My "new" 98 LS400 is idling at about 100 rpm. Shuts off when I come to a stop sign. Any ideas greatly appreciated. Can the idle be adjusted by an average DIYer?

Thanks
hms09

 

If you don't have any codes to read the first thing I would do is clean the MAF sensor.

 

Do you also have a problem of any slow acceleration then an unexpected speed up when you turn a corner or slow down at a speed bump? A badly clogged throttle body causes too low idle and it confuses the ECU to cause this symptom when you speed down then accelerate. If so, you'd better read the page below and follow what MngreLMatt did as was written at the post #41
https://www.clublexus.com/forums/ls-...w-ac-on-3.html

 

No acceleration problems. Engine is smooth and powerful. Just low idle. Drops to 100-200 rpm at stop sign. Doesn't always shut off but does at times.
I pulled the MAF and it looked like new. TB was a little dirty so I cleaned it. Didn't help.. While under there I noticed what I think was a broken fitting on a vacuum line. Theres a rectangular box with 3 hoses. One of them was broken. Not even sure what this rectangular box is but its part of the air system and is on passenger side of engine. Any ideas?

Thanks,
hms09

 

It's a common issue with that box. I repaired mines but read this thread. https://www.clublexus.com/forums/ls-...r-cleaner.html

 

Does anyone know what that box is called and what it does? Is there vacuum on either of the three hoses?

 

Attached is from a JDM manual and it is called "インテークエアコネクター" in Japanese. The English translation is "intake air connector". It is aimed to reduce the intake noise using a method called a sound trap. If there is any air leak, it simply means that the leaking air into the connector is not measured by the MAF sensor and the engine can't work properly.

 

So far I have cleaned the TB, inspected the MAF and found it to be very clean and replaced the engine air filter. Still wont get over 400 RPM at idle. I guess next is to manually adjust the idle. Does anyone know how to do this? I see come cabling on top of the engine and a holder with 2 nuts, one toward the firewall and one toward the radiator. Would appreciate instructions on how to increase the idle.

Thanks a bunch,
hms09

 

In my opinion you are going to create more problems and move backwards by attempting to manually raise the idle.

Fix it right or don't bother.

 

Well, what is the right way? That's what I'm trying to find out.

Thanks,
hms09

 

I used to worry about my car idling at 400 while in drive but don't anymore. The car drives fine and I have not experienced any issue. I believe it's just all the old electronics in the car and unless your going to change every sensor or it's causing codes to be set I would just leave it be. I even adjusted the idle control valve that your alluding to on top of the intake but had to set it back to its original position because it made the car idle too high.

 

Does it idle low right after start or after it warms up?

 

How do you know it's idling at 400 RPM? When I look at my tach, I can't tell at all. It could be 300, or it could be 400 or 500. The lines at the low end of the tach just aren't that accurate.

So, I plugged in my scan tool to check, and it's idling at about 500 RPM.

I have a 1998 LS400 and it runs great. As far as I can tell it runs perfect. However it is currently producing a bit too much NOX to pass smog.

So, how do you know your's is idling at 400 RPM?

 

There is no adjustment at all. The engine management system controls the idle RPM to settle to 750 +- 50RPM at no load. When it is out of the range, it simply means that something is wrong. You need to find out what it is. As far as I have fixed several low idle Celsiors, the most difficult one was to clean the air mix passages. You sometimes need to clean injectors professionally.

 

It idles low after warm up. And I'm depending on the tach to measure it. But I think Sha and others are correct. Its really not causing any issues now that I've changed the air filter and cleaned the TB. It was shutting off at stop signs but doesnt any more after performing these 2 items. So, I think I'll call it a day on this issue and leave it alone.

Many thanks for all the replies.

hms09