I bought a '91 LC with 250K miles that had a ton of problems and I'll post the symptoms and solutions here. Thanks to everyone who makes contributions to this site and more specifically the FJ80!
Symptom: High idle, even after warmed up and inconsistent between Park and Drive. I had a higher idle in park or neutral (900 to 1100 RPM) that would drop to 800-900 RPM (in any gear). In other words, the two idle conditions weren't equal. The ECU controls the idle by allowing or denying air into the intake. It does this with the Idle Speed Control Valve - ISC. This is also known as an Idle Air Control - IAC.
The idle is controlled by the engine computer (ECM). After the truck is warmed-up, the control system should converge on 650 RPM regardless if the truck's in park, drive or cycling A/C. The ECM senses the tach signal and adjusts a stepper motor inside the ISC in real time to control the flow of air into the intake manifold. The ISC can increase the idle by allowing more air (retracting a plunger). Or, it can push the plunger forward and allow less air which will lower the idle. A quick easy way to see if the ISC is working properly is to listen to it immediately after turning off the engine. The ECU keeps the IAC alive for a few seconds and you'll hear the ISC retracting. It's a quick ticking sound for a a couple of seconds.
I found two problems.
#1 problem - the TPS was defective. The TPS should provide a resistance that varies with throttle position. The resistance should slew smoothly and track the throttle. There should not be any erratic or inconsistent jumps in resistance. Mine had different resistances between pins VTA and E2. Sometimes it was 3Kohm and sometimes it was 40K ohm at idle. The TPS is not a repairable item, so I just found a NOS Denso replacement and installed it according to the FSM. It important to get this adjustment correct. The TB must be taken off to do it properly and you will also need a digital voltmeter that can measure resistance. A set of TPS contacts close when the throttle linkage hits the set screw. The closed contacts let the ECM know "hey, we're idling".
When the contacts are closed, the resistance between IDL and E2 should be less than 2.3K ohm That seems high for a closed contact, but the material inside the TPS has resistance, so you'll never see just a couple of ohms. So with this out of the way, next problem....
#2 problem - the golden screw was backed out too much. This is the large brass screw easily seen in front of the EFI. The screw is an air bypass and virtually not discussed in the FSM. In fact, it's not even shown as potential reason for high idle. Surprise, it IS! Someone backed it out too far which allowed too much airflow into the intake all of the time.
Why did they do this? Well, the valves and ignition timing were off (another topic) and they compensated the rough idle by making it too high...past the limit where the ECM could maintain 650 RPM. It didn't matter how far the ECM tried to restrict the airflow through the ISC...the golden screw would still allow too much air. The control system simply will not go far enough to reduce the idle. I easily fixed this by turning the golden screw in (clockwise) until the idle came way down...almost stalling. Then after no time, the idle was correct.
If the system is working properly, the truck will find 650 RPM and I wouldn't put 100% trust into the instrument panel's tach. I checked my actual RPM with an oscilloscope. It was a nominal 650 RPM, but mt L/C tach's needle showed 550. Another quick note and worth mentioning... There is a 2-pin green shrouded EFI temperature sensor tucked underneath the upper radiator hose and impossible to get to without removing some parts. Don't be fooled! There are three sensors under the hose. It's green and really hard to see. If this sensor's resistance doesn't properly track the coolant temperature, the idle will not be correct. I checked it on an 80 degree day. The sensor resistance at 80F should be about 350 ohms (according to the graph in the FSM). I disconnected the sensor connection and simulated a false resistance to the ECM using a 10K ohm resistor across the harness pins, indicating it was about -10F outside. The truck would barely start and the idle was absolutely erratic. I just wanted to prove during checkout that a bad EFI coolant temperature sensor would mess with the idle.
Next...my old school A/C adventure, valve adjustment and an easy way to do the ignition timing!
Symptom: High idle, even after warmed up and inconsistent between Park and Drive. I had a higher idle in park or neutral (900 to 1100 RPM) that would drop to 800-900 RPM (in any gear). In other words, the two idle conditions weren't equal. The ECU controls the idle by allowing or denying air into the intake. It does this with the Idle Speed Control Valve - ISC. This is also known as an Idle Air Control - IAC.
The idle is controlled by the engine computer (ECM). After the truck is warmed-up, the control system should converge on 650 RPM regardless if the truck's in park, drive or cycling A/C. The ECM senses the tach signal and adjusts a stepper motor inside the ISC in real time to control the flow of air into the intake manifold. The ISC can increase the idle by allowing more air (retracting a plunger). Or, it can push the plunger forward and allow less air which will lower the idle. A quick easy way to see if the ISC is working properly is to listen to it immediately after turning off the engine. The ECU keeps the IAC alive for a few seconds and you'll hear the ISC retracting. It's a quick ticking sound for a a couple of seconds.
I found two problems.
#1 problem - the TPS was defective. The TPS should provide a resistance that varies with throttle position. The resistance should slew smoothly and track the throttle. There should not be any erratic or inconsistent jumps in resistance. Mine had different resistances between pins VTA and E2. Sometimes it was 3Kohm and sometimes it was 40K ohm at idle. The TPS is not a repairable item, so I just found a NOS Denso replacement and installed it according to the FSM. It important to get this adjustment correct. The TB must be taken off to do it properly and you will also need a digital voltmeter that can measure resistance. A set of TPS contacts close when the throttle linkage hits the set screw. The closed contacts let the ECM know "hey, we're idling".
When the contacts are closed, the resistance between IDL and E2 should be less than 2.3K ohm That seems high for a closed contact, but the material inside the TPS has resistance, so you'll never see just a couple of ohms. So with this out of the way, next problem....
#2 problem - the golden screw was backed out too much. This is the large brass screw easily seen in front of the EFI. The screw is an air bypass and virtually not discussed in the FSM. In fact, it's not even shown as potential reason for high idle. Surprise, it IS! Someone backed it out too far which allowed too much airflow into the intake all of the time.
Why did they do this? Well, the valves and ignition timing were off (another topic) and they compensated the rough idle by making it too high...past the limit where the ECM could maintain 650 RPM. It didn't matter how far the ECM tried to restrict the airflow through the ISC...the golden screw would still allow too much air. The control system simply will not go far enough to reduce the idle. I easily fixed this by turning the golden screw in (clockwise) until the idle came way down...almost stalling. Then after no time, the idle was correct.
If the system is working properly, the truck will find 650 RPM and I wouldn't put 100% trust into the instrument panel's tach. I checked my actual RPM with an oscilloscope. It was a nominal 650 RPM, but mt L/C tach's needle showed 550. Another quick note and worth mentioning... There is a 2-pin green shrouded EFI temperature sensor tucked underneath the upper radiator hose and impossible to get to without removing some parts. Don't be fooled! There are three sensors under the hose. It's green and really hard to see. If this sensor's resistance doesn't properly track the coolant temperature, the idle will not be correct. I checked it on an 80 degree day. The sensor resistance at 80F should be about 350 ohms (according to the graph in the FSM). I disconnected the sensor connection and simulated a false resistance to the ECM using a 10K ohm resistor across the harness pins, indicating it was about -10F outside. The truck would barely start and the idle was absolutely erratic. I just wanted to prove during checkout that a bad EFI coolant temperature sensor would mess with the idle.
Next...my old school A/C adventure, valve adjustment and an easy way to do the ignition timing!
(Mud salute). The OP (original poster) is discussing an idle problem/fix with his 1991 FJ-80 which has the 3FE engine (1992 also). From 93-97 US spec FZJ-80's have the 1FZFE engine.
