IS300 running cold? High idle always, temp sensor resistor questions - Lexus IS Forum
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post #1 of 5 (permalink) Old 07-17-2012, 10:33 AM Thread Starter
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IS300 running cold? High idle always, temp sensor resistor questions

Car is somewhat heavily modified (Aristo swap, single turbo, bolt ons etc.), but still usually operates and runs like a stock is300.

My cooling system consists of:

Mishimoto Radiator
160 degree thermostat
slim fans
fans kick on 10 degrees colder than normal

However, my car used to somewhat heat up all the way and idle normally and perfect, now the temp gauge doesn't go past 1/4 way up from cold and runs really cold, the idle is up around 3000rpm when cold and never gets down to the normal 1000 rpm that it used to be at. I replaced the thermostat already and it made no change, I have noticed when I pull the connector to the temp sensor the idle drops 1000 rpm and temp gauge stops working. When the car finally gets to halfway between halfway point and cold the idle drops down to 2000rpm but will never get to the midpoint.

I'm assuming my temp sensor is bad, when I pull the connector and jump the connector with a paperclip the idle drops down to 1900rpm, from what I know about these sensors is that when the resistance becomes low, the car is hot and therefore drops the idle down.

To add to everything when I used to drive the car on the highway when everything heated up and acted normally, the temp would drop significantly to 1/4 up from cold and cause the idle to jump up at exit stop lights. I never thought these performance radiators made this much of a difference and it kind of bugs me so I was thinking of adding a resistor to the plug of the sensor therefore making it think it was always at normal operating temp and install an aftermarket water temp gauge somewhere in the vehicle. The question is, what would be the correct resistor in ohm's to buy to make it do what I would like it to do.
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post #2 of 5 (permalink) Old 07-17-2012, 11:31 AM
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Has the coolant system been bled correctly? Do your radiator fans come on anytime before closed loop or before midlevel on gauge?

For a stock GE engine coolant temperature switch 200 ohms is about 176*F and 100 ohms is about 212*F, so since the engine runs at operating temperature around 200*F, i would think the sensor would read around 150 ohms.

'02 IS300 MSM LSD
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post #3 of 5 (permalink) Old 07-17-2012, 03:09 PM
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before you do anything with electrical just jack up the front end really high remove the radiator cap (on a cool motor non pressurized cooling system) and run the engine, see if you get air bubbles. I find it hard to believe the new radiator would keep it THAT much cooler. but hey shit happens!
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post #4 of 5 (permalink) Old 10-16-2013, 01:47 PM
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This happens to me too, except mine is a stock 2001. Had a new radiator put in about a year ago.
Did you ever find a fix or what's causing the issue?
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post #5 of 5 (permalink) Old 04-02-2016, 07:47 PM
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I don't mean to revive an old thread, but I installed a Mishimoto rad with the slim fans as well and now my temperatures are 145-150F. Sometimes it goes upwards of 200F which is normal but not always. I don't think the T-stat would just break because I switched the rad...did you find a fix for your issue though?

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