99 Ls400 Idle Problems...please Help!

[ISRydr]

First let me start by saying that my 99 LS400 in question is my 3rd LS in the last ten years and I'm a Lexus LS fan through and through. I bought this car a couple of years ago and have had a problem with it idling correctly for a year now. Which I should add began within a week of having the 90K service with timing belt, plugs, wires and water pump replaced, The problem is that occasionally when I'm driving and come to a red light the idle will sometimes fall below 300 rpm and will die while in gear. But the problem is sporadic, at times when starting the car, cold or warm, the idle will not get above 500 rpm when in neutral and when I put it into gear I must apply a little throttle just to keep it running. I've replaced the fuel filter, air filter, O2 sensors (front only) cleaned the MAF, had the throttle body cleaned (while still on the car) etc. etc etc......I'm at the point I need the experience of those familiar with the LS400 to chime in with thoughts about what else I need to check. There is one thing I've read else where that indicated I may need to clean the IAC Valve (Idle Air Control) but I'm told that's a time consuming job and want to know before I take it to my mechanic if I should have them check anything else. This idle problem is driving me crazy. There are times I can drive it the whole day and the idle is perfect, or at least close to perfect, idling around 700 rpm, be it in gear or neutral, without change. As of today it idles at 4 to 500 rpm in neutral and drops to 300 rpm or less when I put it into gear. By the way, no errors show up on the ECU when it does this other than the occasional error code showing a front O2 sensor is bad (both are new) which isn't so. So a reset and off again until another check engine light comes on. When it does it will go off by itself in a day or so.

Any ideas would be greatly appreciated.

Thanks in advance for the help and great forum to massage for info.

Rydr

95 LS400 ( Best car I've ever had)

01 LS430

99 LS400

[prix]

I would take it to whomever did the timing belt etc and have them fix the problem (it sounds like) they created.

You will also find a LOT of information on LS400 drivers with similar staliing out issues in a thread on this forum or possibly on the clublexus.com forum. It's a common problem. And stalling out at a stop sign after exiting a freeway ramp can be downright dangerous. I STILL have the occasional problem with this (no longer with stalling after slowing down from a freeway drive though). My solution was just to run the AC which keeps the RPMs up a little higher at idle. To this day, I can stall the car out by turning off the AC while I am driving. The stall happens when I slow down to a stop. As long as the AC is on, the car doesnt' stall. That might help temporarily. I will leave the permanent solutions to the mechanics in this forum.

[1990LS400]

Regardless of what the problem is, a car whose engine dies randomly is not safe to drive. My recommendation is to get this car to a professional Lexus specialist.

The only times I've had the engine in my similar 2000 LS400 die at low speeds or when coming to a stop has been when a battery has begun to fail. I may be losing count but I think at 149,000 miles my car is on its 6th battery with one being DOA and another lasting only one year. Every time, the engine dying was the first sign of the battery failing.

[landar]

I second the opinion of taking it back to your mechanic for diagnosis and fixing. The new problem may be entirely unrelated to the 90k work. On the other hand, maybe some sensor/connector was not put back properly. Rather than throwing ideas at the mechanic, let him/her go after it with their knowledge. Only if stumped, would I make suggestions (or take it somewhere else). A competent mechanic will know what is wrong.

[ISRydr]

Thanks for the suggestions. The idle issue came to existence about a month after the 90k service. I hadn't thought about the battery being an issue as the problem is intermittent but will check it out. I took the car to my local Lexus dealer and of course it ran perfectly and nothing was found. Although it doesn't die often I agree it's a huge safety issue. And I agree that turning on the a/c does keep it idling correct but I really want it fixed right. Guess I'll have to wait until it shows again then get it back to the Dealership.

Thanks to all who replied.

Rydr

[curiousB]

Sticky/dirty IACV and/or vacuum leak.

A vacuum leak is most noticeable when it is a large proportion of the air ingested by the engine (i.e. at idle). This is unmetered air by the MAF so the EFI gets confused and tries to correct a lean condition. At higher throttle the leak becomes negligible in proportion to the metered air so goes unnoticed.

Maybe a cracked hose or your mechanic pulled a hose and connected it back incorrectly?

IACV also very common complaint. If you clean TB you should really do this as well.

[jaswood]

With the gas pedal fully released the engine ECU attempts to maintain the RPM at ~800 RPM. The only thing I can think of that might over-ride that "mandate" would be the need to keep the A/F mixture within the correct range in accordance with the A/F mixture control oxygen sensors.

You have a vacuum leak somewhere.

Don't overlook the PS vacuum line.