Ls400 Idle Problem

[ssls400]

hi i have a 1992 ls400 i know that when the engine is cold it revs approx 1200 to 1600, but when it gets to a quater of the temp on the display on the dashboard the revs dont drop unles i come to a complete stop and then i see the revs dropping to 600 revs then everything is ok.my dad has a 1995 ls400 and his revs about the same but the difference is he doesnt have to come to a complete stop for the revs to drop to normal idling it sort of done automatically when he is driveing the car.i have cleaned the throttle body and looks like new again and i have cleaned the air control valve and the tps.please can someone tell me is this normal or does something else need cleaning.my car drives superb but its the revs that bother me as it revs a little high even when hot and i have to stop the car to get the revs to idle normal

can anyone help

thanks

[ssls400]

can no one at all give me some insight to this please!!!!!!!!

[soundchaser100]

Just a thought, you have to stop the car to get the revs to drop? Is it possible that the transmission is pushing the engine along ( dragging in the torque convertor or something). Paul

[ssls400]

Just a thought, you have to stop the car to get the revs to drop? Is it possible that the transmission is pushing the engine along ( dragging in the torque convertor or something). Paul

hi paul my car starts up from cold at approx 1400 revs then if i am stationery when the temp gets to about a quater of it ways the revs start to drop.now id i was driving and again from cold even when it gets to the same temp the revs dont srop unless say i am at a junction or lights long enough which is about 30 seconds which seems like forever when you are in traffic the revs drop down to normal idle which is about 600.before the revs drop down to normal the car does literally drive on its own only cause the revs are at about 1000 to 1200 revs before normal idle.but on my dads its not like that his is a 1995 and after about 5 or 10 mins of driving again from cold it revs at 1200 to 1400 and when the temp gets to quater and you stop at lights the revs have already dropped.another thing is if i leave my car for about 30 mins from hot it revs about 1000 to 1100 then it drops to normal idle but my dads doesnt even after a hour you start it up from hot and staright away it revs normal idle which is 600.any ideas??

[ssls400]

Just a thought, you have to stop the car to get the revs to drop? Is it possible that the transmission is pushing the engine along ( dragging in the torque convertor or something). Paul

hi paul my car starts up from cold at approx 1400 revs then if i am stationery when the temp gets to about a quater of it ways the revs start to drop.now id i was driving and again from cold even when it gets to the same temp the revs dont srop unless say i am at a junction or lights long enough which is about 30 seconds which seems like forever when you are in traffic the revs drop down to normal idle which is about 600.before the revs drop down to normal the car does literally drive on its own only cause the revs are at about 1000 to 1200 revs before normal idle.but on my dads its not like that his is a 1995 and after about 5 or 10 mins of driving again from cold it revs at 1200 to 1400 and when the temp gets to quater and you stop at lights the revs have already dropped.another thing is if i leave my car for about 30 mins from hot it revs about 1000 to 1100 then it drops to normal idle but my dads doesnt even after a hour you start it up from hot and staright away it revs normal idle which is 600.any ideas??

by the way my ls400 is a 1992

[ROYT]

Possible causes for high idle speed listed in w/manual.

Water temp. sensor circuit

Intake air temp. sensor circuit

Throttle position sensor circuit

Neutral start switch circuit

ECU power source circuit

ISC valve circuit..is this what you cleaned?[air control ]

Dash pot. [stops the throttle closing sharply]

PS idle up

Engine and ECT ECU

This should keep you busy for a bit! :winky:

[terry moore]

Hi, sound like your thermostat is staying open and not letting your cold start off until it is warm.it alway the small things at cause big problems.hope this helps u B)

[ssls400]

already put a brand new thermostat in

[terry moore]

already put a brand new thermostat in

hi, check that the by-pass clear,thats the coolant by-pass,and if it is OK check your thermo-time switch, which mounts close to the temp sensor,disconect the plug 1 by 1, find out which is which and then test the switch with a ohrms meter

temp=20 resistance-ohms2000-3000

=40 ; ; 800-1200

60 500-650

80 ; ; 300-400 , if u don't get the same reading then replace the switch B) also chech your air pipes,pull .apart 1 by 1 and check for cracks and leaks.

[ssls400]

already put a brand new thermostat in

hi, check that the by-pass clear,thats the coolant by-pass,and if it is OK check your thermo-time switch, which mounts close to the temp sensor,disconect the plug 1 by 1, find out which is which and then test the switch with a ohrms meter

temp=20 resistance-ohms2000-3000

=40 ; ; 800-1200

60 500-650

80 ; ; 300-400 , if u don't get the same reading then replace the switch B) also chech your air pipes,pull .apart 1 by 1 and check for cracks and leaks.

hi i dont have a ohms tester.please can you explain to me over the thermo time switch as i am not clued up in that.do you think i have a faulty sensor switch?

thanks

[terry moore]

already put a brand new thermostat in

hi, check that the by-pass clear,thats the coolant by-pass,and if it is OK check your thermo-time switch, which mounts close to the temp sensor,disconect the plug 1 by 1, find out which is which and then test the switch with a ohrms meter

temp=20 resistance-ohms2000-3000

=40 ; ; 800-1200

60 500-650

80 ; ; 300-400 , if u don't get the same reading then replace the switch B) also chech your air pipes,pull .apart 1 by 1 and check for cracks and leaks.

hi i dont have a ohms tester.please can you explain to me over the thermo time switch as i am not clued up in that.do you think i have a faulty sensor switch?

thanks

Hi ,can't say if it is the sensor with out the test,but would think so.These sensors are pricy, so don't just change.make sure that it is the problem.as the sensors get old it take longerand longer to drop the revs, thing to check,the by pass termostat seals,vacuum tubes,and time how long it take for the revs to drop, from cold before going to your sensor B)

[Swampdog]

Hi there. 

I have this EXACT problem with my ls400

Everything works as it should cold start, increased idle etc.

But as original poster describes, the revs will not drop unless the car is bought to a complete stop. 

It's as if the cold start system is linked to a speed sensor or something that prevents the revs dropping to normal once engine is warm unless you come to a stop. 

It's obviously a Common(ish) fault as the original poster had described my symptoms to the letter. 

 

Someone must have come across this and any help would be great. 

Many thanks

Gary.