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...

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 ;)

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 - YouTubeSmoke test on LS400 1990 - YouTubeLooks 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 lolThank you all for your invaluable input. I made friends along the way and learnt something about cars

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

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

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

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?...

This DOME fuse from second picture of post #45 that had 1.7 to 2mV was a result of my MISTAKE: trunk was open during the test thus the light was on drawing 2mV... Once I closed the trunk it went down to 0mV... I updated the picture description accordingly... To summarize: I retested all fuses of MAIN JUNCTION BLOCK (including cartridge fuses) in engine bay - all had continuity and showed 0mV at Vdc. No blown fuses observed. I retested regular fuses of the DASH PANEL 0mV at Vdc and continuity was good. For some reason slot for RR.AC fuse was missing but I am not sure if this fuse was supposed to be in my car. Could not find at all 3 cartridge fuses of DASH PANEL that seem hidden: P/W, DEFOG, DOOR, thus they are not ruled out as potential culprits... The battery may just be bad and not hold charge... I may buy new battery and try to take closer look into Trunk hinge wiring harness.

I always had taillight bulb blowing problem... I am attempting to check the trunk hinge wiring harness right now... But did not trace them yet. Just looking at it. Looks ok. Nothing obvious. I unclipped the clip. Will try to trace the wires but am a bit hesitant as i don't want to cause the problem. I updated post # 45 -additional info under second picture regarding *3 ALT fuse... I checked it. It tested fine - 0mV and continuity.

Dear Randall, Thank you for your message :) It indeed got gradually better. I was driving the car from time to time since summer, but was not in the mood to continue fixing it -needed a break ;) thus I did not post anything... My LS400 once started increases slowly and gradually in rpms until it reaches approx 1200 then it starts to fluctuate between 800 and 1200rpms every second or so when in P or N... in a very regular fashion. In D, R, 2, L it keeps at 600 rpm when no gas pressed. It still pulls and I feel that when I'm slowing down to stop the gears downshift with noticeable jerking of the car... Recently even before installing "new old" throttle body I was driving the car quite often and there were no codes... at least the Check Engine Light was not lit. It used to fluctuate between 1200 and 1600 rpm in P and N - thus it looks like it is slightly better now. It does not pull forward like it used to but it still will not require pressing gas pedal to slowly reach high speed... Also it is pretty bad in terms of fuel economy. No noticeable difference was observed in this behavior after replacing Old Clean Throttle Body to Ebay-bought cleaned "new-old " Throttle Body which was done this past week... (both throttle bodies had original sensors attached which I did not touch/adjust in any way) However, for a little while now I noticed that my BATTERY GOES DEAD nearly overnight - it probably went bad due to problems from the past. For a year or two I had occasional battery going dead, but never thought about it too much as it was occuring once in a while... maybe i got used to it, sometimes I left dome light on. I eventually went through car records... I realized that alternator was replaced twice in this car by my housemate, within 2 years timeframe -I think mechanics didn't diagnose the problem and replaced alternator because it was good money. 2011 September : Alternator replaced 2012 January : Battery replaced at PepBoys 2013 June : Alternator and L/F Oxygen Sensor Replaced Back then I was not interested in car mechanics. Now the battery is 3 years old on its deathbed. I begin to think that maybe there is a parasitic draw which would explain why it goes dead so often. Or maybe the CAPACITORS on ECU cause strange problems -can they discharge battery like that? SIDE INVESTIGATION of BATTERY PARASITIC DRAIN : Today I checked Voltage and Continuity on most fuses, while the smart battery charger (NOCO GENIUS G26000) was connected to battery leads to make sure that my bad battery allows for voltage to flow to expose parasitic draw if any (I try not to make too many mistakes, but am new to electrical diagnostics). I used EXTECH true RMS multimeter. I did not pull any fuses during that test. The battery was connected. I let the circuits go to sleep - the readings were done at least 5 hours after parking the car in the garage. The results were negative: All checked fuses had continuity and there was no measurable voltage (mV) present across fuses... I also checked cartridge fuses under the hood - the same results. [Cartridge fuses were tested in situ as well, after gently popping the top cap off with small screw driver. Basically I just touched meter leads to both sides of it up top with the cap off.] Below are some pictures and explanation: Kick panel / DASH FUSE PANEL fuses... All regular fuses showed 0mV and had continuity. I could not access P/W, DEFOG nor DOOR cartridge fuses, thus I have not checked those... hope the power drain is not there. Not sure how to access them. Those 3 fuses described below: * A P/W- 30 Amp: Preset Steering, Power Shoulder Anchor, Electric Moon Roof, Power Windows * B DEFOG - 40 Amp: Rear Window Defogger * C DOOR - 30 Amp : Theft Deterrent System, Seat Control System, Fuel Opener Control System MAIN JUNCTION BLOCK / FUSES in ENGINE BAY. I checked regular and cartridge fuses here. All checked fuses exhibited 0mV and good continuity. The only fuse I did not check was cartridge fuse described as ALT 120A - I could not pop off the top... *3 ALT - 120 Amp: Fog, Tail & Stoplights, Turn Signal Lights, Defogger & Seat Heater, A/C & Heater System, Power Shoulder Belt Anchorage, Door Lock Control, Power Windows & Power Seats, Antenna Motor, Cigarette Lighter, Combination Meter, Wiper & Washer UPDATE 2015-Dec-11: I managed to pop gently the top of 3 ALT-120Amp Cartridge Fuse and checked it. It showed 0mV and continuity. I also rechecked other fuses in Engine Bay/Main Junction Block -they again showed 0mV and continuity, apart from 7-DOME-10Amp fuse which had 2mV and continuity. I assume that 2mV is not enough to draw battery power. RELAYS listed below I left alone: * A - HEAD Headlight Control * B - ST Starter * C - ENGINE MAIN * D- MG CLT A/C Magnetic Clutch * E - HTR Heater Main * F - HORN* G - EFI Main UPDATE 2012-12-12 12-12AM ;) : This DOME fuse had 1.7 to 2mV, but it was the result of my MISTAKE: trunk was open during the test thus the light was on drawing 2mV... Once I closed the trunk it went down to 0mV... To summarize: I retested all fuses of MAIN JUNCTION BLOCK (including cartridge fuses) in engine bay - all had continuity and showed 0mV at Vdc. No blown fuses observed there. I retested regular fuses of the DASH PANEL 0mV at Vdc and continuity was good. For some reason slot for RR.AC fuse was missing a fuse butI am not sure if it was supposed to be in my car. Could not find at all 3 cartridge fuses of DASH PANEL that seem hidden: P/W, DEFOG, DOOR, thus they are not ruled out... CONCLUSION: maybe my battery is just bad and does not hold charge... schematic layout of Main Junction Block Fuse panel. Measuring Voltage across Cartridge fuse. Popping off top cover from cartridge fuse in order to check voltage and continuity. That's all for now... I post long posts as it helps me keeping track of all those troubleshooting. I apologize if it sounds like rambling on :) . It is potentially all connected and significant. Feel free to comment by including number of post you refer to as my quoted post may be too long to be seen twice or n-times in a row ;)

That's a good point. A while ago I disconnected IACV /ISC valve CONNECTING WIRE to rule out that IACV/ISC is defective. Then I turned the car on. The same behavior was observed. The car idled with gradually increasing rpms from 1350 (when I started the engine) to 1500 rpms which took approximately 40 seconds. Then It started to surge back and forth between 1100 and 1500 rpms in cycles... It was a while ago when the rpms were reaching higher values. Thus I concluded that neither the IACV valve (previous multimeter tests) nor its wire was the problem and there must be something else causing it... but what !

Fluctuating idle problem in lexus ls400 1990. REVIVING this thread... Still wasn't able to track down the cause. But didn't do much since last post - I lost inspiration thus no posts for extended amount of time... To remind you what was done just before problem appeared to be apparent: - new Valve cover gaskets and rear plugs - new spark plugs and new wires (tested wires resistance with multimeter) - new distributors and new distributor caps - cleaned egr pipe in air intake chamber - cleaned cold start fuel injector in air intake chamber - cleaned IAC valve - removed and cleaned throttle body, put new TB gasket After all that I started the car and the problem of fluctuating idle begun in N and P... D, R, 2, L gears were at constant rpms when no gas pressed... To troubleshoot: -I checked vacuum lines, tried propane test with no luck. -I rechecked IAC valve and tested with multimeter and all works within spec -I used EGR blocking plate to rule out EGR valve. No change. -I checked EGR with mityvac mv8000 vacuum pump. Within spec. SINCE MY LAST POST I went for "vacation" from fixing my Lexus as it was upsetting that I could not figure it out... -Recently I bought a new old THROTTLE BODY with new old TPS... I tested it with multimeter and its within spec. After installation it didn't help with the symptom. Thus I assume it was not TP nor TPS... Currently after starting the car and keeping it in Park the rpms still do the same thing... RPMs are at first rising slowly to 1200rpms then start to fluctuate between 800 and 1200 rpm with rapid drop to 800 again in the interval of time that stays pretty much constant... It can be seen on my YouTube links below: https://youtu.be/v9cDyYOeMtU https://youtu.be/yhJf3pdnOIA Thanks for all your help and ideas :)

I'm pretty sure it is sitting flush...

That's true.

OK. I will keep slowly at it. Mechanics don't have time to troubleshoot - they check the codes and throw parts and labor at problems until they solve them costly ;). I probably would do the same if I had to make a living out of it... Below are notes helping me organize my strategy... Idea 1: Maybe I can disconnect IACV and remove the part that I so nicely cleaned out, then cover the hole with something (need to figure out what to use as a plug) and then see if it normalizes rpms in any way, probably making it really high?. Probably stupid idea. By the way, is there anything in lexus that acts similar to Fast Idle Control Valve from Honda, as I read that Hondas have a problem very similar to mine... Idea 2: find method to check Coolant Temperature Sensor... and make sure that my system does not have any bubbles... By the way, if it was the sensor or the bubbles, what would be the symptoms? and would they go away once the the engine got hot. Could any of it cause my symptoms? Idea 3: adjust TPS with filler gauge and continuity to reduce the idle speed?... Idea 4: adjust throttle body screw that is on dashpot, although I don't want to change it too much... (could that help in fluctuating idle?) I'm rambling on a bit in order to get new ideas... I also want to evaluate all the ideas that I already got from all of you in order to get strategy that would allow me solve my problem with the least amount of trial and error lol . I cannot check everything at once... and each test requires me to study a new area of automotive knowledge and to buy new tools :( . Idea 5: can i uncover throttle body inlet and then check what happens with IDLE? But what to do with the air hose from IACV and the little one connecting next to it that connect right after Air Flow Sensor...Would it be too much air for TB and intake chamber or too little? would it eliminate the function of IACV given the airhose from IACV connects right after air flow sensor? I apologize for possibility of wrong terms used here, as I don't know the names of all those parts... Rambling on... hoping for the best...