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

[Aggiesdm]

Not sure if I've gone down the wrong road or what, but can't seem to diagnose my issue. After cleaning, calibrating and bench testing both the TPS and the ISC. I decided to reinstall and pray for a solution. As a note, I made sure that the ISC was fully extended (closed) when I reinstalled it. I then buttoned everything up and made sure all vacuum lines and coolant lines were properly installed.

I cranked it up and the high-idle seemed to have disappeared. I was excited! I decided to drive her around the block and give the ISC a chance to adjust itself. Well, after a 3 minute drive the idle was back up to 1600...back to square one.

So, after not sure if this means that the ISC has opened up again during the drive and just isn't responding properly or what? It passed all the bench test regarding the resistance and it did move when electrified although VERY little. Like 1/8" rotation. Seems like it should be moving more.

Anyone have an idea of how I can get a more definitive diagnosis? Again, this started with a high-idle and a Code 51, which indicates some sort of switch issue or TPS. TPS checks out and was replaced within the last 2 years.

@jonheld I hate to call in the cavalry, but any thoughts or advice on how to diagnose?

 

[george_tlc]

As mentioned, the ISC is a stepper motor type implementation. It will NOT move fully just because you apply 12V to it. It can only move one 'step' for each activation. The ECU will pulse the appropriate connections with the appropriate polarity to either extend or retract. Since there is no 'absolute position' feedback, the ECU just sends pulses as needed to open/close the air path to maintain the RPM it wants. If the ECU wants to put the ISC to a known position (closed), it will just send a LOT of pulses when you first turn the ignition to on to home it to closed.

If the ECU is reporting a TPS issue, then just buy another TPS and install it - worst case you have a spare... I presume all the wiring to the TPS is in good condition?

cheers,
george.

 

[Aggiesdm]

As mentioned, the ISC is a stepper motor type implementation. It will NOT move fully just because you apply 12V to it. It can only move one 'step' for each activation. The ECU will pulse the appropriate connections with the appropriate polarity to either extend or retract. Since there is no 'absolute position' feedback, the ECU just sends pulses as needed to open/close the air path to maintain the RPM it wants. If the ECU wants to put the ISC to a known position (closed), it will just send a LOT of pulses when you first turn the ignition to on to home it to closed.

If the ECU is reporting a TPS issue, then just buy another TPS and install it - worst case you have a spare... I presume all the wiring to the TPS is in good condition?

cheers,
george.

Yeah, I actually have a brand new TPS on the bench, but didn't install it because the other one checked out. Also, when I reinstalled everything, it seemed fine. It was after I drove around that the RPMs gradually crept back up. To me, it seems that something improperly "adjusted" during my test-drive and nullified any changes that I may have made.

Also, when I unplugged the rubber hose going from the ISC to the intake hose and plugged both ends, the RPMs came back down to normal levels. Not sure if this is any sort of definitive diagnosis or what it really means.

Does it still sound like a TPS to you? I'm open for any suggestions, it just seems very different from my previous experience with the TPS where everything was inconsistent and sporadic. The engine is running smoothly and the throttle seems to respond accordingly, the only problem is the high idle.

 

[mct75]

My tps would only fail once it got hot. Very frustrating to have it work fine on the bench then cut out at 70 MPH.

 

[george_tlc]

The TPS has TWO functions and TWO mechanisms.

One is a variable resistor that varies its resistance based on the accelerator pedal position. It's function it to provide a heads up to the ECU about what your are doing with the pedal - accelerating, constant application, removal, etc.

Two is a ON/OFF switch which indicates whether your foot has pressed the pedal even a little bit down.

If the variable resistor is 'dirty/worn' you get inconsistent acceleration etc.

If the ON/OFF switch is faulty the the ECU will think you have the pedal applied or not applied erratically or all the time.

They are two different faults/failures that are not related.

cheers,
george.

 

[Aggiesdm]

The TPS has TWO functions and TWO mechanisms.

One is a variable resistor that varies its resistance based on the accelerator pedal position. It's function it to provide a heads up to the ECU about what your are doing with the pedal - accelerating, constant application, removal, etc.

Two is a ON/OFF switch which indicates whether your foot has pressed the pedal even a little bit down.

If the variable resistor is 'dirty/worn' you get inconsistent acceleration etc.

If the ON/OFF switch is faulty the the ECU will think you have the pedal applied or not applied erratically or all the time.

They are two different faults/failures that are not related.

cheers,
george.

Ahh. Ok, thanks a for the clarity. I'll swap it out and see if that rectifies my situation. Again though...it passed all bench tests, but I'll give it a shot for sure.

 

[Aggiesdm]

Just a note...I started the motor and disconnected the TPS cable. There was no change in the idle speed.

 

[mct75]

To confirm, you do get the clicking on engine shutoff? And the TPS E2-IDL responds like it should?

 

[Aggiesdm]

To confirm, you do get the clicking on engine shutoff? And the TPS E2-IDL responds like it should?

Yes on both of those. I get the clicking noise on shutdown although its not super strong, but I don't have a normal reference either. The TPS E2-IDL was fine when I had it on the bench. I can check it again on the vehicle, but wouldn't unplugging it basically take it's influence out of the loop or does the ECU already have the high-idle command in the cue until changed by the TPS? Does that question make sense?

 

[mct75]

What happens when the connector is unplugged depends on how the transistor logic is set up in the ECU. It might think you are on idle or it might not.

The clicking sound on shutoff is the "high idle in cue" command.

The ECU has no feedback on the position of the ISC valve, all it can say is "hmm I sent some pulses to it and the idle has not changed, better log a code 51."

What I did was remove the rubber boot from the TPS connector and was able to insert multimeter probes into the back of the connector. This let me drive around and read the voltage on VTA while I was going 50 MPH! Of course you don't have to do all that but it's possible to measure it in-circuit. IIRC there's something like 11.5 volts on IDL when you are on the throttle.

 

[Aggiesdm]

What happens when the connector is unplugged depends on how the transistor logic is set up in the ECU. It might think you are on idle or it might not.

The clicking sound on shutoff is the "high idle in cue" command.

The ECU has no feedback on the position of the ISC valve, all it can say is "hmm I sent some pulses to it and the idle has not changed, better log a code 51."

What I did was remove the rubber boot from the TPS connector and was able to insert multimeter probes into the back of the connector. This let me drive around and read the voltage on VTA while I was going 50 MPH! Of course you don't have to do all that but it's possible to measure it in-circuit. IIRC there's something like 11.5 volts on IDL when you are on the throttle.

Thanks MCT75, I'll have to ponder all that. It might be getting above my pay-grade. How did you get you Voltmeter into the cab if you had probes in the TPS connector? Really long leads? Mine are only about 24" long, so don't know if I can do that or not. I do have a bunch of alligator clips I could string together

But, what am I looking for? Voltage at VTA and/or IDL? And when you say "on the throttle" how much throttle are you referring to? Is this something I could do with the engine running in the driveway or does the motor need to be under load?

 

[george_tlc]

So, have you swapped to the new TPS yet?

cheers,
george.

 

[mct75]

Thanks MCT75, I'll have to ponder all that. It might be getting above my pay-grade. How did you get you Voltmeter into the cab if you had probes in the TPS connector? Really long leads? Mine are only about 24" long, so don't know if I can do that or not. I do have a bunch of alligator clips I could string together

But, what am I looking for? Voltage at VTA and/or IDL? And when you say "on the throttle" how much throttle are you referring to? Is this something I could do with the engine running in the driveway or does the motor need to be under load?

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.

 

[Aggiesdm]

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.

Good deal. I'm traveling for business until Thursday, but I'll try it when I get back home. Thanks so much for the info!

 

[Aggiesdm]

So, have you swapped to the new TPS yet?

cheers,
george.

Not yet, I just haven't had the time to do anything other than quickie testing to this point. I'll swap it out on Friday once I'm back from my work trip. Hopefully that's my issue and it'll fix it easily.

 

[Aggiesdm]

UPDATE: The new TPS sensor seemed to work for a few mins, but after driving the truck around for 10 mins, the idle crept back up and I was right back to square 1. I'm stumped...Any suggestions?

 

[mct75]

the two little hard-vacuum lines on the top

EGR (and by extension) evap purge. Enjoy that butt dyno.

 

[Aggiesdm]

EGR (and by extension) evap purge. Enjoy that butt dyno.

Well...I spoke to soon. I'm so confused at what's going on...the new TPS sensor seemed to work when i got it installed, but after driving it for about 10 mins, the idle is right back to 1600-1800 RPMs. Not sure what else to do...any suggestions?

 

[Aggiesdm]

@jonheld I hate to call in the cavalry, but can you suggest any other diagnostics for my situation? Here are the bullet points so you don't have to read through everything:
* one day I started the truck and the idle jumped up to 1400 +/- RPMs and didn't come back down. It had been running fine 30 mins before this. I pulled a Code 51
* I removed the ISC valve, cleaned it and reinstalled - appeared to be working per the bench test in the FSM, but the FSM indicates the plunger should move in and out when 12V is applied in certain order. When I pulse the 12V to it, it moves, but its very very small increments (like 1/16" bump either clockwise or counterclockwise depending on the order of 12v applied to the different receptacle pins). I'm not sure how far its supposed to move, but this seems too little to me. When I reinstalled it with the plunger all the way retracted, the idle seemed to be fine until I finished a 10 min test drive. Then back to the high idle.
*I bench tested the TPS and it tested OK. After other searches and suggestions, I replaced it with OEM part. I calibrated the new TPS properly and bench tested it. Reinstalled today and it idled fine for the first 5mins of a test drive, then the RPMs crept back up to 1400-1600.
*I also removed the hose from the ISC valve to the intake hose and plugged both ends. This seemed to lower the RPMs back into a normal range. Wouldn't this indicate a bad ISC?
* I know that Code 51 indicates a possible switch issue, but I can't seem to find any other ways to actually diagnose them.

The truck has 289K miles on it and all maintenance is up to date. I'm stumped...

 

[jonheld]

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.