....

Runing your car at high speeds with or without a/c get you at higher RPM's. Higher speed=higher engine temp. Thermostat reaching it's critical temp. must allow (by opening) for coolant flow thru radiator causing its temp. cool down and than return to engine block with lower temp.

Same thing in slow traffic....., thermostat being bearly open, there is no flow , no flow=no cooling
Running fans won't help coz there is not sufficient amount of hot coolant coming thru radiator. It might be just enough to activate fan sensor.

Bad thermostat is not always operating in it's full range, that means:
1.It may be stuck at fully open positon, allowing for constant cooling
2.It may be stuck at fully close position, (worst possible) causing overheating at any speeds, any wheater conditions....
3.It may work properly in lower temp. range but not respond at all at higher temp.

Diagnose:

Locate thermostat, if coolant piping coming out of engine right after thermostat is fairly hot or if you're able to hold it for few seconds while engine is overheating, that is a thermostat

I belive #3 is a right diagnose for your car, good luck finding problem

 

LEXSOOS,

Thanks for the help! - I think I'll just go ahead and replace the Thermostat and see if that fixes the problem. They are cheap, and it's only two bolts. I think it's located on the passenger side of the engine, where the bottom radiator hose attaches. Only 2 small bolts. Correct me if I'm wrong.

I used to change Thermostats on a regular basis in the 60's and 70's.

 

If so, you won't have any problems unless you don't know what are you doing.
I'm not sure about thermostat location on GS300 (i have GS400) so if you need more help just take a picture of it and post it so i can verify it for you, cool?

MAKE SURE TO TOP COOLANT OFF

 

with teh red stuff