i have a 1990 ls 400 and a couple of months ago i slipped up and put unleaded gas in it...it jerked a awhile but i put gas treatment and premium back in it. it rides smooths but not as powerful it idle alil lower than usual...it rides pretty good not as strong though

 

All gas in the US is pretty much Unleaded these days. I think you meant to say regular instead of premium. You could check the thermometer by feeling on one side then the other of the coolant hose mine idled weird when my thermometer went bad.

 

I would change the fuel filter and see if that helps. You Can get them for around $16.00 dollars at autozone. I recently just replaced mine and it definently improved on my lex's power.

 

Im thinking you mean thermostat

 

The IACV is a major cause of low idle. Search for it and you'll find alot of info on cleaning it and the symptoms of a bad IACV. Here's a start; http://www.planetsoarer.com/IAC/iac.htm

 

thanks alot guys

 

This is just my experience and I don't know this is related to yours or not, dncerre.
Like you did, I have filled the regular gas when the premium was not available at the country side several times in the past.

I began to have a low idle problem on my 98 Celsior recently and decided to check the throttle valve. I found carbons building up there and I cleaned them using a can of carburetor cleaner branded Musashi Holts Japan.

The result was too successful. I mean the idle was too high around 1600rpm at P when the engine was started. But it settled around 750rpm at P and 550rpm at R and D after driving 30 minutes in the city. The ECU has readjusted the idle speed, I believe.

I don't know my problem was related to low the octane gas or just my car's age but it was a good try to check and clean the throttle valve for me. Now my Celsior runs great too.