smithm92 Posted July 12, 2004 Share Posted July 12, 2004 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. Quote Link to comment Share on other sites More sharing options...
jzz30 Posted July 13, 2004 Share Posted July 13, 2004 a vacume leak some where could cause a high idle. Quote Link to comment Share on other sites More sharing options...
glenmore Posted July 13, 2004 Share Posted July 13, 2004 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? My 1990, 132k, dealer serviced by PO behaves exactly the same. I was thinking that the long warm up period was one of the causes of my crappy MPG. Idle in neutral after cold start, 1200-1300. Cold idle under load, 1000-1100. Idle after car is warm, 600. It seems to me that the the cold high idle stays on for too long. glenmore 1990 LS400 1991 300CE Quote Link to comment Share on other sites More sharing options...
smithm92 Posted July 13, 2004 Author Share Posted July 13, 2004 Agreed, a vacuum leak could cause a higher idle, but in this case the motor does settle down to the correct 600 rpm idle. So, I don't think the problem is vacuum... Quote Link to comment Share on other sites More sharing options...
smithm92 Posted July 13, 2004 Author Share Posted July 13, 2004 glenmore, Thanks for the info on your similar warm-up characteristics. My LS400 actually delivers 20 mpg in city driving and about 27-28 on the highway. In the city I'm drive with a light foot and rarely use my AC. Do you have the same "floating" high idle until you come to a stop and then experience the idle drop to 600? Quote Link to comment Share on other sites More sharing options...
VMF Posted July 14, 2004 Share Posted July 14, 2004 i think it is normail for 1uzfe to get a pretty high idle till it reaches it operating temperature. i always wait till warms up completely or just remote start the car few mins b4 i'm gettin out. make a lil experiment and let it warm up while ur in the car watchin the temp gauge. will it fall to 600 when need will reach the middle? Quote Link to comment Share on other sites More sharing options...
smithm92 Posted July 14, 2004 Author Share Posted July 14, 2004 VMF, Good idea. I'll try that and report what I find. Quote Link to comment Share on other sites More sharing options...
smithm92 Posted July 15, 2004 Author Share Posted July 15, 2004 I did the warm up test this morning and the LS dropped to the correct 600 rpm when it reached normal operating temperature. Based on this, it seems as though the idle speed control valve is operating correctly. So I'm left wondering why my LS maintains a high idle speed until two conditions are met: (1) normal operating temperature; and (2) zero mph. I verified yesterday that the high idle speed remains in effect if I enter the freeway before the car has met both of these conditions (even after the car has warmed up after a couple of miles at freeway speed). The tell tale sign is coming off the freeway, with a fully warmed engine, and still having high idle until after I've braked to a stop and am still for about 4-5 seconds. It sure acts like its another input to the ECM which in turn controls the idle speed control valve. Before I go on a wild goose chase, would others please confirm that their gen I LSs idle properly after running fully warmed-up (but not having had the 4-5 second stopped condition). Thanks in advance. Quote Link to comment Share on other sites More sharing options...
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