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Old 05-13-2021, 07:39 PM   #1
CraigT
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Default Question about how the computer decides on Throttle Position

When my 1998 SL2 intermittently generates excess idle without the throttle being pressed, my scan tool shows 2 to 4 % throttle position.

Question: can a malfunction of something like the Idle Air Control motor ( i think it's referred to as a motor) convince the computer to show a throttle being pressed? Or can some other malfunction?

The idle speed vacuum with no ac running is a steady 20.

I did change my TPS for one from OReilly's and it caused greater problems than I already had.

Then I put in one from Auto Zone, and curiously, some improvement. My excess idle still occurs with no pattern but the actual idle of the car when not overspeeding has dropped to a lovely 850 RPM.

Oh! And, I turn the key off at idle overspeed, restart the car, no overspeed.

Any and all thoughts much appreciated. My "work area" is the street in front of my house.
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Old 05-13-2021, 08:29 PM   #2
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Default Re: Question about how the computer decides on Throttle Position

No, the IAC will not cause a bogus TP reading. Having thrown two TPS units at it, and getting different results with each, I would be looking hard at the TPS wiring (includes the connectors). You could have had two bad TPS units too.
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Old 05-14-2021, 12:54 PM   #3
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Default Re: Question about how the computer decides on Throttle Position

Thank you for your reply. I have used deoxit on the tps end of the wiring. I assume the other end is connected to the computer which I think is on the drivers side under the dash. I've never seen it. Hmmm.
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Old 05-14-2021, 01:56 PM   #4
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Default Re: Question about how the computer decides on Throttle Position

This is a super common problem. If you're already tried a high quality TPS then you'll want to look at the connector.

After decades of ownership I eventually found that too was not enough. So I back probed the wires at the PCM and mounted a mulitmeter left of the cluster gauge so I could watch it. Magic! That solved it. Back probing fixed it. The connector pins at the PCM were not tight.

-Robert
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Old 05-16-2021, 01:24 PM   #5
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Default Re: Question about how the computer decides on Throttle Position

Should one consider that the throttle plate is sticking due to oil/carbon buildup?

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Old 05-16-2021, 01:56 PM   #6
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Default Re: Question about how the computer decides on Throttle Position

As it turns out, cleaning out the throttle body seems to have cured my intermittent idle overspeed......

Thanks for the various thoughts!
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Old 05-16-2021, 01:57 PM   #7
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Default Re: Question about how the computer decides on Throttle Position

Ps: I still can't get over that the throttle position reading said I was pushing the throttle during the idle overspeed events.
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Old 05-16-2021, 02:00 PM   #8
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Default Re: Question about how the computer decides on Throttle Position

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Originally Posted by RobertGary1 View Post
This is a super common problem. If you're already tried a high quality TPS then you'll want to look at the connector.

After decades of ownership I eventually found that too was not enough. So I back probed the wires at the PCM and mounted a mulitmeter left of the cluster gauge so I could watch it. Magic! That solved it. Back probing fixed it. The connector pins at the PCM were not tight.

-Robert
Care to mention your source for realizing which wires to probe at the PCM? Also, did I see somewhere that the PCM can be dismounted while still wired or were you able to see the connector(s) of the pcm just by looking up from the drivers side? I've never looked at it and should probably keep it in mind for future reference... lol
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Old 05-16-2021, 02:38 PM   #9
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Default Re: Question about how the computer decides on Throttle Position

A cruddy TB could (was) preventing the throttle blade and TPS from returning to a consistent "0" position; thus the higher idle (air getting by the not-closed throttle blade) and TPS reading (potentiometer wiper in TPS not returning as close to ground potential). There is no mystery at all there.

Honestly, that "cruddy TB" scenario is so common and easy to check/fix that I am a bit ashamed I didn't direct you there myself; before even checking wiring.

PCM pin-outs have been posted here and elsewhere, could be found without too much searching. However, why bother if cleaning the TB fixed it?
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Old 05-16-2021, 03:55 PM   #10
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Default Re: Question about how the computer decides on Throttle Position

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Originally Posted by billr View Post
A cruddy TB could (was) preventing the throttle blade and TPS from returning to a consistent "0" position; thus the higher idle (air getting by the not-closed throttle blade) and TPS reading (potentiometer wiper in TPS not returning as close to ground potential). There is no mystery at all there.

Honestly, that "cruddy TB" scenario is so common and easy to check/fix that I am a bit ashamed I didn't direct you there myself; before even checking wiring.

PCM pin-outs have been posted here and elsewhere, could be found without too much searching. However, why bother if cleaning the TB fixed it?
For future reference. And oddly, the high idle would sometimes start when the car was just sitting, without having touched the throttle.

Again, thanks for the timely and great thoughts.

Ps: My next challenge with be replacing the blend door actuator.

Last edited by CraigT; 05-16-2021 at 03:59 PM. Reason: afterthought...
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Old 05-16-2021, 04:29 PM   #11
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Default Re: Question about how the computer decides on Throttle Position

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Originally Posted by CraigT View Post
Care to mention your source for realizing which wires to probe at the PCM? Also, did I see somewhere that the PCM can be dismounted while still wired or were you able to see the connector(s) of the pcm just by looking up from the drivers side? I've never looked at it and should probably keep it in mind for future reference... lol
I use AllData.
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Old 05-18-2021, 08:17 AM   #12
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Default Re: Question about how the computer decides on Throttle Position

As mentioned in my thread Iím going through this also.

Throttle body is clean.

I shot the connector with contact cleaner.

Both the napa and BDW TPS worked great for a few months and then progressively got worse. This last BWD would slowly go from 0 at closed throttle to 10% over my 32 mile drive to or from work.

Since it was a warranty I put a new BDW on last night. Sadly I had to give them the old one so I couldnít dissect the bad one.

If/when this happens Iíll look at the connector pins. I already tried squeezing the top cover tight incase it was loose.
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Old 05-18-2021, 12:33 PM   #13
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Default Re: Question about how the computer decides on Throttle Position

When failing, do they still reset to zero at every key-on cycle? Did you check the failed ones with an ohmmeter? Did that checking involve heating the TPS with a hair drier?
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Old 05-18-2021, 01:02 PM   #14
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Default Re: Question about how the computer decides on Throttle Position

To me the best testing is watching it on a scanner. At normal idle there will be some percent value that is set each start up. Maybe 15%. If you see 20% when the idle is high that is an indication that the signal from the TPS isn't right or something is hanging up. Typically its either the TPS or the connector (either at the TPS or at the PCM).
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Old 05-18-2021, 05:30 PM   #15
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Default Re: Question about how the computer decides on Throttle Position

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When failing, do they still reset to zero at every key-on cycle? Did you check the failed ones with an ohmmeter? Did that checking involve heating the TPS with a hair drier?
If you are referring to me, cycling the key it zeros. However, it continues to creep up. Over my drive home it drifted up to 10% if I didnít restart it at a redlight. I returned it for a new one and it is now good again. It was a lifetime warranty. Iíve been stuck doing overtime for people taking vacation so I really didnít get to mess with it or go junking and get some OEM sensors. I work a 12hr shift and leave the house at 4:40am and get home at 7pm.
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Old 05-18-2021, 06:54 PM   #16
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Default Re: Question about how the computer decides on Throttle Position

Yes, I was referring to you. You should really start a new thread, or bump up an existing one. Regardless of time restraints, you are going to have to do some troubleshooting, or keep throwing parts at it. I can't offer any other choices (other than take it to a shop).
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Old 05-18-2021, 07:48 PM   #17
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Default Re: Question about how the computer decides on Throttle Position

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Yes, I was referring to you. You should really start a new thread, or bump up an existing one. Regardless of time restraints, you are going to have to do some troubleshooting, or keep throwing parts at it. I can't offer any other choices (other than take it to a shop).
I just thought Iíd mention Iím going through the same thing. I keep TPS dialed up on my scangauge.
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