Very high idle, intermittent problem

Tiny
FIXTHISONE
  • MEMBER
  • 2002 BUICK CENTURY
  • 3.1L
  • V6
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
This is a problem that doesn't occur all the time. It is intermittent.
No DTC's.
It happens when the car has warmed up, but not 100% of the time.
I have replaced the following:
Fuel regulator. Because it was leaking gas. Okay
PVC
PVC hose and U tube ( U tube was collapsing it is okay now)
IAC valve ( thought may be old on the tip some pitting) cleaned seat of tip
TPS ( it rode up and down smoothly in % ( Autel reader) and Volts (DMM) I thought that there was a short inside the TPS sensor, therefore the lack of a DTC, but it would send a high signal to PCM and if this happened at the closed position, the PCM would interpret the voltage reading as throttle open some %.
Cleaned throttle body
Throttle linkage appears free.
Cleaned MAF
Have disconnected:
IAT
MAF
IAC
TPS
Vacuum hose from brake booster and plugged.
PCV valve from valve cover and plugged PCV valve.
There appears to have no significant effect on RPM.
Last episode RPM's went from 1300-1400 then 1800, then 2000, then 2200, then 2800 then 1400, 1700, then 3400, then 2000. The changes happened without any reasons that I can tell what is causing it. The RPM's are all over the place,
Prior to taking everything apart I pushed the RPM and around 4000 the car would cut off and catch up again.
I have to put in neutral when coming to a stop and then put in drive and when shifting back neutral these variations occur.
I check the TPS connector with the connector unplugged with a DMM and got to read about 1.6V on the signal leg (dark blue B con. #66)
I thought I should be reading close to 0 volts.
5 volts is present on the reference leg (grey wire B con. #33)
No volts at ground leg.
Voltage presence on signal leg.
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Friday, September 25th, 2020 AT 11:55 AM

9 Replies

Tiny
ASEMASTER6371
  • EXPERT
Good afternoon,

The signal wire at the TPS should read .5 to .75 volts at closed throttle.

If it is out of adjustment, it will not cause the high idle.

https://www.2carpros.com/articles/engine-idles-too-high

I am concerned the throttle body plate is not seating in the bore allowing too much air in to the engine.

https://www.2carpros.com/articles/throttle-actuator-service

I would start by cleaning the throttle body itself and retest.

https://www.2carpros.com/articles/how-a-throttle-body-works

I attached the connector diagram for the TPS. Check or back probe the low reference wire with the key on and closed throttle.

Roy
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Friday, September 25th, 2020 AT 12:06 PM
Tiny
FIXTHISONE
  • MEMBER
Thank you for taking the time to address my question.
As you stated the TPS voltage output(signal) will not cause the idle to go that high. I don't know enough to either accept or decline that statement.
You further stated that the throttle body plate is not seating (correctly) in the bore.
You suggested that I clean the throttle body ( I have done it and also cleaned the IAC sensor seat) I will do it again, just in case the first time didn't clear everything.
Would you help me understand the following:
Is there a command from the PCM that actually opens the throttle or is this event done strictly by the accelerator pedal cable?
When I start the engine (cold). It goes through the warm up process. Around 1300 RPM and then it comes down to around 750 RPM. This period the engine is "normal", once it warms up the RPM starts to go all over the range (1,300 to 3,400 RPM's). The only change I can perceive is the increase in temperature ( I don't know if there is a sensor that will trigger the PCM to increase RPM, maybe through the IAC valve. I don't have a guide to what should the IAC voltage should be. It would be great if you can provide such information, so a can back probe the IAC and see what the voltage is doing.
I suspected two things the PCM is at fault, or this is a mechanical problem. The mechanical problem would be if the throttle body plate is somewhat damaged (out or round or warped ) or the actual throttle body has been damaged and is not matching the shape of the plate. As temperature increases the dimensions are changed enough to cause binding between the throttle plate and body causing the plate not to seal completely and avoid a "leak" causing the PCM to read a lean condition and injecting additional fuel causing the RPM to go higher. I don't know for sure what commands the throttle plate to open other than the accelerator linkage or in the absence of plate opening, if the IAC will provide the additional air. There is no question that air is coming in somehow, the engine is a air pump, without the additional air it can't increase the RPM.
Any clarifications will be greatly appreciated.
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Saturday, September 26th, 2020 AT 7:06 AM
Tiny
ASEMASTER6371
  • EXPERT
Okay, the ECM does control the operation of the idle air control. From your statement it is working correctly

When hot it sounds like the throttle plate is opening whether from the throttle body itself or the ECM is commanding it open.

There is no cable. It is all electronic controlled now.

I suspect the ECM is the issue

Have you checked for any codes?

Roy
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Saturday, September 26th, 2020 AT 7:29 AM
Tiny
FIXTHISONE
  • MEMBER
In this case there is an actual steel cable that rotates on a cam and opens the throttle plate and that is why I asked if the ECM has a way to command the opening of the plate. This is not a step motor controlled throttle. On one side there is the TPS unit and on the other the plastic cam with the steel cable, attached to it, that is operated by the accelerator panel.
I can't tell how the IAC sensor is positioned, therefore I can't figure out if the air is coming through the IAC or the throttle plate. Another possibility is the EGR valve being open and letting air in once the engine warms up and the amount varying because the EGR valve may be getting stuck on different positions.
Thanks for the help.
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Saturday, September 26th, 2020 AT 8:53 AM
Tiny
ASEMASTER6371
  • EXPERT
You would need a scan tool to see the movement of the IAC.

No to the EGR. It would code.

I still think we are heading to an ECM.

Roy
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Saturday, September 26th, 2020 AT 9:34 AM
Tiny
FIXTHISONE
  • MEMBER
Thanks again.
Is there a way to check the ECM without a scanner?
I don't have a scanner at this moment.
I'm looking at Autoenginuity. The scan tool is about $250.00 with domestic package about $800.00 and with Domestic, Asian and European packages it is about $1,500.00. I was looking at the parameters that this tool, with the Enhanced package for GM, it shows over 100 parameters. Do you have any comments on the Autoenginuity scanner?
Thank you.
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Saturday, September 26th, 2020 AT 9:50 AM
Tiny
ASEMASTER6371
  • EXPERT
That is a good scanner and will give you the information you need.

The ECM needs to be checked by someone with the special equipment needed.

Roy
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Saturday, September 26th, 2020 AT 9:52 AM
Tiny
FIXTHISONE
  • MEMBER
Thank you, I will look for someone to check the ECM.
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Saturday, September 26th, 2020 AT 10:55 AM
Tiny
ASEMASTER6371
  • EXPERT
You are welcome.

Always glad to help.

Roy
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Saturday, September 26th, 2020 AT 11:00 AM

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