Sc400 Inconsistent Idle

[GeminiII2]

I have a 1992 Lexus SC400 that runs flawlessly except for one problem----The idle speed does not stay constant. When cold, the engine will not start unless I feather the gas pedal, keep it at 1000 rpm for 30-45 seconds and then slowly back off the gas until it idles at around 450 RPM. After the engine warms up, it increases to about 700 RPM and all appears to be normal until I drive for a short period and then the idle speed goes up to 1200 rpm and cannot be kicked down. I have checked for sticking linkage, lubricated all moving parts on the throttle body, adjusted the idle speed screw, removed the IAC and completely cleaned it, and added gas treatment to premium only gasoline. What is the problem? ECU?

GeminiII2

[GeminiII2]

..................I forgot to mention that if I turn the car off and back on, sometimes the idle will go back to normal and stay that way as long as I am moving 20mph or greater. If I get stuck in traffic it will occasionally stall and restart can only take place if I give it gas and slowly back off. Cold fast idle does not work anymore. Any expert ideas?

[fhb5032]

It is probably caused by one of the many sensors that feed in to the ecu. could be coolant temperature, intake vacuum or?? Might be a bad connection to a sensor or a failing sensor.

I've had various driveability problems when I had a leaky water pump and low coolant levels in the cylinder heads. The temperature sensor was intermittently in air then coolant. BTW, you might want to check that the cooling system has been properly bled of air. (Check to see if the coolant is full at the plug on the thermostat housing at front center of the engine.)

Least likely culprit is the ecu.

You might try to find a shop manual for the engine.

Good luck.