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Another Idle problem thread. 1997 LS400 Idles too low when warm and in gear

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Old 03-06-11, 11:17 PM   #1
Entropic86
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Default Another Idle problem thread. 1997 LS400 Idles too low when warm and in gear

Hi everyone, I'm new to the forum but have been reading quite a bit here over the last couple weeks since I got my new 1997 LS400 Coach with 100k on the clock. The Car is essentially perfect, never spent a night outside, guy got it detailed several times a year, timing belt service was done already....anyways, I got a screamin' deal on the car, but it has one small problem:

The idle, when the motor is warm, drops to about 400rpm or lower when in D or R.... Low enough to make the power steering heavy. Out of gear or past idle, everything seems to work great and it's warm idle is right at 650rpm. I've checked a little bit for vacuum hose leaks but haven't found anything. I have the factory service manual, but it isn't very helpful, or at least the cheap copy I bought isn't. I tried resetting the ECU, but this didn't do anything. I'd like to spray out or use some sort of solvent in the IAC without taking it off, but am not sure which tube to spray. Any help or other suggestions would be greatly appreciated. I'll post pics of my find next time I get it cleaned up. Thanks much
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Old 03-08-11, 09:41 AM   #2
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I feel your pain. I have a 1996 LS400 and I am experiencing the same issue. I replaced the Idle Air Control Valve (bought used on E-Bay) and it did the same thing. I also checked the vacuum hoses for leaks but I could not find anything. I am leaning towards the Throttle Position Sensor (located on the throttle body). I am going to take it off and test the resistance.

If anyone has resolved this issue, we would greatly appreciate your input.
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Old 03-08-11, 10:03 AM   #3
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PRMs on my LS400 (1998) were also low, but got higher after the throttle body cleaning. Get it done, its easy
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Old 03-08-11, 01:35 PM   #4
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As above clean the throttle body,there is a tutorial on www.lexls.com for the earlier model but it should give you the basic process.
In the meantime having the AC on normally raises the idle speed a little which may help until you get the TB done.
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Old 03-09-11, 08:06 AM   #5
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I cleaned my throttle body, replaced spark plugs, wires and pcv valve but the results were the same. As I mentioned earlier, I even tried another Idle Air Control Valve - no luck.

Hopefully this weekend I can test the Throttle Position Sensor. I am leaning towards this as the culprit. One of the reasons I am targeting this sensor is because I am experiencing the throttle shock syndrome that has been discussed in length on the ClubLexus Forums. When driving at speeds above 40 mph, I will let up on the accelerator pedal and I can feel the vehicle surging forward and then kick back. I have replaced the ECU as well as the cruise control computer (as was recommended in the Lexus notice regarding this issue) - no luck.

I will keep everyone informed.
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Old 03-11-11, 09:30 PM   #6
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when my ICV started going bad it acted in the same way; very erratic idle, surging, etc... test your ICV by taking it off and running jumper wires for power to the 2 center prongs and grounding the outside 4 one at a time. the valve should move in and out. spraying cleaner in it wont work unless it has so much carbon buildup that its jammed up. I have a 97 coach as well with about 185k. I ended up having to replace the ICV and it works perfect now. rockauto.com had the best deal on the part. hope this helps.
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Old 03-16-11, 07:09 PM   #7
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Thanks for the tips so far guys. I took the throttle body off and cleaned it, but didn't have a replacement gasket on hand, which of course broke into a half dozen pieces when I took the TB off. I'm replacing all the vacuum hoses while I wait for the gasket to arrive. I also ordered an IAC gasket so that I can replace that right after I clean it. I really fear that it might be an expensive computer problem seeing as the problem only appears when the engine is warm and in gear.

Anyone know why vacuum hoses go from the power steering pump to the air intake? Seems strange to me, but I'm used to older, simpler designs. Thanks again,

Dave
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Old 03-16-11, 07:35 PM   #8
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Quote:
Originally Posted by silos5 View Post
PRMs on my LS400 (1998) were also low, but got higher after the throttle body cleaning. Get it done, its easy
How do you clean the throttle body? is it safe?
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Old 03-16-11, 08:52 PM   #9
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Quote:
Originally Posted by kuziwk View Post
How do you clean the throttle body? is it safe?
http://www.lexls.com/tutorials/EFI/throttlebody.html

There is a tutorial for the first generation models which is close enough for me. Is it safe?...I would say it's only worth doing if you have a good reason to believe it will fix a known problem.
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Old 03-16-11, 09:11 PM   #10
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I am having the same problems i changed everything plugs wire cleaned tb, egr, and replaced the icv. For some reason when i first start the car turn the ignition the car just goes back off then i give it some gas for a little bit and runs fine but still idles low when i get to stop lights... Any help please
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Old 03-18-11, 12:01 AM   #11
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I pulled out my IAC today. It looked ok, see pictures below. I tested the various resistances and everything was within spec. But, I couldn't get a peep of noise or any movement out of the motor/valve...I tried with a 9V battery first, then the car battery. I'm guessing I need to suck it up and get a new one.

http://i720.photobucket.com/albums/w...6/P3170200.jpg
http://i720.photobucket.com/albums/w...6/P3170201.jpg

You can't see it in the pictures, but the valve was stuck open. Also, it didn't make any noise immediately after shutting off the engine like the repair manual says it should...Definitely need a new one I think

How did the car manage to idle so well outside of when warm and in gear without the IAC motor working?

Last edited by Entropic86; 03-18-11 at 12:05 AM..
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Old 03-18-11, 06:12 AM   #12
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Same.

'93 LS, had 130k when I fixed this.

Bypassing the IACV did absolutely nothing. When I took a toothbrush to the throttle body the RPMs went from ~200-300 to a solid 550rpm.

For what it's worth, I never noticed a difference from when it was idling at ~250 to 550. Car never stalled out on me, and felt and sounded the exact same in park.
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Old 03-18-11, 10:15 AM   #13
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The idle air control valve could be the culprit, it will cause a low idle if it's stuck closed when the car is at idle and the throttle butterfly plate is closed. Something else that can cause strange idle behavior is the mass air flow sensor going bad. Usually it will be accompanied with poor fuel economy. It can (potentially, not necessarily) change its behavior based on engine and/or ambient air temperatures. Luckily it seems your IACV is the smoking gun here. The TB getting a good cleaning really only helps if the channels for bypassing the throttle plate are clogged with some kind of residue.
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Old 03-18-11, 01:46 PM   #14
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Hmmmm, I got the IAC to work today using a 12V, 500mA DC wall outlet converter. It takes about 20 cycles of the s1-s4 leads to get it to go from fully open to fully closed, but it works just fine.

What now? Maybe I should get it all back together and see if the throttle body cleaning and vacuum line replacement helped.
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Old 03-20-11, 03:18 AM   #15
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I got everything back together last night. Problem is gone! Idle is just over 600 when warm and in gear....I can park it with one finger now. It could have been the vacuum hoses that fixed it, or the throttle body cleaning, or the fact that I removed and played with the IAC...who knows. I also noticed the PCV was clogged on the left valve cover, which I cleaned, and the right valve cover breather was supposed to be connected to the throttle body by a hose, which I just installed. It also could have just been that this is the first warm day I've driven it...who knows. I'm just glad it's fixed. Thanks for the tips.
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Old 03-20-11, 03:18 AM
 
 
 
 
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97, body, cleaned, engine, high, iacv, idle, idles, lexus, low, ls, ls400, problem, thread, throttle, warm

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