Help! Idle loops up and down from 1100rpm to 1600rpm every 3 seconds
06-26-15, 06:38 PM
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Help! Idle loops up and down from 1100rpm to 1600rpm every 3 seconds
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 behavior like that?
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 behavior like that?
06-26-15, 09:55 PM
#3
Do you mean a "lope" like a racing cam idles? I don't know what an idle "loop" is, a loop is like a round ring, or a program that loops, meaning ends and then restarts.
Did you disconnect the battery when doing all those repairs?
Maybe you left something unplugged or broke a wire, things get pretty brittle after the years and heat under the hood.
Did you disconnect the battery when doing all those repairs?
Maybe you left something unplugged or broke a wire, things get pretty brittle after the years and heat under the hood.
06-26-15, 10:37 PM
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Do you mean a "lope" like a racing cam idles? I don't know what an idle "loop" is, a loop is like a round ring, or a program that loops, meaning ends and then restarts.
Did you disconnect the battery when doing all those repairs?
Maybe you left something unplugged or broke a wire, things get pretty brittle after the years and heat under the hood.
Did you disconnect the battery when doing all those repairs?
Maybe you left something unplugged or broke a wire, things get pretty brittle after the years and heat under the hood.
I wasn't sure how to call it. I called it a loop as while in idle it would start at 1100rpm, go to 1600rpms and then restart at 1100rpms thus I figured a loop would be an appropriate description of the problem...
I disconnected the battery. I was careful with wires. But the car is old... who knows. Maybe someone came across similar symptoms and knows the solution... Thus I posted.
06-27-15, 01:02 PM
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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 found the video in this thread that actually shows nearly if not identical symptoms to my problem: https://www.clublexus.com/forums/ls4...lp-please.html
The above thread suggests that my problem could be IACV valve. However, 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 is a culprit?...
I will probably also check throttle position sensor next, however I don't want to mess up any more components...
Regards and Thanks!!!
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 found the video in this thread that actually shows nearly if not identical symptoms to my problem: https://www.clublexus.com/forums/ls4...lp-please.html
The above thread suggests that my problem could be IACV valve. However, 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 is a culprit?...
I will probably also check throttle position sensor next, however I don't want to mess up any more components...
Regards and Thanks!!!
Last edited by Magda; 06-27-15 at 01:22 PM.
06-27-15, 03:22 PM
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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 idles with gradually increasing rpms from 135000 to 1500 rpms which took approximately 40 seconds. Then It 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 !
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 idles with gradually increasing rpms from 135000 to 1500 rpms which took approximately 40 seconds. Then It 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 !
Last edited by Magda; 06-27-15 at 04:40 PM.
06-27-15, 06:18 PM
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I took it for a spin.
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.
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.
Last edited by Magda; 06-27-15 at 06:37 PM.
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06-27-15, 08:41 PM
#8
Does this car have trac? I ask because I was wondering what would happen if the two throttle position sensors connectors were reversed? I guess I should just go look and see if that is possible.
06-27-15, 09:24 PM
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. Fortunately I am confident I haven't mixed those connectors for sure. First of all I labeled them while still connected and secondly, I took pictures and can definitely say that they are reconnected in correct order. 
06-29-15, 12:49 PM
#10
Didn't want to reread all this, did you do anything with the engine coolant temperature sensor? I would on principle put a new one in. Its a pretty key sensor for proper idle and engine operation.
Do you have a DMM? Digital multimeter. If not harbor freight has them for free on occasion and will work good enough. With it you could check the o2 voltage at the service plug on the engine.
Do you have a DMM? Digital multimeter. If not harbor freight has them for free on occasion and will work good enough. With it you could check the o2 voltage at the service plug on the engine.
06-29-15, 10:14 PM
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Didn't want to reread all this, did you do anything with the engine coolant temperature sensor? I would on principle put a new one in. Its a pretty key sensor for proper idle and engine operation.
Do you have a DMM? Digital multimeter. If not harbor freight has them for free on occasion and will work good enough. With it you could check the o2 voltage at the service plug on the engine.
Do you have a DMM? Digital multimeter. If not harbor freight has them for free on occasion and will work good enough. With it you could check the o2 voltage at the service plug on the engine.
I haven't done anything with the engine coolant temp sensor. I may consider it.
I have DMM. I may take a look at O2 sensor. I need to figure out where it is located and how to test it.
Thanks for your input
06-30-15, 02:33 AM
#12
They can be checked at the diagnostic plug on the engine, by just poking the probe in the correct hole.
If its correcting for a lean condition volts will be high, like up to .8 or so.
This video shows what is normal voltage sweep and he mentioned it was pegged high before he started the video. And not saying the O2 is the culprit but it will help show what could be. If there is too much air then the O2 volts will go up to correct that. And do the opposite for less air. And I suppose to accurately compare the rpm to the change is o2 volts you would need at least a 2 channel oscilloscope to see if that is causing it to idle up and down. Yamae will be much better at explaining this than I am.
If its correcting for a lean condition volts will be high, like up to .8 or so.
This video shows what is normal voltage sweep and he mentioned it was pegged high before he started the video. And not saying the O2 is the culprit but it will help show what could be. If there is too much air then the O2 volts will go up to correct that. And do the opposite for less air. And I suppose to accurately compare the rpm to the change is o2 volts you would need at least a 2 channel oscilloscope to see if that is causing it to idle up and down. Yamae will be much better at explaining this than I am.
07-03-15, 02:25 PM
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They can be checked at the diagnostic plug on the engine, by just poking the probe in the correct hole.
If its correcting for a lean condition volts will be high, like up to .8 or so.
This video shows what is normal voltage sweep and he mentioned it was pegged high before he started the video. And not saying the O2 is the culprit but it will help show what could be. If there is too much air then the O2 volts will go up to correct that. And do the opposite for less air. And I suppose to accurately compare the rpm to the change is o2 volts you would need at least a 2 channel oscilloscope to see if that is causing it to idle up and down. Yamae will be much better at explaining this than I am.
If its correcting for a lean condition volts will be high, like up to .8 or so.
This video shows what is normal voltage sweep and he mentioned it was pegged high before he started the video. And not saying the O2 is the culprit but it will help show what could be. If there is too much air then the O2 volts will go up to correct that. And do the opposite for less air. And I suppose to accurately compare the rpm to the change is o2 volts you would need at least a 2 channel oscilloscope to see if that is causing it to idle up and down. Yamae will be much better at explaining this than I am.
I am educating myself with potential causes of the observed idle malfunction. I will let you know on any developments. It's a spare car and I work on it with no time pressure thus infrequent updates. I appreciate all the help from ClubLexus members
07-05-15, 01:18 PM
#15
Problem is that is for a gen2, no one has actually given a photo or specific advice of where to actually do the connections for the test on a gen1 that does not have the same pin outs on the test connector under the hood. Coolant temp sensor was mentioned but that requires getting the coil out of the way and some other junk, we need a less of a hassle way to test for ripple. What about the egr ? Maybe the tps? Either way it requires piercing a wire to find the signal or having a wrecking yard connector setup with pinouts.