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Binari

My '90 Idles High

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I really enjoy reading the posts on LOC. This site has saved me over a $1K so far, and maybe you guys and gals can help me figure out an annoying problem.

Anyway, my LS idles at 1200 after starting, and will not settle into 600 RPM until fully warmed, and sometimes I have to shift to N or P at a stop light before the idle will fall to the correct level. When it idles at 600 RPM at full stop she rides and shifts just fine. When she is warming up or idling high, she coasts with a high RPM and does not decelerate.

For example, if I take my foot off the gas approaching a freeway exit, she will maintain the RPMs and speed as if in cruise control. I have to apply the brakes to slow down and the RPMS will come down, but if I take my foot off the brakes before the engine down shifts, like when the engine is at 1400 RPM she will coast at that RPM and that same speed until the transmission down shifts. On flat ground it will maintain 40mph with foot off the gas with no deceleration.

Most times after coming to a stop the engine will idle high unless I put it in park or Neutral and then for some reason the RPM falls to 600 and then she runs properly (I.E. she decelerates without the cruise control like coasting).

I started thinking it may have something to do with one of the air control values, or a vacuum tube to the transmission.

To complicate things, I bought the LS a month ago with 99.4K miles and the first thing I did was the timing belt/Water Pump, Spark Plugs/Wires/Rotor/Cap replacement. So I have no history on how the vehicle drove before I did the 90K maintenance. I may have goofed something up but she gets 23MPG on highway, and seems to run smooth and well other than the issue I noted above.

Any ideas are appreciated.

Thanks,

Ed

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IMO, this is normal behavior. My '97 with 91k miles does the same thing, and I noticed it also does have quite a bit of power at that 1200 rpm idle--it can pull the car up a 2% grade or so if I take my foot off the gas.

I believe the ECU sets the idle high until it warms up when it's cold (cold weather, that is). If you idle it in your driveway, it will stay like this for about 10 minutes and then will methodically decrease to the desired about 600rpm.

If you find anything that contradicts what I just said, I'd sure like to know!

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I had a 1990 LS400 and currently a 1991 LS400 and both idle like this. Its designed to idle at that rpm until the engine warms up a little, then it settles down as the engine warms up.

Sounds like a nice car... not even at 100K, that car is not even broken in yet. :)

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My new-to-me 96 LS400 with 171k does the same thing. Idles at 1200 in cold Boston mornings, and gradually goes down once warm. As long as it's not a hunting (erratic) idle, I'm not worried.

Because of this I actually start driving a little sooner than usual after starting... if it's gonna sit at 1200 no matter what, I might as well start power-rolling through the side-streets in the general direction of my destination :D

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Thanks for that. I have been trying to find things wrong with this car and I just can't seem too... Glad to know she's acting normal, just a bit quirky.

My only concern was that when she is coming off freeway speeds she tends to hold the rpm and doesn't decelerate. I like to coast to save fuel as much as possible rather than use my brakes. Plus I notice she will idle high when warmed up, but will drop to 600 rpm when I shift to N or P when stopped. Then when I put in D and drive she will come back to 600 rpm at the next stop.

When I have time I think I will take off the IACV and give it a cleaning, it may help. Could be a spring or valve sticks until the vacuum dynamics change - e.g. manually shifting the transmission.

BTW she is really clean inside and out. No cracks in the leather or dash, though the leather is a bit stiff from drying out. I put two applications of Lexol on it and that seems to help.

Cheers,

Ed

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exactly what my 90 does. At startup, it stays at 1600 for quite awhile then slowly drops off. I have noticed that often when driving during warmup, even after the car is almost up to temp, the idle stays high when stopped at lights. I have found that slipping it into nuetral makes the idle drop down in a few moments. Almost lie something in the transmission causes it to hold a high idle longer. This has always done this and I've had the car over 5 years.

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Yep, Mine does same as VBDenny's '90. Something about putting it in N or P after it is warmed up or close to warmed up.

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Welcome to the LOC! :cheers: Hope you continue to save $$$$!

Shouldn't have to wait until engine is at normal temps before idle starts to drop. If it's taking too long to reach normal temps, check your stat and coolant level. Checking the IACV is a good idea. It can gum up and become slow to move. The ISC might be a bit slow as well. Another possible cause is your cold start circuit is slow to disengage. Also check you coolant temp sensor. If it's sending the wrong readings, your ecu can keep a high idle until the sensor sends a normal temp signal.

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this might also be a throttle cable. does the car shift hard into drive or reverse. it sounds like the car thinks the gas pedal is being pressed when it isn't which is a common issue with an out of alignment throttle cable. do a search and you will find more info on this.

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Thanks, for the other ideas. I probably will clean the IACV this weekend. It occurs to me that testing the Coolant Temp Sensor would be to disconnect the electric plug and see if it makes a difference. Is there another test that would make better sense (electrical resistance)?

Since the engine does idle at 600 when fully warmed up, how do I know whether the coolant temp sensor is working or barely working? I'd rather not fix something that doesn't need it.

What is the ISC?

Thanks,

Ed

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Hey Guys,

On my 91 lexus ls400 , along with the high idle until the car warms up, when I turn the air up it idles back up to the same level as when you start it in the cold. Once it is warm . I can start air and it will build up the idle and stop when it is less than 1 slot on the rpm, if i turn air off , it idles back down. What could cause this.

Thanks

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