Postby ramishrrr » April 8th, 2025, 6:32 pm
The fast idle system on earlier Toyota engines with a cable-operated throttle is not too difficult to understand,
Upon starting of a cold engine the ECU takes feedback from the coolant temperature sensor to bring the engine to operating temperature quickly.
A solenoid-operated fast-idle valve is located on the throttle-body, This is a unit with a cylindrical brass plug, rotating on two bearings, The plug has a slot, that aligns with two ports.....one before the throttle butterfly, and one after the butterfly. When the ECU sets the engine to fast-idle, the slot on the plug turns to connect the two ports. The vacuum of the intake manifold pulls air through the ports to cause the engine to fast-idle, independent of the throttle butterfly. As the engine comes up to temperature, the fast idle valve will get a signal from the ECU, causing the plug to rotate, eventually cutting off the air that caused the fast idle. It is a black plastic body with a two-wire socket.
This fast-idle valve is also used by the ECU to speed up the engine when the AC compressor cuts in. It should be taken out and cleaned of carbon buildup if you have an idle problem. Lubricate the bearings with motor oil.
A problem that causes the engine to fast-idle for a prolonged period is if you have a coolant thermostat sticking in the open position. Because of an open thermostat ( it is actually a water valve) the coolant circulates through the radiator keeping the engine cool, preventing it from achieving running temperature. So, the ECU will keep the fast-idle valve open, trying to achieve the temperature. A stuck-open thermostat can be identified if the engine takes more than ten minutes to reach centre mark on the dash temperature gauge.
Because of the stop-and-go driving in our country, a trip from San Fernando to Port of Spain involves depressing and releasing the accelerator hundreds if not thousands of times. The throttle valve (butterfly) opens and closes hundreds of times each day. This promotes wear on the butterfly shaft, and the butterfly itself. When the butterfly is worn, excess air passes through the throttle body, and ultimately causes a fast idle. There is no cure, but to replace the throttle body. If the throttle-body is removed from the engine and held up to the light, and there is a gap between the butterfly circumference, and the body (where light passes through). Then the throttle-body should be replaced.
While the solenoid-operated fast idle valve has a small adjustment, there is an idle adjustment screw on the throttle body, pushing against the quadrant where the accelerator cable attaches. Of course both adjustments should be made on a warmed-up engine.
Hope this helps those with older model engines !
Last edited by
ramishrrr on April 11th, 2025, 6:02 pm, edited 1 time in total.
