gw204

My daughter is currently driving around in an '01 ES300 with just shy of 226K. Ever since we got the car about 3 years ago, the engine temp gauge has never registered much above 1/3 up from the bottom, so I've always suspected it was running too cool. I recently read coolant temps with my bluetooth obd2 reader and the Torque app and things fluctuate between 168 and 176...which seemed way too cold to me. I'm used to 4.0 liter Jeeps running 195 to 200. So I stopped at the local Toyota dealer and bought an OEM 90916-03084 thermostat, thermostat gasket and water inlet seal. $49 later I wondered if I should save a little money and go aftermarket...

I looked up part numbers on Stant's website and according to them, the OE temp is only180 and there's an option for a 170 degree thermostat as well. Now I'm wondering if it's actually running to cool, if someone just stuck in a 170, if the gauge is lazy or some/all of the above. I'll have to go back and look at maintenance records, but I don't believe the thermostat has ever been changed. I seem to recall validating that temp with an IR gun on the upper rad hose before, so I don't think the sender reporting things to the ECU is confused.

Questions...

1. Does the gauge get it's data from the ECU or does it have a separate sensor?
2. Are those temps actually too cold? If not, I don't necessarily want to put $90 worth of parts and coolant into solving a problem that doesn't exist. The car is old and I'm not sure how much time it has left.

Thanks.

Brian

LeX2K

Replace the thermostat with an OEM unit. FYI the temp gauge uses info from a specific sensor but it is interpreted by the ECU. This is done so the gauge doesn't move around constantly.

gw204

Did some more data gathering last night. As soon as the car was parked, temps were reading around the 170 mark. As I let it sit and idle, it slowly got hotter until the both fans kicked on...so at least I know they are working. I don't remember what the temps were at that point but they would start up and then shut off. Temp gauge climbed to what you see below and at that time, the ECU was reporting a hair over 199, and an IR gun on the base of the water outlet confirmed that. The boss that the sender threads into was reading about 10 degrees cooler than that, but I attributed that to the ability of aluminum to quickly disperse heat. At least I know that the sender is reporting accurate temps.

I meant to shoot the area of the thermostat to see what the temps were there as well, but it was getting dark and I had other crap to do.

Anyway, if the coolant coming out of the motor is only at 170 or so after highway travel, that's too cold. At first it seemed strange that the temps would be higher at idle if the thermostat was stuck open but it makes sense the more I think about it. It's most likely stuck open enough to NOT provide adequate restriction at highway speed where the pump is turning fast and moving a large volume of coolant, but is open just enough to allow it to maintain that higher temp at idle when coolant flow is much lower.

Going to stick the new OEM thermostat in, flush out the existing coolant and refill with red Zerex Asian.

KDAG

Mine, 1998 es300 at 148k runs really cool too. Doesn't really go past 1/3 --> 1/2 up the temp guage. I always thought it was because I used synthetic oil and fresh coolant. It's never been an issue for me though the last 40k miles ive owned the car. Ive since learned that the temp gauge is not a measure of the engine temperature itself rather the temperature of the coolant. Your engine can be at a specific temperate and the specific brand of coolant may be more heat resistant which may not be indicative of the engine temp. I know that at highway speeds the water pump is making the coolant flow a lot faster thats why at idle temps tend in increase. A fresh air filter and PCV valve will also help keep the temps cool.

Mrv8q

My ‘99 ran cool for a couple years, I could actually see the temp gauge lower while at speed... strange!

As above, replaced hoses, used OEM thermostat and O ring. The coolant pipe pulls straight out.

Wound up replacing the original radiator a couple months later, due to leaking plastic top; got to do the red coolant again.

Also replaced the original hose in the intake valley while redoing the knock sensors a couple years before. Not a fun job, but at least all the hoses have been replaced on a 20 YO car.