Difficult problem - looking for expert advice.

Joined
Nov 1, 2015
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Magnolia, TX
I have an odd and seemingly unresolved problem... no matter what I've tried.
First things first... I have an FSM, I have used all of my Google superpowers, replacement parts are all OEM (even the replacement tubes, lines, sensors, etc) and now I'm asking for help.

The problem:
I have a 96 LX450 (205k) in pretty good nic but it has a nagging problem. After about an hour of drive time the throttle gets what I can only describe as sticky or jerky and does not have a linier or smooth response that it normally does and the idle surges to and stays at about 1500 RPM (while in gear). When it's out of gear and in park the idle will hunt / surge up and down.

What I've done so far:
1.) Removed and throughly cleaned the throttle body and MAF, pulled the negative and let the computer reset (several times)
2.) Recently Replaced - accelerator cable (it was sticking), intake tube, ICV, TPS, all vacuum lines, coolant temperature sensor (not the one for the gauge), new fuel pump and screen.
3.) Within the last 6 - 8 months - full tuneup, fuel filter, EGR (new grommet and tube too), new brake booster and booster check valve. Regular oil changes, etc.

Everything runs and idles normally until the vehicle is sufficiently warm. It never gets hot / overheats. I've tried spraying starter fluid around all of the vac lines and seals and I'm not getting any signs of a vac leak (again almost every hose has been replaced)

The only next steps I can think of is to hook up the old smoke machine and see if there are vac leaks I have not been able to hit with my starter fluid. What am I missing?

Thanks for the help.
 

Cruzilla

 
 
 
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Mar 25, 2002
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Boise ID
You could also look at the IAC connection and the dreaded wire harness next to the EGR for melted wires and behind the glovebox for chafed ones.
 
Joined
Nov 1, 2015
Messages
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Magnolia, TX
You could also look at the IAC connection and the dreaded wire harness next to the EGR for melted wires and behind the glovebox for chafed ones.
I have this as a next step but I have not noticed any wire issues yet. The harness and connectors seem to be in good condition but I know that I have not seen all there is to see yet.
 
Joined
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Messages
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Magnolia, TX
My first step would be; check the TPS connector for corrosion, good connection, and test the switch.
To my eye everything looks clean, when you remove the TPS with the engine running it activates when throttle is applied and it’s ohms out according to the FSM.
 

nukegoat

Should have bought a Jeep
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To my eye everything looks clean, when you remove the TPS with the engine running it activates when throttle is applied and it’s ohms out according to the FSM.
Id look at all the OBD 2 output when the problem is occuring. Look to see if the throttle position is jumping or how the o2 sensors are reading or the air flow is being measured etc. One of those sensors should likely give you insight into where the problem might be stemming from.
 
Joined
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Messages
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Id look at all the OBD 2 output when the problem is occuring. Look to see if the throttle position is jumping or how the o2 sensors are reading or the air flow is being measured etc. One of those sensors should likely give you insight into where the problem might be stemming from.
Yes, I’ll be pulling the fuel trim and other info today to see if I can suss out where the issues is
 
Joined
Feb 10, 2017
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Seattle
I'd second checking out the harness/TPS. May be a long shot but could be the culprit. I had TPS issues after my rebuild. Turned out to be an improperly installed TPS after i took it off for cleaning (my mess up). I'd definitely check that the sensor is installed and clocked properly. I used my OBDII reader to verify that it was at ~10% throttle setting (IIRC) with the pedal fully back which was an awesome recommendation from @Tools R Us I think. Though this was an issue with the truck cold or warm, which makes me think it could be harness related too on your end.
Does the truck idle down once warm without driving?
 
Joined
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I'd second checking out the harness/TPS. May be a long shot but could be the culprit. I had TPS issues after my rebuild. Turned out to be an improperly installed TPS after i took it off for cleaning (my mess up). I'd definitely check that the sensor is installed and clocked properly. I used my OBDII reader to verify that it was at ~10% throttle setting (IIRC) with the pedal fully back which was an awesome recommendation from @Tools R Us I think. Though this was an issue with the truck cold or warm, which makes me think it could be harness related too on your end.
Does the truck idle down once warm without driving?
Thanks, I will check the harness. Once warm idle remains high no matter what... ingear about 1500rpm, in park the idle hunts up and down.
 
Joined
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Thanks, I will check the harness. Once warm idle remains high no matter what... ingear about 1500rpm, in park the idle hunts up and down.
In my situation idle was sitting high because the TPS was installed incorrectly. There is a tab you have to catch and then rotate into position to the right accel %. My idle was also sitting very high and not lowering. Worth checking out if you've ever removed the TPS.
 
Joined
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Messages
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Long shot but try your ECU coolant temp sensor. Not the gauge sensor.
Unplug, clean connector and try again.
I've done that, it was one of my first attempts to fix the problem. The connector appeared to be clean and corrosion free when I did the replacement, its a bitch to get to also...
 

Tools R Us

 
 
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To my eye everything looks clean, when you remove the TPS with the engine running it activates when throttle is applied and it’s ohms out according to the FSM.
Both the idle switch and throttle sweep are as expected? Did you do the tests at the connections on the ECU to confirm the same as at the switch?
 

Bambusiero

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I'll throw in an idea, seemingly from left field.

> Find all the electrical harness chassis ground connections (there are several) - unscrew, wirebrush / clean all metal surfaces, re-install.

This is 99.9% taken for granted, which is about 99% justified - usually fine, no problem...but that remaining 0.9% will mess you up.

Why? The ECU makes electrical measurements on a lot of sensors, and makes decisions based on those measurements.
Every electrical measurement the ECU makes is a relative differential measurement - it measures sensor voltage, as compared to a reference voltage. In most cases, the differential is implicit, meaning that it is made relative to ground. Ground is the negative reference voltage. In other words, the chassis is reference zero volts for everything the ECU does. But if some harness ground connection is aged / oxidized / corroded / loose, then that source of negative reference voltage is unreliable, and could change due to the most subtle of influences, like engine compartment temperature, for instance, or whatever.

I (re)learned this lesson the old fashioned way, by painful experience, on another Toyota vehicle.
OBD2 codes blamed the TPS sensor, which is really just one component in an analog sensor/ digital control / servo motor / mechanical gear loop.
Well over a year, many many hours, many $100s spent on replaced components.
Even replaced the ECU - no small matter on a Lexus with encrypted key codes.
Around and around in circles, almost no discernible pattern to the problem.
Wire brushed the harness ground connections - problem solved, just like that.
Should have done that first.
 
Last edited:
Joined
Jun 3, 2006
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Panama city, Rep of Panama
Gonna tell you one simple thing...check transmission...either low on oil or old oil or poor maintainance...if he transmission is too hot, it WILL control the engine...I have seen it happen...this could be your problem...just check that...better yet, change oil and filter...
 
Joined
Nov 1, 2015
Messages
94
Location
Magnolia, TX
I'll throw in an idea, seemingly from left field.

> Find all the electrical harness chassis ground connections (there are several) - unscrew, wirebrush / clean all metal surfaces, re-install.

This is 99.9% taken for granted, which is about 99% justified - usually fine, no problem...but that remaining 0.9% will mess you up.

Why? The ECU makes electrical measurements on a lot of sensors, and makes decisions based on those measurements.
Every electrical measurement the ECU makes is a relative differential measurement - it measures sensor voltage, as compared to a reference voltage. In most cases, the differential is implicit, meaning that it is made relative to ground. Ground is the negative reference voltage. In other words, the chassis is reference zero volts for everything the ECU does. But if some harness ground connection is aged / oxidized / corroded / loose, then that source of negative reference voltage is unreliable, and could change due to the most subtle of influences, like engine compartment temperature, for instance, or whatever.

I (re)learned this lesson the old fashioned way, by painful experience, on another Toyota vehicle.
OBD2 codes blamed the TPS sensor, which is really just one component in an analog sensor/ digital control / servo motor / mechanical gear loop.
Well over a year, many many hours, many $100s spent on replaced components.
Even replaced the ECU - no small matter on a Lexus with encrypted key codes.
Around and around in circles, almost no discernible pattern to the problem.
Wire brushed the harness ground connections - problem solved, just like that.
Should have done that first.
Good suggestion. I did notice the ground cable lug on the battery ground looks to be in poor condition and is not making a good seat. I ordered some mil spec battery terminals last night and will get this corrected.
 
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