Sounds like a big-ole vacuum leak to me. Put some smoke into the system and see if/where it starts coming out. I've had good luck with a home made 'smoke generator' - coffee can and cigar along with my air compressor regulated down to like 2psi. Kind of like this:

 

Yeah with them stinky things you could smell the leak. Unless you got a stuffy nose like I got right now . I agree a nice $1400 smoke machine would be nice to hunt for the leak, and you could use a dye with it too.

 

If the problem still persists, I had a similar issue and never cared to pay attention;
If your videos the car is not warm, however upon initial start up of our cars, they idle at 12-1400 rpms to heat up the engine, even when in gear and slowly decreased until at operating temperatures;
I solved this issue by replacing a coolant temperature sender.

Worth a try maybe?

 

I did change the ECT sensor and torqued it to 14 ft lbs for a new one. I had to buy 19mm deep socket from Craftsman (bought a set) as my cheap deep sockets were not deep enough in hexagonal crossection.



Attached are couple of pics from changing the ECT sensor.
However it did not change any behavior in fluctuating idle in Neutral and Park. Maybe my problem is a vacuum leak after all...

Once an old ECT sensor was out I checked resistance between two prongs of connector and when cold it was 2000 ohms when heated gently with a flame it dropped to 400. This suggests that old ECT sensor was probably good... But good to know


I also checked visually trunk hinge wires due to some suggestions. I took the insulation out and plastic pieces on the bent of it. All wires looked intact. Thus for now I will leave it without checking continuity on those wires destroying insulation. The second set of pics is for that trunk wiring harness..

 

Trunk hinge Wiring harness after removing electric tape insulation. Pics

 

Trunk Hinge Wiring harness before removing electric tape.

 

I think I should try coffee can with cigar smoke machine next... It probably is a vacuum leak after all...

 

Its a good thing to know the ECT is in good shape, since it can cause problems if it isn't.
Check hose to brake booster, make sure throttle body gasket didn't get messed up.

Did you ever try to read the codes?

 

Another revival of my thread.
First of all I would like to thank all of you for suggestions and great support!!!
I didn't do much with LS400 1990 of mine until recently. I guess I needed a Lexus detox after being unable to find a problem.

First thing I did was a SMOKE TEST:
I performed a smoke test with home made machine and it was a success!!!


Homemade smoke machine




Homemade smoke machine:
-aquarium pump
-some vacuum lines
-cheap cigar
-empty glass bottle with plastic cap (drill holes for vacuum lines)





Homemade smoke machine: hose feeding smoke to the air intake chamber





Smoke test with homemade smoke machine - YouTube
Smoke test on LS400 1990 - YouTube

Looks like there are no major air /vacuum leaks - I was afraid that valve cover gaskets and plugs were installed with airleaks but they came out tight!
Two little leaks were present at IACV and its air hose which goes to air intake plastic black box.
Also some air was coming from EGR-VM vacuum modulator from its cap which may be normal as it vents air or sucks air through the filter - not sure...

Based on smoke test I decided to buy used IACV valve.
I tested new old IACV with multimeter for resistance - B1 to S1 or S3 and B2 to S2 and S4 which read around 23 Ohms and with 9v battery for function where I observed closing and opening. I installed it. The car started to run normal and smooth. But the rpms in idle are very low now, around 200 to 300 rpms.

My theory : IACV was stuck open and delivered air to the engine all the time causing ECU to add FUEL to maintain proper fuel to air ratio... This is why the rpms fluctuated, engine was running rich and car was trying to go faster by itself. There was too much air coming to the engine via stuck open idle control valve...

But now IDLE speed is really low: D or P idle is around 200 to 300 rpms... Way too low. Maybe I should adjust TPS?... But I tested TPS and it seems to be adjusted properly.
I've read through a lot of posts on this subject but i've hit a wall of sorts.
What if IACV is stuck closed now?... After all I installed a new old one .

It didn't stall yet but what if it does... It runs quiet.
It does not fluctuate anymore it accelerates nice and smooth. It runs smooth. Still needs some testing. I will drive LS400 for a while and see if ECM adjusts its settings and idle.

By the way, later I may get another IACV to see if it improves idle rpms, but at this point I'm happy with solving haunting / fluctuating idle!!! What a relief!

One day I may take ECM to see condition of capacitors but for now I just want to drive and not push my luck, given that my soldering skills are be that great lol

Thank you all for your invaluable input. I made friends along the way and learnt something about cars

 

Great work Magda. I'm glad you came back and updated us, and nice to hear you made some head way on the problem. I haven't dug into mine yet, its still on the back burner, since the weather is getting better it will be easier to work on it. And as you have discovered I was leaning towards the IACV and the EGR as culprits. I was going to plug in my extra IAC and watch it to see what it does to regulate after warm up. I know at shut down they are suppose to open all the way for the next start up. Disconnecting the hose to the egr diaphragm should close it, easy to try and see if it makes any difference.
And what a nice smoke machine, you know they cost around $1400 or so, yours gets the job done. Huge thanks for the video's, I'm sure others will find it use full.





I'll post what I find as well, and maybe just add it to this thread.

 

Thanks Dicer your suggestion was good!
I will try to fix the low idle problem now, but at least haunting is gone

 

Nice work on the smoke machine.

 

Thanks you should have seen me when the glove exploded with smoke when pressure was slightly too high lol

 

I've been driving my LS400 since I fixed fluctuating idle / haunting idle issue. It runs pretty much fine but that new issue i mentioned in previous post bothers me a bit.

When I start the engine from cold when in P or in D, not driving, idle stays around 700rpm.
But once engine warms up I read around 200rpms in P and in D idling when car is not moving... It worries me a bit. But interesting fact is that so far the car hasn't died yet on its own or stalled even once... I am puzzled.
Another thing is when I cruise at 55 to 60mph I observe only 1200rpm. Is it normal for V8 of LS400? I don't remember how high this number was way before my repairs... V8 is powerful so maybe it's normal...

I am not sure what is the reason for such low idle rpms ... I was thinking that maybe now the "new old" IACV may be stuck closed but it was working in tests with 9v battery so it may as well be working fine... I realize that LS400 1st generation (1990) tend to have a low idle rpms but mine is ultra low.

Does anyone know what can be the drawback of such low idle? Is it detrimental to the car or to the fuel economy?
The cool side-effect is that at such low rpms engine is very quiet... Good for meditation

Thanks for any suggestions... Should I make a new thread on this problem? I stayed in this thread given it may be a result of preexisting conditions besides I already described at length certain repairs and condition of my car...

 

Sounds like your tachometer is showing far less than the actual rpm because it should show nearly 2000rpm at 60mph. I suggest you to calibrate the tachometer.