I have a 1990 st184 with the 5s-fe motor. After the engine has got up to temperature the idle speed drops down to 600-700rpm which from what I have read is to low, it should be at 800rpm. But if i turn on the parking and or headlights it jumps up to 800rpm and stays there. And if i turn off the lights it drops again. I have had a new timing kit put in and replaced the spark plugs, but this has made no difference. I have checked vacuum lines i can not find any leaks. I adjusted the idle speed up to 800rpm but this only works for a day as the next day once at temp the idle is low again.