Low Idle Speed Issue

[lexusnutt]

I'm posting this from the clublexus forum.

http://clublexus.com/forums/showthread.php?t=239568

Full credit goes to Jayjw for his post!

My car is 1994 LS400.

The low idle speed problem is so common among the earlier LS400s. It is very difficult to pin point the problem. I have experiencing the low idle speed problem for about 6 months. Whenever the car stops at traffic light or stop signs, the idle speed was very low, sometimes it was below 200. With AC on it was a bit better. There was a strange symptom, when the air humidity was high like raining, the problem went away. I did quite a lot of study on this forum about this problem. There are many many LS400s having this problem. And people did all kind of things trying to fix it, cleaning the throttle body, replace or adjust the throttle position sensor, clean the idle speed control vavle, etc. but still having the problems.

There is one article giving me a lot of hint. Here is the URL.

http://www.autoshop101.com/forms/h21.pdf

It is talking about the engine air intaking. In it, it mentioned the "Power Steering Idle Up". In short, the power steering draw quite a lot of horse power from engine at low speed. To compensate this power, it use the "Power Steering Idle Up". Basically it is a valve, it opens when the car is moving at very low speed or idling. It gives addtional air to the engine and keeps the idling up. The LS400 has this valve. If you open your car hood, you will see a thin hose from the air intake (right after the Mas Air Sensor) goes to the power steering pump, and another hose gose from power steering pump to the engine air intake, right above the Idle Control Valve. After many years, this valve stops working. It stops giving the addtional air to the engine. And I think it is the LS400 engine ECU programming flaw, when the engine runs at low idle speed, it is only programmed to control the Idle Speed Control Valve in addtion to the power steering idle up valve. It is not programmed smart enough to detect the low idle speed below 650, and to let the Idle Speed Control Valve to give enough air to raise the idle speed to 650. In short, the ECU can not handle the situation when the power steering idle up vavle stopped working.

The above is my analysis after the studying.

To fix the low idle speed is really simple. Just to re-connect the two hose I mentioned above ( one hose from the air intake (right after the Mas Air Sensor) goes to the power steering pump, and another hose gose from power steering pump to the engine air intake, right above the Idle Control Valve).

Here is the steps to solve your low idle speed problem. Refer to the attached picture.

1. Disconnect "hose1" from "nibbles1".

2. Disconnect "hose2" from the Power Steering Pump Idle up valve (this end of hose2 is not shown in the picture).

3. Connect "hose1" (the side just disconnected from "nibbles1") to the Power Steering Pump Idle Up valve ("hose2" just disconnected from it).

4. Connect "hose2" (the side just disconnected from the Power Steering Idle Up valve) to "nibbles1".

It is pretty difficult to disconnect and connect the hoses to to power steering idle up valve because there is not much room down there. What I did was removing the air filter housing, it will give you some extra room to work there.

After I did this, the low idle speed problem went away. Man, do I feel good every time when I see the speed right at 650 in front traffic lights? You bet.

You may ask some questions, does this cause any problem when this hose keeps sending air to engine all the time? I think no. Just think it is equivalent to your gas peddle, it simply give the engine a little bit more air when you drive at high speed, or you can think you lift your gas peddle a tiny bit since this hose give some addtional air. But I can bet you won't feel any difference. When the car moves at low speed, the hose suppose to open, so there is no difference here. Also this hose is right after the Mass

Air Sensor, so it won't cause any unmeasured air issue.

There is an addtional benifit by doing this. The LS400 also has a known problem, leaking Power Steering Fluid to the engin intake. The problem of that is also caused by the Power Steering Idle Up valve. By loop it back, you prevented this future problem.

I think the Toyora engineers should totally eliminate the Power Steering Idle Up valve. Instead, they should use the ECU to detect the low speed and sending the addtional air through Idle Speed Control Valve.

[mehullica]

That is probably one the the f-cking stupidest things I've heard of. Why not check the idle speed control motor in front of the intake? Clamp off the hose and if the idle doesn't change, it's not working! Why not repair the problem? Come on now. The idle-up valve is there for a reason, as is the idle speed control motor.