FJ80 High Idle Problem Solved

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Mar 9, 2011
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. old school A/C adventure, valve adjustment and an easy way to do the ignition timing!
Mine idles exactly like yours did. I'm going to go take a look at these. Thanks for the write-up!
Merc, thanks for the write up... I have been chasing this down for awhile. I have a '92 and it is doing the exact same thing.
Did you have to tighten the brass screw in the front of the EFI and then back it out or tighten it until snug?
Thanks, Jake
Outstanding first post!

Great write up, pics can happen after 10 posts or the purchase of a silver star :cheers:
Thank you for posting. I have the same problem (mine is a 91 w 260k miles). I feel better knowing bc I had mechanics saying I needed to do crazy/expensive stuff to fix it.
This is great info. Thanks.

finally i found what the golden screw is! It is in fact located just under the efi and to the left about an inch. it is just a round screw that is golden and looks as if it is flush but when tightened i was able to turn mine three rotations. It lower my idle and also did help the bottom end, havent checked mpg yet! I hope i can get away from the 11's and start hitting the 14's. I have a 92 LC 3fe. This site has saved me alot of heartache and also much needed $$$. I need an IH8MUD sticker!!! :steer:
FFGarcia: Welcome to Mud :flipoff2: (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.
Kernal- I understand that the OP position. I was just wondering if there was a remedy for the 1FZFE. I have the exact issue as described in the original post. The only thing I was able to find was tinkering with the IAC.
I was hoping for a quick and easy fix like the golden screw. Sounds like no suck luck. Thanks for the reply though. Much appreciated.
Mine is a 96, and I'm having a lobing idle after warmup and some highway miles. Its intermittant, but just started recently. Even though I have over 300k on the odometer, the truck runs great. I havent read the ohm value for the TPS but I'm heading that way.
Mine is a 96, and I'm having a lobing idle after warmup and some highway miles. Its intermittant, but just started recently. Even though I have over 300k on the odometer, the truck runs great. I havent read the ohm value for the TPS but I'm heading that way.
Search high idle when warm and you should gets some information regarding your issues with your 96'. A particular thread started by @Flank has good troubleshooting info in it.
Man this helps out a lot. Exact thing Im looking at. After reading everyones advice and digging up nuggets like this it's all starting to make a lot more sense. Although Im a total rookie with resistance and multimeters I think I can figure it all out. Thank you!
I just tried this on my new to me 91 FJ and it did adjust the idle, turning the screw. Mine was not golden, but in the exact location as described.

My idle was pretty high around 1200 in P or N and any other gear it went down to about 800/900.

I actually had to turn the screw all the way (clockwise) to get it to adjust to a better idle. Now after it starts up it goes from 1200 and slowly down to 650-800 rpm and in moving gears it goes to 500-650 per the instrument panel. I'm working the accuracy of the panel.

My question is, if I have to turn it all way in (could back in out a 1/16 at a time to see effect) should I be thinking of the TPS issue?

*also maybe worth noting, the fuse panel in the cruiser is missing the ECU-B fuse. Does this haven't anything to do with whats discussed here? I've been searching for answers on that.
Always worth doing the onboard TPS multimeter test. :shrug:

I don't think I have an ECU-B fuse either.

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