Intermittent idle issue.

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Hello. I’ve been having a intermittent issue with my 1992 3fe when fully warm. Every so often when in park and I rev the engine to 2000 rpm and let my foot off the gas it will drop idle almost to stalling out before it catches itself. Sometimes it doesn’t catch and dies. This has happened when coming to a stop also. One thing I noticed while looking for vacuum leaks (none that I could find). If I disconnected the pcv hose and create a vacuum leak it would run a little rough but not almost stall out when reving. I did clean the throttle body and isc valve recently. Before my current issue appeared.

(It does idle at the correct speed when warm) 650 rpm in park and drops to about 600 when in drive.

Any help would greatly be appreciated.
 
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Right. When I “cleaned” the throttle body, I didn’t remove it. Basically I sprayed cleaner and used a shop rag opening the butterfly. It wasn’t terribly dirty and cleaned up well. Pulled the isc valve and cleaned it with a toothbrush. Put it all back together and had a high idle that ended up being a too tight throttle cable. I did try to adjust the gold screw on the throttle body when I was trying to figure out my slight high idle. I set it back to where I think it was before. 🤔
 
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Right. When I “cleaned” the throttle body, I didn’t remove it. Basically I sprayed cleaner and used a shop rag opening the butterfly. It wasn’t terribly dirty and cleaned up well. Pulled the isc valve and cleaned it with a toothbrush. Put it all back together and had a high idle that ended up being a too tight throttle cable. I did try to adjust the gold screw on the throttle body when I was trying to figure out my slight high idle. I set it back to where I think it was before. 🤔
CHECK THE INTAKE PLENUM FOR CRACKS/SPLITS FIRST.

The ISC valve has a gasket between the boss of the intake and the valve body. If you buggered the gasket or if the gasket is brittle and fractured you'll need a replacement.
There are coolant lines that can become clogged. They should be clear.

The 3FE does not have an "idle adjust screw". The "golden screw" will have a TEMPORARY effect on idle, but the ECU will continually try to push the ISC to get the idle speed correct. Idle speed is set by the ECU and ISC.
The correct setting for the golden screw is 1/4 to 1/2 turn from fully seated. It sets the amount of air entering the manifold at idle, assuming that the throttle plate is closed and resting on the end stop. If the TPS is not at idle position, none of this will work well.

All of the above is only valid if the mechanical adjustments are correct. The 3FE is first generation EFI, 1986 technology. The ECU has VERY small correction windows to pull things back into spec. All mechanicals must be correct first. If there are ANY vacuum/air leaks on the top half of the motor, the idle will go to crap. That includes the PCV valve and grommet, the valve cover gasket and grommets, the oil fill cap gasket, the oil dipstick gasket, the side cover gasket, the intake plenum.

Proper adjustment of the throttle cable is to have someone floor the accelerator pedal in the cab and adjust the throttle cable until the throttle plate is just touching the WOT endstop. At idle, there should be some slack in the cable and the throttle plate should be resting on the opposite endstop.
 
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CHECK THE INTAKE PLENUM FOR CRACKS/SPLITS FIRST.

The ISC valve has a gasket between the boss of the intake and the valve body. If you buggered the gasket or if the gasket is brittle and fractured you'll need a replacement.
There are coolant lines that can become clogged. They should be clear.

The 3FE does not have an "idle adjust screw". The "golden screw" will have a TEMPORARY effect on idle, but the ECU will continually try to push the ISC to get the idle speed correct. Idle speed is set by the ECU and ISC.
The correct setting for the golden screw is 1/4 to 1/2 turn from fully seated. It sets the amount of air entering the manifold at idle, assuming that the throttle plate is closed and resting on the end stop. If the TPS is not at idle position, none of this will work well.

All of the above is only valid if the mechanical adjustments are correct. The 3FE is first generation EFI, 1986 technology. The ECU has VERY small correction windows to pull things back into spec. All mechanicals must be correct first. If there are ANY vacuum/air leaks on the top half of the motor, the idle will go to crap. That includes the PCV valve and grommet, the valve cover gasket and grommets, the oil fill cap gasket, the oil dipstick gasket, the side cover gasket, the intake plenum.

Proper adjustment of the throttle cable is to have someone floor the accelerator pedal in the cab and adjust the throttle cable until the throttle plate is just touching the WOT endstop. At idle, there should be some slack in the cable and the throttle plate should be resting on the opposite endstop.
I will check over those things and report back. Golden screw is 1/2 out. I will recheck ISC valve gasket and check coolant lines. When you say intake plenum, do you mean rubber intake hoses (which look to be fine, inspected yesterday) or intake manifold to block?
 
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The hose that goes from the AFM to the throttle body. If it's 30 years old, then just looking at it funny will crack it. If you lifted the air cleaner lid, look at the underside of the bellows.
 
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Hoses from AFM to throttle body seem good. Pulled off and inspected. ISC looks to be flowing coolant as they are both the same temp when engine is fully warm. (If that’s a simple way to tell?) Gasket for ISC looks fine.
 
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Ok. So as I was looking back at this issue of mine I forgot to mention that I pulled the tps when I was originally trying to fix my high idle after cleaning throttle body. I thought I put it back in the same spot that I removed it. I have code 51. I’ve been trying to spec it back out from the fsm but, no luck. I have it fall into the correct parameters when calibrating. Button everything up and drives and idles pretty good for about 30 miles then after a shutoff and restart I get the above issues. Maybe I’m doing something wrong.😕
 
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How spot on does the tps have to be? It seems like it would have a tiny bit of wiggle room based on the fsm. In theory at least. Also, is there a way to verify that it hit the mark without having to bolt everything back up. Like jumpering the diagnostic box and seeing if the code disappears.
 
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I have code 51.
1643207988887.png


Code 51 is likely the TPS, not the ISC. Read the FSM. It's telling you that the ECU is not getting the idle position signal from the TPS.
Do you have some slack in the throttle cable at idle position?
Is the throttle plate fully closed at idle position?

Again, mechanicals have to be correct first.
 
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I thought I had some ability to fix vehicles but this issue is stumping me. Everything points to TPS. LC was fine until I decided to clean the throttle body. Very frustrating
 
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Did some more poking around.
vc-e2 give .OL on meter
Vta-e2 full open gives .OL on meter. Right before full open it gives 5-6k
Everything else falls into parameters
 
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Assuming you followed the procedure in the Toyota FSM, either the TPS is faulty or installed incorrectly.
How can you install it incorrectly? I had it counter clockwise and rotated up. I did ask a question yesterday about installation. Whether the prongs on the throttle body side need to go into the c-shaped groove of tps or just rest on the flat part.
BEB7E234-2F38-47F7-B230-AF664422B576.jpeg
 
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Thank you. So to be clear, the prongs from the throttle plate should rest on the highlighter area below?View attachment 2906238
Boy, that sure seems wrong to me. Those cut outs on the TPS sure look like they want the tabs from the throttle plate mated to them.
If you installed it according this picture, then I would try it the other way and see if your readings are a little better. I would also guess that it's why you're not getting the IDL signal to the ECU, because the TPS is never reaching the idle position.
From what I've read, the FSM does not address this.
 
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TPS doesn’t fit into the tabs. Can only lay on the out side. Put everything back together and cleaned off the tps contacts and code 51 was gone only to be replaced with 41.
 

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