3FE code 51 high RPMs...I'm stumped

[Aggiesdm]

I really haven't been following this, so I'll ask some basic questions. If it's been covered please forgive me.
Have you verified proper mechanical operation of the throttle cable?
Have you verified proper mechanical operation of the throttle valve? IOW, without power applied, does the throttle plate fully close and come to rest on the endstop? Does it open all the way to the endstop and operate smoothly throughout the range?
Is the dashpot stuck in an open position?
It all has to all work properly mechanically first. Remember 1986 technology. If there's a piece of carbon on the intake preventing the throttle plate from closing, or any mechanical fouling at all, the electronic adjustments go right out the window. It's not smart enough to know the difference.

Thanks for responding Jon. Here are answers to your questions:

1. Throttle Cable is working properly
2. Throttle valve is operating properly and resting on the endstop when no throttle applied.
3. The dashpot appears to be working properly, but I'll double-check it

I removed the throttle body and cleaned it really well. The plate appears to be opening and closing properly and there are no obstructions.

The most intriguing thing I've experience is when i remove the air hose from the IAC valve. When its removed from the IAC and then plug up the IAC and the air hose, the idle drops down to near normal.

Would this indicate that the IAC is not functioning properly?

 

[jonheld]

Thanks for responding Jon. Here are answers to your questions:

1. Throttle Cable is working properly
2. Throttle valve is operating properly and resting on the endstop when no throttle applied.
3. The dashpot appears to be working properly, but I'll double-check it

I removed the throttle body and cleaned it really well. The plate appears to be opening and closing properly and there are no obstructions.

The most intriguing thing I've experience is when i remove the air hose from the IAC valve. When its removed from the IAC and then plug up the IAC and the air hose, the idle drops down to near normal.

Would this indicate that the IAC is not functioning properly?

It's an ISC valve.
You said that it's already been replaced. I have a hard time believing that 2 are faulty exactly the same way. There is a diagnostic on page FI-40 of the FSM, so I would start with that.

I would also run a continuity check between the ISC connector and the ECU connector to make sure all 4 coils of the stepper motor are receiving the pulse from the ECU as well as +12. The ECU pulses to ground to run the stepper, so you want to check for a short to ground as well as continuity. If one of the coils is hanging low, that's going to screw the pooch.

 

[Aggiesdm]

It's an ISC valve.
You said that it's already been replaced. I have a hard time believing that 2 are faulty exactly the same way. There is a diagnostic on page FI-40 of the FSM, so I would start with that.

I would also run a continuity check between the ISC connector and the ECU connector to make sure all 4 coils of the stepper motor are receiving the pulse from the ECU as well as +12. The ECU pulses to ground to run the stepper, so you want to check for a short to ground as well as continuity. If one of the coils is hanging low, that's going to screw the pooch.

Sorry...I was confused whether it was called ISC or IAC! I has not been replaced yet. It was the TPS that was replaced. I did take the ISC apart and clean it and bench-test it per the FSM. I applied 12v to the pins in the sequence designated, but as I mentioned earlier when I applied 12V to the pins, the movement on the "plunger" was very minimal less than 1/16" of a rotation. Is that a normal amount of movement or should it be moving more?

I'll try to run the continuity check to the ECU plug. Stupid question...where is the ECU? Is it behind the glove box or is it in the driver kick-panel? Fortunately I've never had to mess with it before.

 

[TomOsborne]

But my issue was more transient than yours. On idle you need to be seeing the voltage between IDL and E2 be close to 0. When you open up the throttle the voltage will be like 11 volts. If this is different, the TPS is not adjusted right or broken.

This is basically the FSM test, but with the ECU in the loop.

This is awesome.

 

[TomOsborne]

I really haven't been following this, so I'll ask some basic questions. If it's been covered please forgive me.
Have you verified proper mechanical operation of the throttle cable?
Have you verified proper mechanical operation of the throttle valve? IOW, without power applied, does the throttle plate fully close and come to rest on the endstop? Does it open all the way to the endstop and operate smoothly throughout the range?
Is the dashpot stuck in an open position?
It all has to all work properly mechanically first. Remember 1986 technology. If there's a piece of carbon on the intake preventing the throttle plate from closing, or any mechanical fouling at all, the electronic adjustments go right out the window. It's not smart enough to know the difference.

Good time to insert a question if you would oblige. My rig has the dashpot removed. It used to idle fine at 550 +/- than incurred problems that I have slowly improved upon (vac, tps adjustment, etc) but not solved; i idle at about 1600 now. The way I understand it, the dashpot is more an emissions device that smooths out the throttle plate from slamming shut on deceleration. My throttle plate does rest on the endtop, but does not completely close. Probably stays open a good quarter inch while sitting on the endstop. Problematic?

 

[jonheld]

Sorry...I was confused whether it was called ISC or IAC! I has not been replaced yet. It was the TPS that was replaced. I did take the ISC apart and clean it and bench-test it per the FSM. I applied 12v to the pins in the sequence designated, but as I mentioned earlier when I applied 12V to the pins, the movement on the "plunger" was very minimal less than 1/16" of a rotation. Is that a normal amount of movement or should it be moving more?

I'll try to run the continuity check to the ECU plug. Stupid question...where is the ECU? Is it behind the glove box or is it in the driver kick-panel? Fortunately I've never had to mess with it before.

ISC, IAC, 220, 221...whatever it takes.
I wouldn't expect lots of movement from the plunger per step of the stepping motor. You're applying a steady +12 to each step. The ECU will pulse each stepper coil sequentially to achieve the desired result.
The ECU lives above the glovebox in all 80 Series. It is mounted horizontally way up high.

 

[jonheld]

Good time to insert a question if you would oblige. My rig has the dashpot removed. It used to idle fine at 550 +/- than incurred problems that I have slowly improved upon (vac, tps adjustment, etc) but not solved; i idle at about 1600 now. The way I understand it, the dashpot is more an emissions device that smooths out the throttle plate from slamming shut on deceleration. My throttle plate does rest on the endtop, but does not completely close. Probably stays open a good quarter inch while sitting on the endstop. Problematic?

The dashpot is exactly that. An emissions device.
Your idle should be 650. 550 is low and 1600 is not right by any measure. Always check the mechanicals first. You did something that set the idle high in your quest. Backtrack your steps.

And it really isn't good form to ask about your problems in a thread that someone else started. It's always best to start your own thread.

 

[Michael Hanson]

Hi, Look for loose wires or cracked hoses. Our 1991 has almost 400,000 miles and has needed few replacement parts .Almost always it has been something simple . Get a Toyota manual ,it's the best help you can get . Mike

 

[Aggiesdm]

ISC, IAC, 220, 221...whatever it takes.
I wouldn't expect lots of movement from the plunger per step of the stepping motor. You're applying a steady +12 to each step. The ECU will pulse each stepper coil sequentially to achieve the desired result.
The ECU lives above the glovebox in all 80 Series. It is mounted horizontally way up high.

HA!! Mr. Mom reference. Love it.

 

[Aggiesdm]

Hi, Look for loose wires or cracked hoses. Our 1991 has almost 400,000 miles and has needed few replacement parts .Almost always it has been something simple . Get a Toyota manual ,it's the best help you can get . Mike

Thanks. I have the FSM already, but there are some things that just don't make sense to me sometimes.

 

[TomOsborne]

The dashpot is exactly that. An emissions device.
Your idle should be 650. 550 is low and 1600 is not right by any measure. Always check the mechanicals first. You did something that set the idle high in your quest. Backtrack your steps.

And it really isn't good form to ask about your problems in a thread that someone else started. It's always best to start your own thread.

Okay thanks for answering my question, and sorry for interjection. Lurk mode = on

 

[TAURO75]

Is your problem solve? Whats make the fault?

 

[red66toy]

I just want to chime in as I had a similar issue. I had a high idle issue with my 3FE recently. I made sure there were no vacuum leaks, checked throttle cable. It already had a new TPS and cleaned throttle body. I ended up cleaning the IAC and cleaning the electrical pins on the IAC and the harness plug with electrical cleaner. No weird idling since.

 

[Aggiesdm]

Is your problem solve? Whats make the fault?

So, here is where it stands. I believe its fixed, but I’ll know more this weekend.

I took the throttle body off and recalibrated the TPS to make sure it was in the right position. I recently read a thread that suggested that the Dash Pot could also be the culprit. I had previously eliminated that concern, but thought I’d check it out a little further. I ended up spraying some PB Blaster on it as I wasn’t sure how far it was supposed to move. Well, after letting it sit for a few minutes, the plunger loosened up and moved way father and smoother. So, fast forward...I reinstalled everything and my RPM’s are now back down NEAR normal. I’m idling at about 900 vs normal 650. What I realized is that I calibrated the TPS with the dash pot in the stuck position, so I’m going to remove it again, recalibrate with the newly-freed dash pot and hopefully bring everything back into spec.

I’ve chased this thing around for a while and it appears that it was a stuck dash-pot, which is funny and frustrating as I thought I had already ruled it out, but I didnt’ realize how far the plunger was actually supposed to travel. Another lesson learned.

 

[mct75]

Impressive! I have never heard of a stuck dashpot before. Maybe remove it entirely and see if that 100% fixes the idle?

 

[Aggiesdm]

Impressive! I have never heard of a stuck dashpot before. Maybe remove it entirely and see if that 100% fixes the idle?

I know! I would not have considered it, but sure enough...it was stuck in the open position. It would only compress about 1/8” until I hit it with the PB Blaster and now it travels 1/2”+. I’ll have to recalibrate the TPS now that there is proper “rebound” with the throttle. I honestly think that will solve the problem.

 

[CJF]

I'm convinced that most 3FE high-idle issues are simply from the throttle plate not closing completely for one reason or another.

 

[red66toy]

I know! I would not have considered it, but sure enough...it was stuck in the open position. It would only compress about 1/8” until I hit it with the PB Blaster and now it travels 1/2”+. I’ll have to recalibrate the TPS now that there is proper “rebound” with the throttle. I honestly think that will solve the problem.

Did you just spray it with it mounted or take it out? Definitely going to look into this. Thanks!

 

[Aggiesdm]

Did you just spray it with it mounted or take it out? Definitely going to look into this. Thanks!

I took it out and sprayed it while it was on the bench. Worked it back n forth a little and it freed up.

 

[Aggiesdm]

I'm convinced that most 3FE high-idle issues are simply from the throttle plate not closing completely for one reason or another.

Agreed. Mine was clearly not closing all the way with the dash-pot stuck in the open position. Much better now, but I think I can improve it a littte more by recalibration get the TPS.