The only thing that I can think of is that you may have damaged the wire filament in the MAF meter.
If you know someone else w/ a GS400 maybe you can swap and see if idle issue goes away.

 

Thanks GILL, I will try that. I think there is someone I know at my gym with a GS400. My idle is ONLY high (around 1200) when the car is anywhere below the 1/4 line on the temp. guage. As soon as the temp. rises above the 1/4 mark my idle is at 700 rpm (in park) and 500 rpm (in drive, brakes applied). BUT and here is a BIG but...the car sometimes NEVER gets above this 1/4 line on the temp. guage. I know it's cold here in NY (15 - 20 deg) but I would think after 25 minutes of driving to work in the morning that the car SHOULD reach normal operating temp. Is this correct? or not necessarily. Thanks - Nick

 

I have not had my Lex out in a long time in the really cold weather but it sounds like your car is doing alot better since it does not idle high all the time now.
I would think that that 25 minutes would bring it to normal op. temp. But w/ 15-20 degrees temp. and driving speeds ( wind will factor) it might be keeping the temp. gauge below 1/4. Your temp gauge is taking its reading from your water temp.
A test to try. On cold morning let car warm up to over 1/4 on temp gauge and rpm drop to normal and then drive to work. while your driving to work see when or how soon the temp.drops the gauge below the 1/4 mark. when you finally stop, and your car is idleing it should raise to norm temp quickly3-5 minutes.
Just FYI I know mine is a 430 but for reference:
I went out to my garage (its not 20 degress but its 42 degrees). When I 1st started the car it idles at about 1100rpm, when the temp gauge was mid-way to 1st mark (1/4) it dropped to about 950, at 1/4 mark 850 and slightly above 1/4 mark to 2nd mark it stayed at 725- 750rpm.

Also how many miles on your car? When was the last time your thermostat was replaced? just a thought if it is not working righ, it could keep your engine temp down but 15 degrees is pretty clod.

 

Thanks again... I have 87K on the car now. I bought it as a pre-owned Lexus from my local dealer almost 4 years ago. I have never replaced the thermostat. IF my thermostat is faulty and it IS keeping my car in "cold start-up" mode (with higher idle) would that explain my terrible gas mileage lately? I have only been getting about 215 miles on a tank of gas (approx. 16.5 - 17 gallons is used when I fill it up) - mostly local driving.

 

I've been following this thread. Honestly, something doesn't sound right. It's been below zero here in Chicago on several days over the last few weeks, and my temp gauge always gets up to the normal position after about 10 minutes of driving. I'm not sure what are the symptoms of a bad thermostat, but I'm guessing that would be a good place to start. I have to believe others on CL have had this problem.

 

Generally if your thermostat is stuck open, your car won't heat up to get to normal temp as fast, most thermostat will be closed until the temp reaches somewhere in the 190 degrees and then at that point, it will open up and let the coolent through the engine.

 

Its definitely the thermostat. I have a 94 camry 4cyl and it does the same thing in the cold and lately here in jersey its been in the teens. You have to let the car warm up completely before driving. In my camry the temp would actually drop back down while driving. I took it to my uncle (who Is a toyota mechanic) and he said its the thermo its just stuck open, replace it and the car should warm up fine. As for the idle even if the engine got warm and then cold again the rpms should not go nack up unless it drops extremely low. But it sounds like its not a big deal just that thermostat.

 

Thanks for the reply guys. It's exactly what you guys are mentioning. The car sometimes reaches normal operating temperature more so than before. But every once in a while the temp. guage will go back down to a little below the 1/4 mark after it has been at the halfway mark. I will have the thermostat checked out. Why do these things go BTW? Could I have screwed something up with my throttle body cleaning (using carburator/choke cleaner "not intended for fuel-injected vehicles" - stupid) or is this NOT related? The car runs fine and my gas mileage is almost back up to where it was previously. 230 miles on a full tank (17 gals. of fill-up; local driving) is this what everyone else gets on a GS4xx???

 

I didn't have the thermostat checked as of yet but I noticed one other thing yesterday morning. When I started the car up I was hearing a loud "swishing" noise and heard it more when I started to drive. Then it went away. I guess this is the coolant that I am hearing...never heard this before; is this normal? I had my timing belt done about 10K miles ago however at that time I DID not change my water pump.