I'd appreciate your thoughts on this one... When cold, my '90 LS400 (132K, dealer serviced all of its life by PO) will high idle at 1200-1400 in park. Once I shift into drive, the idle will drop to 1000-1100. All fine so far... After a couple of miles the motor will warm to normal operating temperature where I would expect the idle speed to drop to 600. What I am experiencing, though, is that when I brake the car to a stop the idle remains at 1000 for 4-5 seconds before falling to 600. Is this normal? Do other LS400s operate this way?
I've found somewhat of a temporary side-step to the issue: when I brake the car to a stop, I shift into neutral. This momentarily raises the idle but then drops the idle to 600 in 2-3 seconds.
It's as if the ECM must sense that the car is at 0 mph before dropping the idle to the normal 600 setting (or the ECM is overriding some errant sensor input to bring the idle speed down when it senses normal operating temperature and 0 mph). I've seen other topics on the Idle Speed Control valve and considered whether this might be the cause of what I'm experiencing, but what throws me is that the idle does eventually come down.
A secondary symptom I'm experience, that I think may be related to the above, occurs when I jump on the freeway before the motor has warmed to normal operating temperature and can I brake the car to 0 mph in order to "set" the idle to 600. In this situation, the car seems to maintain a higher coast rpm (like 1400) rather than dropping to a more relaxed 1000 or less setting. When I'm slowing down on the freeway, this causes the transmission to downshift but then slightly accelerate as the higher rpms catch the lower gear.
I have yet to check the EGR and PCV valves. Are these inputs to the ECM that could cause what I'm experiencing? Any other sensor inputs to control idle speed?
Thanks again.