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

[Magda]

LS400 idle loops up and down from 1100rpm to 1600rpm every 3 seconds

My 1990 LS400 runs high but even idle for approximately 15 seconds upon cold start. But then there is a moment when it changes... The idle goes up and down in a loop from 1100rpm gradually to 1600rpm then sharply drops back to 1100rpm every 2 or 3 seconds. When idle reaches 1600rpm the car makes subtle electric buzzing sound and idle goes sharply down to 1100 rpm... And it does it over and over again...

Below is a recap of repairs/maintenance I've just completed in my car:

I did clean following components from CARBON DEPOSITS:

1. throttle body (removed the TB to clean plates and interior),

2. EGR VALVE

3. air intake chamber including COLD START INJECTOR and EGR little pipe above cold start injector (which is inside the air intake chamber and can be seen from TB inlet),

4. IACV valve ( I also tested the IACV function with 9v battery -see below).

All those components were super dirty with CARBON DEPOSITS accumulated across the span of 25 years...

Throttle body plate had approximately 2mm or 1/8 inch of deposit...

The EGR little pipe that was supposed to be hollow was completely clogged with carbon thus I assume that EGR valve air passage to air intake chamber was completely blocked.

At the same time I replaced following components:

1. spark plugs and ignition wires,

2. distributor cap and distributor rotor.

3. valve cover gaskets and rear camshaft housing plugs in LH and RH valve covers.

4. I cleaned and reinstalled PCV valve and replaced PCV hoses for PCV valve (LH) and throttle body (RH).

The problem began after completion of all those repairs...

I turned the car back. It turned on. It run high idle for approximately 15 seconds. But then there was a moment when it changed... The idle RPM went up and down in a loop from 1100rpm to 1600rpm every 2 or 3 seconds. When idle reached 1600rpm the car ma subtle electric buzzing sound and idle went down to 1100 rpm...

I checked vacuum hoses and all seem to be connected.

Now, I thought that the IACV / ISC valve which I cleaned might be a culprit, so I took it out and TESTED it according to instructions I found:

The protocol for testing that I used (may be for ES300, not sure...) :

"1. INSPECT VALVE RESISTANCE
Using an ohmmeter, measure the resistance between the terminals
(B1–S1 and S3, B2–S2 and S4).
Resistance:
30–50
If resistance is not as specified, replace the IAC valve.
2. INSPECT IAC VALVE OPERATION
(a) Apply battery voltage to terminal B1 and B2, and while
repeately grounding S1–S2–S3–S4–S1 in sequence, check
that the valve moves toward the closed position.
( B) Apply battery voltage to terminal B1 and B2, and while
repeately grounding S4–S3–S2–S1–S4 in sequence, check
that the valve moves toward the open position.
If operation is not as specified, replace the IAC valve assembly"

Resistance of IACV was approximately 25 ohms. The battery test worked as the valve was moving according to the book when the voltage was applied to B1-B2 and S1–S2–S3–S4–S1 or S4–S3–S2–S1–S4 grounding sequence was implemented.

My resistance was 25 ohms but I found another instructions that said anything between 10 and 30 Ohms should be fine... I assume that IACV is fine. I reinstalled it...

I reinstalled IACV valve. Reconnected everything. Started the car up.

But the IDLE LOOP is still a problem...

Please, help with your suggestions :) Thanks!

What could cause a behaviour like that?

[landar]

I am kinda waiting for the guys on CL to come up with a solution. ^_^

But until then...

Have you tried driving the car in this condition?

What happens when you put the car in 'drive' (foot on the brake). With a load, does it act the same way?

I am wondering if something might be wrong with the MAF but just speculating at this point.

Have you checked for any codes? (Even though the CEL may not be on).

After working on a car, the most likely suspects are a missed wire or vacuum line. Go back over those areas with a fine-toothed comb and make absolutely certain all connections are firmly back together.

[landar]

Oh...and since you cleaned the throttle body, you might check the throttle body position sensor.

[Magda]

Dear Landar,

Thanks for your suggestions.

I checked the engine codes using hairpin wire method and found no engine codes. It may be because the engine got battery reconnected after a long time and I didn't really drive the car after that.

I rechecked connectors and vacuum lines.

I turned the car on in the morning. I kept it in P (park) and let it idle. At first the car was at 1000rpm and was steadily but slowly increasing in rpms. After approximately 80 seconds it reached 1500 rpms and then it begun its cycle of idle from 1100rpms to 1500rpms every 2 to 3 seconds... over and over again.

It would drop to 1100rpms then increase to 1500rpms, when it would make a buzzing electrical sound in the engine bay and then it would drop to 1100rpm... and repeat again... After cycling like that for 2 minutes I turned the car off...

It is very frustrating. I wonder what this buzzing sound could be?

Is there any possibility that new distributor rotor and distributor cap may cause symptoms like that?

Maybe cleaning EGR valve unblocked some vacuum lines in a strange way?

Maybe IACV cleaning unblocked something?

Maybe cleaning and unblocking 3 clogged vacuum holes which are at the top of throttle body, where the throttle plate rests in closed position.

I will probably check throttle position sensor next, however I don't want to mess up any more components...

Regards and Thanks!!!

[Magda]

I've just found the video in this thread that actually shows nearly identical symptoms to my problem:

http://www.clublexus.com/forums/ls400/700910-92-ls-400-idle-surging-help-please.html

The above thread suggests that my problem could be IACV valve. I removed and cleaned IACV as well as checked its function with multimeter and battery as explained in post number 1 of my thread. Thus I am pretty confident that IACV is in working condition. Maybe its connecting cable?...

[Magda]

I performed another test.

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 1000 to 1500 rpms which took approximately 40 seconds. Then idle started to surge back and forth between 1100 and 1500 rpms in cycles...

Thus I conclude that neither the IACV valve nor its wire is the problem and there must be something else causing it... but what !

[landar]

It seems that the computer is 'hunting' for a correct RPM probably because of a bad sensor or connector. The computer tries to close the 'loop' based upon all of the input sensor data. The computer also has to 'relearn' settings whenever the battery is disconnected for long periods but I do not think that is causing your problem.

Did you try driving the car or putting it in drive to see how it behaves? Take it for a spin around the block. Let it fully warm up so that the computer is in the (hopefully) close loop mode. Then see how it idles.

[Magda]

Hi Landar,

I took it for a spin according to your suggestion.

Here is link to my YouTube Channel demonstrating what happens when I change gears. The engine was fully warmed up at the time of making this video (see coolant temperature dial):

Observations:

1. Idle surges in PARK like stated before . The same behavior is in NEUTRAL (no pressing on gas pedal)

2. It does keep steady 900rpm in REVERSE, DRIVE, 2, L when the gas pedal is not pressed on.

When I drive the car in DRIVE mode, I don't need to press on gas, I have to press on brakes... The car wants to go. I guess for some reason it wants to get to 1500rpm or something...

By the way, this was my first big set of DIY repairs on Lexus LS400. I read a lot about vacuum system and all those components that I was working with. But I still am a newbie.... Thus bear with me if I'm not wording my posts 100% correct. I am expanding my mechanical vocabulary as we speak...

I previously replaced radiator / fan / coolant flange / heater core on my Volkswagen MK3 Cabrio... I also did small tune-up before (spark plugs, air filter, MAF sensor cleaning). My automotive repair expertise is limited but substantial ;) And I always extensively research each and every new repair I'm thinking of doing myself.

[landar]

Thats good that the car idles smoothly in drive or reverse, albeit a bit high (900 is a little higher than 650). And it is normal for a car to want to move when in drive or reverse and you let off the brake. It won't move very fast but it will move. I 'idle' out of my garage in reverse every day. So, thats just normal stuff.

I am beginning to believe that, despite 'good' tests on the IACV, it is still misbehaving...sticking perhaps. What I would do next is to disconnect the IACV and see what the RPMs do. Granted, it may idle too high or too low but see if the 'loping' is changed or gone.

BTW, I am very impressed with your automotive skills, Magdalena!

[Magda]

Hi Landar,

Thanks for the compliment ;) It is very encouraging. I was actually happy that the car started up after all those repairs ;)

I know that Lexus LS400 drives itself in D without the need for pressing on gas pedal. I experienced this before my repairs when the idle oscillated around 300rpm (it was that low before my repairs). However, now when I am in Drive it really wants to go. I was on local roads where the speed limit is 20mph and had to constantly press on brake pedal lol Maybe because now the idle for drive is 900rpm ?...

In post #6 I disconnected IACV wire, but left IACV valve attached to the car (not sure if it was in open or closed position). Then I turned the engine on and it was still looping high and low rpms... Thus I thought that IACV valve or its wire is probably not the reason for my problem. I can disconnect IACV again, and take it for a drive to see how it goes... Hope I will find the problem. I want my car back on the road ;)

[sha4000]

Car was revving like this before I cleaned my MAF. In drive with foot on the brake it would surge sometimes but not as extreme as your case. After cleaning the MAF and cleaning my battery cable and posts it cleared up.

[Magda]

Thanks for this suggestion... I didn't clean MAF as I don't know how to clean this particular type. The MAF sensor spray cleaner from AutoZone/AdvancedAutoParts is for those other kind of sensors and LS400 1990 has FUJI sensor. I didn't search for the cleaning methods for it. I may try it.

[landar]

Sorry, I missed that post on unplugging the IACV.

So, it sounds like the engine runs ok in drive but the idle is high. I would look at the throttle body position sensor and the MAF as suggested. One at a time, however.

I would look at this first -> http://www.lexls.com/tutorials/EFI/tps.html

Maybe even this to make sure your throttle body butterfly is not sticking(make sure you properly cleaned it) -> http://www.lexls.com/tutorials/EFI/throttlebody.html

Then there is the MAF (attached)...Be very careful with these sensors as they can be somewhat delicate.

[sha4000]

If I remember correctly those early gen MAF can't be cleaned. I'm not even sure it's called a MAF it's called a AFM which the file that landar linked states.

[landar]

Whatever the technical name, it is a measure of the "mass of air" going thru the throttle body. In any case, its job is to indicate to the computer how much air is flowing thru the system and how much fuel to mix with the air. Now, the IACV is kind of a 'bypass' around the throttle body...a controlled vacuum leak if you will. However, the IACV 'leak' does not show up on the air flow reading. The computer would of course know what value it is setting the IACV, primarily thru the RPM signal.

I still wonder if the IACV is not the culprit. Perhaps some missed carbon on the value or seat?

[Magda]

Sunday Update:

1. AFM Airflow Meter: I inspected it before I started this thread as I took its assembly off of the car when I was doing throttle body cleaning and valve cover gaskets. I did not clean it as I didn't find a protocol for cleaning this type of sensor.

Today I checked resistance on the AFM connector between THA and E2. At the temperature 70F it read 2.36kOhm thus it is within range.

2. Throttle body: I took it off the car and cleaned it before I started this thread. Then I reinstalled it with a new gasket. The TB plate had approximately 1-2mm (1/8 inch) of carbon deposit on the engine air intake chamber side. It was a lot of cleaning. I was careful not to change the positions of TPS... I also checked that the plate are moving when the cables are pulled.

When I put TB back on, I installed new PCV hose and water-by-pass pipe no.4:

• ventilation hose no.2 of PCV system fat hose under the throttle body, part #: 12262-50010 $23 at dealership #
• HOSE for water by-pass pipe no.4 part number: 99556-10050 # rubber hose for cooling water by-pass from right side of throttle body housing

Today I decided to check the condition of throttle position sensor (TPS) by checking the resistance between the terminals using multimeter and 0.4mm and 0.65mm thickness gauges between the throttle stop screw and stop lever. I used the instructions from pdf supplied by Landar (thank you).

Everything is within suggested values:

0.00mm VTA-E2 : 0.55kOhm

0.40mm IDL-E2: 150 Ohm = 0.15kOhm (my additional thickness gauge)

0.50mm IDL-E2: 160 Ohm = 0.16kOhm (my additional thickness gauge)

0.55mm IDL-E2: infinity

0.65mm IDL-E2: infinity

Throttle valve fully open position VTA-E2: 3.5kOhm

VC-E2: 4.9kOhm

Everything is within suggested values thus I didn't unscrew the TPS sensor to adjust its position.

3. Next I will try to see if there is any troubleshooting for EGR. EGR Valve was very dirty and its pipe in air intake chamber was clogged with carbon deposits. I cleaned it before I started this thread.

I may post some pictures later to show you what kind of dirt I encountered here...

[Magda]

I just came across Mazda Miata forum http://forum.miata.net/vb/showthread.php?t=388269 and it gave me an idea of unplugging TPS for troubleshooting and see what happens with my idle fluctuating up and down while in Park or Neutral... however I need to read about it more to see if it will not cause any damage in lexus...

[Exhaustgases]

Hunting will be caused by the system trying to correct for something. Like too much air getting in and causing the idle to go up. Then the ecu says no, your supposed to be idling at this speed and corrects it.

With out a good way to check the various signals its going to be a hit and miss deal. When its cold does it do this or just when it warms up.

[Magda]

If I turn the engine on, after overnight rest, it will start doing this only after steadily idling for a minute or two... Once it reaches certain RPM it starts the cycle of high-low idle every 2 seconds... So it looks like it does it only after the engine warms up a bit and only in Park or Neutral.

I wonder what happens every 2-3 seconds. Very regular thing.

[sha4000]

Have you thought about pulling the ECU and doing the caps? I would think the check engine light would come on at some point.

[landar]

The CEL might come on...if she drove the car on a regular basis. Thats what I would be doing despite the high idle. I would drive that puppy and see what happens when it is fully hot and see if I could encourage the computer to throw a code.

[Magda]

Hi,

I didn't do Capacitors... I am not fluent in soldering yet... I didn't inspect ECU. I may do it later/soon.

Fortunately this car is not my daily driver thus I have time to go slowly with next steps, reading about potential problems.

I didn't have the fluctuating idle before the repairs/cleanings I did. Thus I thought it may be a problem with some parts I touched during the repairs rather than the ECU... The list of repairs I did just before the idle problem started is in post number #1.

The reason why I didn't drive the car a lot after my repairs is because I didn't want to do more damage by driving it without learning more about potential causes... I agree with Landar that I probably should drive it and let the computer spit the codes...

Today I drove the car for 10 minutes. It started the fluctuating idle at Park after less than a minute = way before the car got to operating temperature. So I assume it was still in so called OPEN LOOP (?) ... In drive it was keeping stable rpms of approximately 950rpms when stopped and holding foot on the brake. After approximately 5 minutes of driving in my neighborhood it did get to the operating temperature (=CLOSED LOOP?) . I drove the total of 10 minutes. It feels like it is pulling. If I am in drive and don't press on gas pedal or brakes, it will eventually go to 45mph before it stops increasing the speed. Not sure if it's normal. The only way to slow down is to hit on brakes. I don't want to burn my brakes too much. They get hot from driving even for 5 minutes as it requires constant braking...

Next I put the car in Park, let it idle and did test for air leaks with PROPANE TORCH (no flame of course, just propane). I went through the vacuum lines and other potential sources of air leaks... But I didn't observe any change in fluctuating idle pattern. Maybe I should repeat the test.

Once I was done (5 minutes later), I turned the car off and looked underneath the car. I noticed some smoke coming from left and right junction of Catalytic converters with the exhaust pipe located closer to the rear of the car. This smoke/exhaust went through the cat and then some of it out on this junction. Not sure if it is bad. It probably qualifies as exhaust leak. I hope I didn't ruin catalytic converters.

I probably should drive more for the ECU to generate CELs...

[sha4000]

Yes plz drive the car more if possible. Try to get it on the highway to push the rpms some but pay close attention and be careful. 950 rpms is too high when in drive with your foot on the brake. This tells me that the computer is compensating for some type of lean condition. The car has to be driven a certain amount of time for it to generate a code.

[Exhaustgases]

Warm doesn't always mean closed loop. On the newer obd2 cars its easier to figure out with a scan tool if it is or not, though some fault codes on them that turn on the cel will take it out of and keep it in open loop. I would think if the O2s were fluctuating then it would be in closed loop. I guess another way to know is if the timing is fixed and not advancing or retarding. And that is your next assignment, use a timing light and see if the timing is changing when it does the surging.

[Magda]

Hi,

Thanks for all the advice so far...

I recently was busy with my other stuff, thus not too much car work to show for... I am a bit afraid to drive it due to the fact that it kind of goes to 45mph without hitting gas pedal --> don't want to ruin brakes.

Just wanted to rule out simple things first... Also given my very limited mechanical experience, it is a learning experience thus be patient with me. Sometimes I need to read a lot before I do something.


Since my last post I did some simple tests:


1. replaced PCV valve --> no change


2. Checked if EGR valve itself responds to applied vacuum --> I connected vacuum pump to its top vacuum connection and applied vacuum.
The valve opened and held vacuum, thus it looks like valve can be opened and closed by vacuum. See pictures below:









I made blocking plate from Coke can foil that i installed instead of
regular metal seal, to rule out that the EGR valve stays stuck open.
Blocking EGR didn't remove the fluctuating idle phenomenon, thus it
looks like EGR is not the cause of it.











I made blocking plate from Coke can foil to rule out that the EGR valve stays stuck open.









I tested EGR valve when the car was off. Here no vacuum applied and the valve is closed.









I tested EGR valve when the car was off. Here when I applied vacuum to EGR valve the valve opened thus it works.









Opened EGR valve when the vacuum applied is shown in this picture.


3. I looked carefully on vacuum lines and didn't find any obvious vacuum leak.

4. I disconnected battery a few times and each time it relearns, it ends up fluctuating in Neutral or Park after 1 to 2 minutes. When the car is already wormed up it starts fluctuating nearly immediately once I turn the car on. It now fluctuates between 950 and 1300rpm in Park. I may decide to take it for a drive, but am a bit affraid about the car going too fast as brakes on 1st Gen LS400 are relatively small.





UPDATE WITH PICTURES FROM THE MOST RECENT WORK FROM BEFORE THIS THREAD:







Air Intake Chamber during cleaning from carbon deposits.









Throttle body (TB) was a nightmare to clean.









It was very dirty... I cleaned it scraping it before I used TB
cleaner. If you use TB cleaner first, the dirt just sticks to the metal
better...









EXCAVATION PROJECT: Air Intake Chamber during cleaning from carbon
deposits. EGR little pipe was completely clogged!!! I thought it was
solid metal rod at first... not a pipe.









EXCAVATION PROJECT: EGR/PCV inlet housing (not sure its name) was totally filled with carbon deposits,









EGR little pipe and Cold Start Injector after cleaning. Not 100%
clean, but it was very difficult to get to this point. I put towels in
to make sure dirt does not go into the engine...









clean at least









Distributor cap and rotor new (left) vs old(right)









Distributor cap and rotor new (left) vs old(right)

Old had cracks in plastic next to metal T-shaped part, which are visible in this pic.









Old distributor cap.









Visible cracks in plastic.








New rotor.







I also cleaned IAC (Idle Air Control) valve, because as you can see it was dirty.









IAC passage









IACV after cleaning.









I also inspected the bearings. Here the valve is in closed / extended position.









Here valve is in open (short) position.

Once I assembled the IAC valve, I tested it with 9v battery according to
Lexus instructions. It was slowly moving back and forth in small
increments.









I replaced PCV valve and grommet.









I changed valve cover gaskets and sparkplug seals. Also halfmoon
seals and cam rear bearing cap with its plug. I used old bolts with new
washer for valve cover gasket and for cam bearing caps...




Cam bearing caps, halfmoon plugs, old bolts with new washers.

I also replaced spark plugs and all its wires, triple checking the proper order and placement of wires.

TEMPORARY CONCLUSION:

I realize that all those repairs I did just before the problem appeared point to something I recently did. I still suspect a vacuum leak, but where is it...

Before repairs, the idle was really low, around 350-400 rpms... but there was no fluctuations. Now idle is around 900rpm and fluctuates between 950 and 1300rpm in Park or Neutral. Those fluctuations are very regular (every 1.5 or 2 seconds - I may record it to show you, but didnt get to it yet) and each time when 1300rpm is reached, I can hear an electric buzzing sound, engine shakes a bit to decrease rpm and rpms go down to 950 to repeat... like something wants to lower it back to 950rpm and something else is dragging it up.

In D (drive) it will gradually speed up by itself to 40-45mph without touching gas pedal. When in D on stop light, rpm will sit at around 900.

Maybe I cleaned something too well. By the way, I torqued everything to proper torque with beam torque wrenches or a clicker if beam wrench was too bulky for the particular spot...

I will gradually follow your suggestions. There is so many of them... I have some ideas and doubts.

Thought A. ECU capacitors which may be bad from what I heard must wait a bit. I have no soldering skills yet. I can use digital multimeter a bit... I have to learn more before I attempt that.

Question B. Checking timing with timing light is a bit mysterious to me, but it crossed my mind that timing may be bad, though it seemed fine before. Is there a possibility that timing itself may cause my symptoms?

Question C. Coolant Temperature Sensor -I understand the idea behind it, but wouldn't it cause fluctuations that would be a bit unpredictable, as air bubbles would go through sensor in irregular way, causing computer to fluctuate idle in irregular way? I have very regular fluctuations...

Question D. I installed new distributor caps and rotors, new wires (that were tested for resistance) and new sparkplugs (that were tested for proper gap). What would happen if one rotor was not installed correctly and got damaged... would it cause the car to not turn on at all or would it fluctuate in idle like mine does? I dont think this is it, but I didn't unscrew new distributor caps to inspect new rotors...