Car speeding up on its own

Tiny
SREPW
  • MEMBER
  • 2003 BUICK CENTURY
  • 3.1L
  • 2WD
  • AUTOMATIC
  • 175,000 MILES
A while back my car started speeding up by itself with foot off pedal. A week later it happened again. Started happening more often and car started stalling after just a few blocks.
I finally got around to buying a MBD-II scanner and read a P0121 code. Throttle/Pedal position sensor/switch a circuit range/performance. I also had a freeze frame P0001 code fuel volume regulator control circuit/open. I was getting a rpm's of 1700-1800 at start and idle. Throttle showed open forty percent (that is right) at start and never really went below thirty seven percent. Hitting the gas got somewhat erratic movement of throttle.
I replaced the TPS. (There is some sick locktite on those screws)
Seemed to be fine for about a week. After a week same thing happened again. What I did realize is that if I started car with air conditioner off the car would last much longer before stalling and would usually have a normal 700-800 rpm's at idle. Same problem with throttle open and stalling. I checked the throttle and was not open at idle, but still read at forty percent.
Thought I possibly got a bad TPS. Tried reading the voltage with a multi-meter. I do not know what was I was doing wrong but I could not get any reading at all, not even on the battery. (On my friends 2010 Hyundai Sonata I got regular readings.) I obviously was doing something wrong, but I did not know what it was.
Time to call in the experts. I have made somewhat friends with my local mechanic. (Always a smart move! The guy really knows his stuff and is a real nice guy.) I brought it in for diagnostics. Comes back and tells me that the TPS is bad. Sounds like he read it and graphed the voltage. Says I bought a defective part. (This time I think I finally took too much advantage of him. Unlike in the past he charged me for diagnostics.)Time to bring the TPS back to advanced and exchange it.
Replaced TPS and erased codes. Same problem.
What's up over here? I am thinking must be a wiring problem with the TPS as otherwise he would not have gotten messed up voltage readings for the TPS. That is bad because I clearly do not know what I am doing with electrical wiring (I could not even get a reading with a multi-meter) and I guess this is going to cost me a pretty penny.
Am I right about this? Is this a wiring issue? Can I really do what is necessary myself?
Thanks in advance for all your help!
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Wednesday, July 13th, 2016 AT 5:42 PM

13 Replies

Tiny
KEN
  • ADMIN
Hello,

Sorry for the slow response, there is one of two things that I think is happening, either the PCM is bad or the throttle actuator is bad, here is a wiring diagram so you can do a pin to pin check.

Please let us know how it goes so it can help others

Best, Ken
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Sunday, July 17th, 2016 AT 7:47 PM
Tiny
SREPW
  • MEMBER
Thank you for the response. I gave up and sent it in to my machanic. Waiting to hear back from him. Hopefully tomorrow. Will try to remember to post what he says tomorrow. Thanks again.
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Sunday, July 17th, 2016 AT 10:19 PM
Tiny
KEN
  • ADMIN
That would be great so it will help others

Best, Ken
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Monday, July 18th, 2016 AT 8:55 AM
Tiny
SREPW
  • MEMBER
Ok. As promised I'll update and pose another question. Sent it to mechanic and he calls back to say that they replaced the throttle body, but my starter went! I'm not sure what they did to it, but since it was under warranty, I returned and replaced myself. Lo and behold same issue.
Now mechanic says it's a PCM issue exactly as 2carspro ken wrote. I asked him how he knew it wasn't a wiring issue and he said he put it another PCM and no longer got the 40 percent open throttle reading at start.
Two questions here. 1- Is he right? I'm beginning to doubt him by now. Can it still be a wiring issue or some corrosion on the original PCM?
2-Is there a way to flash/reprogram the old one. I am currently unable to afford such a costly charge. He says spoke to one guy who said he can't do it, but will look into it again tomorrow. Says the guy claimed he can't do a Buick Century.
Any and all advice would be appreciated and thank you again 2CAR PROS KEN!
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Tuesday, August 2nd, 2016 AT 8:14 PM
Tiny
KEN
  • ADMIN
Hi and thanks for getting back to us,

Is there any codes in the computer? If not the computer is bad. If so then a short or high resistance can be the issue. The computer has a built in fail safe to not allow the engine to rev on its own, that why I think it the ECM that has failed.

Please let me know what you find

Best, Ken
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Wednesday, August 3rd, 2016 AT 8:16 AM
Tiny
SREPW
  • MEMBER
I'm not sure I understand the question. The ECM that is in the car now was giving me a Throttle Position Sensor code (P0121). My understanding is that the other PCM he tested it with wasn't giving him a 40 percent open throttle at start. The second one couldn't be used to drive the as it wasn't programmed for this car specifically. He therefore got the info re the throttle opening at start, but doesn't know if the codes would come back. He's theorizing that this is the issue as we always got about 40 percent throttle open at start until now.
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Wednesday, August 3rd, 2016 AT 8:56 AM
Tiny
KEN
  • ADMIN
This has to be a wiring problem, can you do a pin to pin check to see if there is any high resistance?

Here is a wiring diagram to get you started.

Please let me know what you find.
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Wednesday, August 3rd, 2016 AT 10:16 AM
Tiny
SREPW
  • MEMBER
Still trying to figure out how to do a wiring check! Never did this before!
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Thursday, August 4th, 2016 AT 2:32 PM
Tiny
KEN
  • ADMIN
I will make an article tomorrow and send it your way okay?
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Thursday, August 4th, 2016 AT 8:31 PM
Tiny
SREPW
  • MEMBER
Wow! Thank you!
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Thursday, August 4th, 2016 AT 9:07 PM
Tiny
KEN
  • ADMIN
Sorry got tied up today will finish it tomorrow though.
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Friday, August 5th, 2016 AT 3:51 PM
Tiny
SREPW
  • MEMBER
Thank you!
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Friday, August 5th, 2016 AT 4:03 PM
Tiny
KEN
  • ADMIN
Sorry it took an extra day,,, here you go

https://www.2carpros.com/articles/how-to-check-wiring

Let me know if you have any questions.

Best, Ken
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Sunday, August 7th, 2016 AT 11:51 AM

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