Ls400 Idle Loops Up And Down From 1100rpm To 1600rpm Every 3 Seconds

[Magda]

Did you use the water bottle spray method to diagnose vacuum leaks?

Spray all intakes and hoses and any other possible leakage sites that come to mind.

That surging is almost always the IACV or a bad ECU.

The other thing that I modified in the past was the TPS , my car also had TRACS so it had dual sensors on the intake.

Lastly, cleaning the throttle body helped - all the Power Steering fluid sucked through a defective Power Steering Actuator valve had gummed up the throttle plates.

thanks for your suggestions.

I used propane to detect vacuum leaks. With no luck. Did not try water. Question: if it is from lets say manifold gasket, then how do i get to spray everywhere around it -its pretty inaccessible. Also i dont have smoke machine access.

I cleaned and tested IACV with multimeter... seems fine. I disconnected its connector and it did not remove the fluctuating idle symptom.

I replaced throttle body and sensors with Ebay-bought "new old" throttle body with sensors. Engine still fluctuates idle up and down...

Maybe it is bad ECU / ECU capacitors?...

[Magda]

I tested voltage Vdc between OX1 and negative battery post as well as between OX2 and negative battery post while in P.

Below is the YouTube movies and excel sheet graphs I made based on movies. I wrote down the multimeter readings and correlated then with RPMs at subsequent time points. The engine was not very warm as you can see from the temperature dial of the dash. However my problem occurs at those conditions.

Observations presented here were made BEFORE changing ECT engine coolant temperature sensor.

Vdc OX1-negative terminal and RPMs (in this video I turned the car with cold engine after a few days in the garage so you can see how RPMs are gradually going up until the fluctuating idle begins):

Graph:

http://www.clublexus.com/forums/attachments/ls-1st-and-2nd-gen-1990-2000/376455d1450937282-help-idle-loops-up-and-down-from-1100rpm-to-1600rpm-every-3-seconds-20151222_oxygensensor1_voltagetestwhilefluctuatingidle_readings.pdf

The same was done for OX2:

Graph:

http://www.clublexus.com/forums/attachments/ls-1st-and-2nd-gen-1990-2000/376456d1450937343-help-idle-loops-up-and-down-from-1100rpm-to-1600rpm-every-3-seconds-20151222_oxygensensor2_voltagetestwhilefluctuatingidle_readings.pdf

The values are approximate and sometimes it was difficult to catch all changes so keep it in mind

Also I assumed that .0L means 0mV !!!

I wonder what's going to happen once I install new sensor.



Happy Holidays to all of Lexus Fanatics

[Exhaustgases]

Keep up the good work. Your going to be the LS400 expert on this website.

So how did you reach the cold start injector? What seals it in the manifold? Did you have the intake off at all?

Try wiggling some of the wires and see if that affects anything.

[Magda]

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 before removing electric tape.

Trunk hinge Wiring harness after removing electric tape insulation. Pics

[Magda]

Keep up the good work. Your going to be the LS400 expert on this website.

So how did you reach the cold start injector? What seals it in the manifold? Did you have the intake off at all?

Try wiggling some of the wires and see if that affects anything.

Thanks :)

Cold Start Injector probably is not cleaned entirely as I did not remove it from the Air Intake Chamber. I cleaned the visible part of it when the Throttle Body was out for cleaning... That may be not a very thorough cleaning but better than nothing. It is shiny and clean on the outside lol

[Banshee365]

Get the car mostly warmed up, shut it down, unplug the ECT sensor the see how it runs with it unplugged. Remember there are 2 ECT sensors. One for the ECM and one for the cluster. Make sure you're dealing with the one for the ECM.

[Magda]

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!!!

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 

 

 

 

[Exhaustgases]

So have you applied to be a tech at your local Lexus dealership?  You have probably accomplished more than most of the auto techs could. If you want to improve your solder skills just get some old junk computer parts and experiment with them first. At the rate you are going soon you will be able to open your own repair shop, keep up the good work. And don't take such a long vacation from these lexus sites. You have done some very impressive things. Just remember there are lots of stories of people taking their cars to a shop and spending thousands and they never get it fixed. So you really have beat some of the shops out there with what you have done.

[Magda]

On 4/5/2016 at 9:03 PM, Exhaustgases said:

So have you applied to be a tech at your local Lexus dealership?  You have probably accomplished more than most of the auto techs could. If you want to improve your solder skills just get some old junk computer parts and experiment with them first. At the rate you are going soon you will be able to open your own repair shop, keep up the good work. And don't take such a long vacation from these lexus sites. You have done some very impressive things. Just remember there are lots of stories of people taking their cars to a shop and spending thousands and they never get it fixed. So you really have beat some of the shops out there with what you have done.

Thanks for your kind words and encouragement - I really needed it  :) I haven't applied to be a tech yet ;) . I realize, many good people have to learn how to cut corners to make it in that industry, thus I would set myself for failure given I don't like to cut too many corners. I would be too slow doing everything according to bentley manual and triple checking protocols lol'
Indeed, I needed vacation from my Lexus - it was super frustrating to do first bigger set of repairs combined together just to face a very unfamiliar problem with no clear answer. I am pretty sure this repair would have cost me a lot at the local shop and it probably would have been rushed and not fixed properly. I have some horror stories with mechanics I carefully selected to service my cars in the past- unfortunately it's very difficult to find honest mechanics who do the job up to spec. I got fed up with being perceived as a girl who does not know about cars each and every time I went to service my car. Now I know about torque, wrenches, special tools and some car components and systems... If only I had discovered my mechanical capabilities sooner I would have had more fun maintaining my cars over the past. 

It's great to be a part of this community - you guys are awesome :)

I will try to fix LOW IDLE next which is kind of the opposite problem, but other than that my LS400 runs nicely given its mileage of little over 300 000 miles ;)

[Exhaustgases]

Maybe the tachometer is wrong, it maybe idling faster than it says. The engine would barely keep running at such a low speed. It is not uncommon for these tachs to do that, Tanin autoelectric knows the fix for that.

[Magda]

That's an interesting suggestion. If it's true then I don't think it's worth fixing especially given the cost of a dashboard and 300.000+ miles on my LS400 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 after driving for a few minutes 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 is 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... 

[Exhaustgases]

Post a sound video of it idling, I'll know if it is right or not. With what you say about the highway speed rpm, that is not the correct rpms so the tachometer is not reading correctly.  Most all tach's I am familiar with use the ignition pulse signal, and since the engine runs you have that just fine. It could be a wire or connector problem, and could be a circuit problem in the tachometer electrical components. It could be similar to the ecu capacitor problem and just a bad capacitor on the tachometer circuit board.

And so cool you got the idle surge sorted out. There have been a lot of LS owners with that problem and no resolution yet, well you solved it. Good work.

You have done things that some so called auto mechanics can't do.

 

Heres a tach problem

 

 

[pawpaw92128]

Gooood morning, I don't know if you are still  having the problem of fluctuating..I had the same type of  problem some years ago with my Toyota van..I have a 1990 LS400,I live in Chicago,so I don't drive it in the winter..Two weeks ago I started it up,and it was fluctuating between 1000 and 1500 rpm..I couldn't figure it out..Finely it dawned on me (I'm 69) it's the MAF..mass air flow sensor..It's located on top right behind the air filter,it should be secured via 3 small screws..be careful taking it out..you can go to ace hardware and get a can of CRC/ QD electronic cleaner and spray it...it will usually just air dry,but make sure it is dry before you reinstall it..I'm 99.44 percent sure it will solve the problem..You could go to Lexus and get a new one if cleaning doesn't work..Don't know how much it cost,on some vehicle's it could be $$/$$$...Hope it works for you..Have A Blessed Day...

[M.Yasir]

On 4/19/2016 at 8:58 PM, Magda said:

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 is 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... 

With all the photos and the hard work it is a good job done, you should soon open your own garage and start fixing lexus

have a look at RPM's level when cruising around at

60MPH

70MPH

80MPH

Hope it will help you in some way (mine is '98)

[Exhaustgases]

On Wednesday, May 18, 2016 at 8:21 AM, pawpaw92128 said:

Gooood morning, I don't know if you are still  having the problem of fluctuating..I had the same type of  problem some years ago with my Toyota van..I have a 1990 LS400,I live in Chicago,so I don't drive it in the winter..Two weeks ago I started it up,and it was fluctuating between 1000 and 1500 rpm..I couldn't figure it out..Finely it dawned on me (I'm 69) it's the MAF..mass air flow sensor..It's located on top right behind the air filter,it should be secured via 3 small screws..be careful taking it out..you can go to ace hardware and get a can of CRC/ QD electronic cleaner and spray it...it will usually just air dry,but make sure it is dry before you reinstall it..I'm 99.44 percent sure it will solve the problem..You could go to Lexus and get a new one if cleaning doesn't work..Don't know how much it cost,on some vehicle's it could be $$/$$$...Hope it works for you..Have A Blessed Day...

There is NO cleaning the air meter on the 1990 to 1994 LS400's. Its a Karmen vortex type unit, and a cleaning attempt will ruin it. They may be able to be cleaned if they are completely disassembled so you can access the mirror to clean it. But then the calibration maybe ruined.