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Discussion Starter · #1 ·
My 97 Deville has a high idle and will shift hard into the gears - drive itself down the road when you let off the brake - harder than normal to slow the car down. I’ll explain what led up to this issue but first a couple key issues. B2750 and B2780 IPC needs to be replaced a second time since I accidentally put the wrong key in or reprogrammed with a tech2. My starter enable relay is rewired and manually grounded - my PCM is programmed Stage 3 from NSP with anti theft removal.

November 2019 it was raining, after I left work at 11pm I stopped for fast food, shut my car off and ate in the parking lot. Went to start it up and it wouldn’t. I had “car nah not restart”randomly leading up to this time. I brought it to a shop that was inexperienced and fiddled with the car for 5 months.Since then I have verified and replaced or swapped with a known good part of the following. Entire interior, wiring from my ignition switch to the IPC. Your usual spark plugs, wires, coil pack,IAC, TPS,IAT,ECT, I have since had to replace starter, alternator, battery.

Im a rookie certified mechanic even though that doesn’t mean much to some people. I’m stumped. I thought I crossed everything that makes sense on alldata, factory service manuals and every single forum on this site and every other one, even for other makes and models. Whats interesting is when I unplug the TPS the idle drops from its usual 1200-2000rpm to the normal 650 I checked for vacuum leaks with a smoke tester didn’t find anything. Blocked the intake while the engine runs and it dies out like it should. Key on Ignition off I have three O2 Sensors showing Lean and 1 was Rich(I changed that sensor) it was bank 2 sensor 1. Fuel trims are in the negative on the scanner -15 or -25 I forget which it said. I’ll have to double check or find my screen shots. Injectors aren’t leaking, fuel pressure regulator is new. Fuel pump is new but fuel pressure showed around 45psi off the rail, but I also didn’t have the pressure gauge T’d off to bleed air out for an accurate reading. The IAC counts are somewhere around 135 which it’s retracted allowing more air in. I swapped IACs didn’t make a difference. If I unplug the TPS idle drops. TPS has been replaced. Engine runs rough after unplugging TPS as that’s probably normal? I must be overlooking something, this shouldn’t be this difficult. Any ideas?
 

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2011 CTS4 Coupe, 2014 ELR, 2018 XT5 AWD
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What is the reading from the TPS sensor when you look at it with your scan tool? It seems like there is a problem with that signal that is causing the IAC to retract. Look at the sensor data and make sure it is correct. Is it adjusted properly? Is it wired correctly?

Just thinking of what might be wrong.
 

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Discussion Starter · #7 · (Edited by Moderator)
What is the reading from the TPS sensor when you look at it with your scan tool? It seems like there is a problem with that signal that is causing the IAC to retract. Look at the sensor data and make sure it is correct. Is it adjusted properly? Is it wired correctly?

Just thinking of what might be wrong.
On this vehicle the TPS is non adjustable and it’s plugged in and seems to be reading correctly on the scanner. If I haven’t mentioned already, an odd thing my buddy and I seen(he has the scanner, not me) is we can’t change the IAC counts because it says “engine not at idle” so it seems like a TPS issue, but TPS, seems fine and I’ve replaced it before too.

Checked TPS?
C SIMME01 beat me.
Been swapped with a known good and a new one

Is it safe to assume that it is a quality part and not an AutoZone Don'tLast part?
I swapped it with an OEM known good part and also a new one from a parts store, doesn’t make a difference. This entire issue has stumped me, not sure what to think. Definitely don’t want to send it somewhere to be figured out but I’m close to giving up.
 

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Is there a mechanical low idle stop on the throttle body?
The engine not at idle message is definitely a clue..
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Yep - either set screw or the actual plunger can be screwed in/out of the ISC solenoid body.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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If the 1997 has an IAC then it's right in the crossover time frame to the "new" throttlebody. The IAC "top hat" plunger screws into the solenoid body - check that for tightness, clean the IAC bore and small air passages, and make sure the throttle blade is clean and shiny; also the TB bore.

Also check the PCV dirty air hose for vacuum leaks.
 

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How much vacuum is it making?
Should be making enough, mechanical the engine is good

If the 1997 has an IAC then it's right in the crossover time frame to the "new" throttlebody. The IAC "top hat" plunger screws into the solenoid body - check that for tightness, clean the IAC bore and small air passages, and make sure the throttle blade is clean and shiny; also the TB bore.

Also check the PCV dirty air hose for vacuum leaks.
I did clean everything, I think it’s an electrical problem or a fuel trim problem because if I unplug the TPS the idle drops. It runs rough and sluggish and no acceleration if I drive it like that though.
 

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Re reading this you have swapped a lot of parts.
That doesn't mean the correct signals are making it to the ECM. You say the scan tool data for the TPS looks good. Have you looked at the service manual and verified it is correct,
I would also check the service manual for the procedure to install a new IAC. If I remember correctly is is more than just plugging in a new part. I think there is a zeroing or indexing procedure that is required. I also think the idle speed reduction when the TPS is disconnected is what it is supposed to do. Since the computer doesn't know the throttle position it faults to doing everything it can to make sure the engine is not going going to be able to overpower the brakes. That would just drive the IAC as far as it can go to idle. If there is a procedure for the IAC that hasn't been done that could cause the high idle.
 

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Discussion Starter · #20 ·
Re reading this you have swapped a lot of parts.
That doesn't mean the correct signals are making it to the ECM. You say the scan tool data for the TPS looks good. Have you looked at the service manual and verified it is correct,
I would also check the service manual for the procedure to install a new IAC. If I remember correctly is is more than just plugging in a new part. I think there is a zeroing or indexing procedure that is required. I also think the idle speed reduction when the TPS is disconnected is what it is supposed to do. Since the computer doesn't know the throttle position it faults to doing everything it can to make sure the engine is not going going to be able to overpower the brakes. That would just drive the IAC as far as it can go to idle. If there is a procedure for the IAC that hasn't been done that could cause the high idle.
You’re right. I checked Alldata. For IAC I believe it’s key on engine off 30 seconds, and key lock 30 seconds. With the TPS it’s the same but no touching the gas pedal. I thought I did that, I’ll have to figure out how to do it again. Seems like none of those procedures work on my car. I even tried the 30 min process for the anti theft key relearn, and nothing. Maybe it’s the same for my IAC and TPS. Or not. Going to try the TPS relearn again not sure if it’ll work or not, and even before that disconnect my battery.
 
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