3fe bouncing idle when started with AC on - high idle otherwise (1 Viewer)

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My '91 3fe FJ80 I've owned since 2012 has developed a high idle that I'm just beginning to diagnose. I'm looking for any useful insights to make solving this easier. Here are my symptoms:

Very high idle:
The truck idles at 1500-2000 RPM's regardless of temperature​
Things I've checked (vehicle is desmogged)​
Check engine light is not illuminated​
Visually checked vacuum hoses​
Visually checked intake tube (which was replaced a few years ago before they went NLA)​
Tightened / replaced clamps on vacuum tubes, and intake tube​
'Golden screw' is all the way in​
Throttle cable and kick down cable have a little bit of slack when the throttle cam is all the way closed​
Valves were adjusted 10k miles ago​
Timing adjusted 10k miles ago​
VC gasket, and associated bolt / PCV valve gaskets replaced 10k miles ago​
Things I have yet to do (in order of priority)​
Check with vacuum gauge (where's a good spot to hook one up? What reading should I expect?)​
Check vacuum lines with starting fluid (or similar) to find potential leak points​
Remove and clean TB​
Remove and clean IAC (which does click when the vehicle is shut off)​
Test TPS per FSM procedure​
Check dashpot throttle stop (I can't imagine this would get messed up on it's own)​
Severe bouncing idle when starting the truck with the AC on:
I have not seen this discussed on 'mud, and I'm wondering if this will point to the issue. If I start the truck with the AC off, it will idle high. If I start it with the AC on, it will bounce rapidly between 1k and 2k RPM's until I put it in gear. The AC does work, and is not overcharged.​
Weak acceleration until 2700 RPM's:
I'm getting the typical weak acceleration until 2700 RPM's normally associated with a vacuum leak​

@jonheld or other 3fe experts...

Am I missing anything obvious? Is the air conditioning issue pointing toward a culprit that I'm not seeing? Should I adjust my priority of repairs, or add another step?
 
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I had that problem - new coolant temp sensor and took off the IAC valve and cleaned all the carbon out of it helped. I still have a bit of a rough idle when cold though. I cant point to exactly what the problem was. Good luck!
 
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The truck idles at 1500-2000 RPM's regardless of temperature
That sounds like the throttle plate is not closed all the way. I know you said there was slack in the throttle cable, but make sure the throttle plate is actually sitting on the end stop.
Remember, tractor motor from 1986. Always look for the simple solution first.
 
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If I start it with the AC on, it will bounce rapidly between 1k and 2k RPM's until I put it in gear. The AC does work, and is not overcharged.
There is no "manual" adjustment for idle on a 3FE. The ECU pushes the ISC according to the input from various sensors.
The AC idle up is controlled by the ECU/ISC. The fact that it "hunts" tells me that the ECU is getting the wrong information from the ISC.
The TPS is not in play UNLESS the throttle plate is not at the idle position resting on its end stop.

Again, the mechanicals have to be correct first. The sensors have a very limited window of operation.
 
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You're an amazing asset @jonheld . I added some slack to the throttle cable, and the rpms dropped to 950.

I removed and cleaned the IAC and the truck no longer hunts for idle during startup with AC. (The IAC wasn't that dirty).

Is 950 too high? The fsm seems to specify 650.
 
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You're an amazing asset @jonheld . I added some slack to the throttle cable, and the rpms dropped to 950.

I removed and cleaned the IAC and the truck no longer hunts for idle during startup with AC. (The IAC wasn't that dirty).

Is 950 too high? The fsm seems to specify 650.
It should be 650 at normal operating temp.
Again, idle is controlled by the ECU which pushes the ISC. If adding slack helped lower the idle, then the throttle plate is not closing all the way.
Make sure the mechanical operation of the throttle is smooth end to end.
Make sure there's not a clump of carbon or some other crap keeping the plate open.

The mechanicals have to be correct first.
 
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As an FYI, the correct way to adjust the throttle cable is to have someone floor the throttle pedal. Then adjust the cable so that the throttle plate juuust touches the WOT endstop. That's it. Any excess slack in the cable at idle is meaningless.
 
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I removed the TB and thoroughly cleaned it. I bench tested the TPS and it checked out. I reinstalled the TB, and I still have a high idle.. between 950 and 1200 randomly. I adjusted the throttle cable per @jonheld procedure, and I'm very confident the butterfly is completely closed.

I tested the temperature sensor (not sender) and it came up as .28K ohms when the engine was hot. I'll check it cold in a little while.

So, the current situation is: the truck doesn't bounce with the AC on when starting, the 'turbo' feeling at 2700rpms is gone, but I'm still getting a high idle.

if the temperature sender checks out cold, I'm going to have to check continuity between wiring harness connections, I suppose.

Any other tips?
 
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Took her for a short ride, CEL came on with codes 22 and 51.

51 seems vague.

I'll update the thread when I figure it out
 
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TPS is the most likely cause for code 51 with your specific symptoms, I would say. It may bench test decent and have an intermittent issue? Good luck. I also have a '91 3FE, so I am always interested in how things work out.
 
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At this point I think I have a wire issue. The TPS bench tested good, so unless it's intermittently acting up it should be fine.

I've read about wiring under the intake manifold getting baked, and going bad.

I need to look at the EWD section of the FSM and find either a splice location, or the ECU pinouts for both the TPS and the IAC.
 
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Using the very scientific method of jigging the wiring harness vigorously, my idle returned to a steady 650 RPM.

It started hard,and still had a cel but I only moved it across the driveway.

Anyone else think I have bad wiring under the IM?
 

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