1991 ls400 idles great when cold, but the idle varies and dies when it warms up

[mb31454]

Have a 1991 ls400 that idles well when cold. Once the temp rises to normal operating temp, the idle becomes erratic on the low side, stalls and dies. Changed out the water temp sensor, cleaned the throttle box, air filter is ok, fuel filter is new, pcv valve is new. 

[Exhaustgases]

I have this same problem on a 1990, ECT was a guess of mine too. I also had a air meter code and did have a dirty airfilter. I have not spent much time on it yet.

I think the normal idle when cold rules out the IAC.  I did a visual check of the caps in the ecu and they looked okay, I know that doesn't mean much. I do plan on doing an oscope to check it better, and wanted to see what the O2 sensors are doing as well.  Please keep this updated and if you figure it out let us know.

Mine runs for about 10 minutes or so I am guessing, before it then acts up, is that what yours does?

[mb31454]

Mine will run an hour or so sitting in the garage, idling fine and responsive to throttle, prior to the idle becoming sporadic. I initially ruled out the IAC, since idle when cold was good, but from an earlier post, if the IAC is stuck open, too much fuel can be an issue and I'll pull it this afternoon and update what i find. 

[VBdenny]

The ICV or Idle Control Valve is located by the throttle body and adds extra fuel when the engine is cold which increases your idle speed. After about 10 minutes, the ICV closes and shuts off the addition flow of fuel which would cause a rich mixture and stalling. My guess is that the Idle Control Valve is not closing all the way.

The thing is wicked difficult to get at however there is a method that sometimes works. I've kind of forgotten how to remove the air intake tubing but once you can see the throttle body intake, there is a small hole at the bottom of the intake. Squirt some carb cleaner in that hole. You don't have to overdue it as the stuff will have to burn off. One caution is to be very, very careful removing the MAF valve, they break easy. I think you can disconnect the tubing without disturbing the connections to that.  

Sorry I don't have a picture, sold the LS400 several years ago.

 

Denny

[mb31454]

Thanks Denny... I think the ICV is at the front of the throttle body, behind the tray with spark plug wires. If that's it, the problem i hear is that the little screws need a # 3 screw driver and sometimes a small vise grip to break free or else the heads strip out. You mentioned this in an earlier post and that made me think it might be stuck open or not fully closed. Yesterday, the car idled for an hour or so before the idle started oscillating up and down between 200 and 400 rpm. After that, about all I can think of is the ecu/ecm behind the glove box. As for the small hole in the throttle body, i might have seen it yesterday, but i spent most of my time cleaning the two flaps in the throttle body. As for the MAF, I try to be gentle with that assembly, but that hard plastic doesn't let go easily. 

[VBdenny]

Most welcome Mike,

Yeah, best to be gentle with older parts. If you can get the stick of the carb cleaner in that hole, give it a squirt and put all the stuff back on. Then start it up and see what it does. You might have to repeat a few times. I read up on the way Toyota designed their control valve and in many cases the valve doesn't slide back and forth due to being gummed up. Seeing as how they are really difficult to replace and not inexpensive either, if you can clean it, might work. A lot of it comes from using NON-Top-Tier fuels as they often are lacking certain detergents.

 

There is a hose on the bottom you can disconnect which will allow the fuel cleaner spray to drain through. You can do it without removing the tube but then whatever you spray in will have to burn off so go easy if you don't pull the hose. Of course it will make it run like crap when you start, but not for too long. 

 

Back to Clemson and Alabama

 

Denny

[datarecovery]

One of the methods that has worked for me is to adjust the Throttle Position Sensor. Too tight and it shuts off air to the engine and oops the engine shuts off.

Too open and the car idles too high. Cleaning the throttle body is and has already be recommended. Taking it full off will allow you to do a more thorough cleaning than stuffing screwdrivers and rugs in from the passenger side.

I am in Sarasota, if you need help just PM me.

Datarecoveryinc

[Exhaustgases]

Book calls it ISC valve, and it has nothing to do with adding or taking away fuel. It by passes the throttle body airflow. And after the coolant temp is at 158 degrees F the valve is turned off. The ecu controls the fuel by pulse width, as well as the ISC motor.

 

I'll have to study what all controls hot idle.  When mine dies it will start right back up, and run with some throttle in it like about 1500 or so plus rpms.

And yeah maybe if the ISC valve is leaking air then it would hunt like a vacuum leak, since it would be leaning it, and the ecu would be trying to add fuel to compensate.

[Exhaustgases]

So here is my plan when time permits. I will plug in a used ISC valve when the engine is cold, and watch what it does as is warms up. If it does not close all the way, I will test it on a known good car and see what it does. I would think it should close and if it doesn't then it becomes a vacuum leak of sorts. Magda has a similar problem that cropped up after she did a bunch of work and cleaned the valve. Maybe it was not reassembled correct and became the problem. Its either the valve or it is not being operated correctly by the ecu. If it was an egr problem then it should show up when cold I think.