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Sudden RPM surges - TPS issue

Tiny
AKANYR
  • MEMBER
  • 1995 FORD MUSTANG
  • 5.0L
  • V8
  • AUTOMATIC
  • 218,800 MILES
Once warmed up the RPM's will surge between 2,000 to 3,000 with my foot steady on the gas pedal. Sometimes the car jerks hard and will stay at 3,000 RPM's for 5 seconds. Sometimes it jerks and go's to 2,000 RPM'S. Sometimes after the RPM's come down to normal it goes back up after I put my foot on the gas pedal again. I always take my foot off the pedal as soon as the surges start, keeping it off until RPM'S come down.
Twice I put a new TPS (non adjustable type) in and each time it reads 1.34 volts and I only get it to go as low as 1.24 volts. It should be 1 volt.
There are no trouble codes, no check engine light and the repair shop couldn't find the problem.
I cleaned and checked the IAC, MAF, Throttle body/butterfly, lubed the cables and spring. Changed the transmission filter and fluid, new fuel pump, filter and strainer. New temperature sensor and radiator. Cleaned the air filter, there's a high performance air intake in the car.
I also changed out the dry vacuum hoses and smoke tested the engine for leaks.
I've increased the mpg from 14 to 17 with all this work but I can't stop the RPM's from surging. I've been fighting this issue for over a year and I'm at my wits end.
Any help you can give will be deeply appreciated. Please help Penelopy my pretty purple pony.
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Sunday, September 1st, 2019 AT 5:37 AM

52 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning.

Can you monitor the system and tell me the long term rim reading on the O2 sensor? You could have a vacuum leak not allowing the engine to return to idle.

If you disconnect the IAC, does the idle drop to normal?

Has anyone checked the EGR valve for being open? If it is, it will create a massive vacuum leak and cause this issue.

The TPS voltage should be.5 to.75 volts at closed throttle. Are you sure the throttle plate is not partially open?

Roy
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Sunday, September 1st, 2019 AT 5:46 AM
Tiny
AKANYR
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Thank you for your quick reply and willingness to help.
I don't know how to monitor system to give you the long term rim of the O2 sensor.
When I disconnect the IAC the idle does drop down to normal. The EGR valve opens and closes fine. I also checked the throttle plate and that is fully closed.
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Sunday, September 1st, 2019 AT 3:57 PM
Tiny
ASEMASTER6371
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Okay, The IAC is fully retracted and making massive vacuum leak. Check the throttle body for carbon build-up.

If all is good, replace the ECM. That controls the IAC.

Roy
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Sunday, September 1st, 2019 AT 4:05 PM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
I cleaned the throttle body and got all the black Coke out. I will check it again to make sure it's not all blacked out again. When I clean the IAC it was moving up and down fine I will check that again. How much would a new ECM cost?
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Sunday, September 1st, 2019 AT 4:16 PM
Tiny
ASEMASTER6371
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Rock auto has them for $136.00. The EM is a plug and play, no flashing required.

Roy
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Sunday, September 1st, 2019 AT 4:22 PM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
There was a little trail of blackish brown dry fluid in the opening from throttle body to the IAC. Must've been left from when I cleaned the throttle body, I cleaned IAC again. I drilled out the screw holes for the TPS and set it to.86 volts. I set it like that cause a few you and a few other others say set it to.75 volts and other's say.98 volts. So.86 is the mid way between the two. The car is quieter and runs a bit smoother but RPM'S still surged. Not as much or as high as before. Before I left work to drive home I checked the volts again. It jumped between.86 and.89 and settled at.89, not sure if it cause my multi-meter old and the probe ends have been soldered back on. I'll check it with my brother's Klien multi-meter. I set my base idle at 750 - 800 RPM'S but when I put it in drive/reverse it drops to 600 RPM'S, is that normal? Should I set base idle to 1000 RPM?
Thank you for all your help, I appreciate it so much. You're the greatest.
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Tuesday, September 3rd, 2019 AT 5:06 AM
Tiny
ASEMASTER6371
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The base idle without the IAC hooked up should be 500 RPM. You have it set too high.

Adjust the base idle and let me know.

Roy
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Tuesday, September 3rd, 2019 AT 5:10 AM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
Okay, I'll adjust it on Friday and let you know.
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Tuesday, September 3rd, 2019 AT 1:46 PM
Tiny
ASEMASTER6371
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Sounds like a plan.

Roy
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Tuesday, September 3rd, 2019 AT 2:07 PM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
It has been raining Friday and Saturday so I didn't set base idle but I did pull codes. 564 (fan control circuit failure) then a single flash and 332. Not sure if 332 is memory code. My engine fan motor is 2 months old and goes on to cool engine so I don't know why 564 code shows.
Last night my check engine light was on 3 minutes then off 3 minutes, over and over as I drove. Today it goes on 10 seconds then off 2-3 minutes, over and over. Second day little to no rpm surges though.
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Sunday, September 8th, 2019 AT 1:44 AM
Tiny
ASEMASTER6371
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Let us deal with one issue at a time.

Adjust the base idle as I described and see if the one issue is resolved.

Roy
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Sunday, September 8th, 2019 AT 4:41 AM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
Okay. Do I adjust base idle with the stop screw or the idle bypass screw?
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Sunday, September 8th, 2019 AT 6:40 PM
Tiny
ASEMASTER6371
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Stop screw on the throttle body.

Roy
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Sunday, September 8th, 2019 AT 9:06 PM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
Sorry Its been a while since my last reply. I've been experimenting with several TPS settings. I'm down to a setting of.72 and I still keep getting the surges. I've made two discoveries. #1)When the engine is cold and I pulled the IAC off the car stalls immediately. It stays running when the engine is warm and I pull off the IAC although kind of weak. #2)When the engine is cold I get a steady TPS reading of.72 but when the engine is warm I get an erratic bouncy reading between.72 and.73 on the voltmeter. Staying mostly towards.72, I'm using a good Klein tool multimeter. I'm not sure if my discoveries has any relevance to my problems but I thought I'd let you know. Also I did as you said and put the base idle at 500 RPMs with the IAC off
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Tuesday, October 1st, 2019 AT 6:50 PM
Tiny
AKANYR
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Here's a video of the reading bouncing erratically.
Video (Click to make bigger)
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Tuesday, October 1st, 2019 AT 7:07 PM
Tiny
ASEMASTER6371
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The TPS voltage is good. That is not causing the problem.

Can you upload a video of how it idles?

Roy
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Tuesday, October 1st, 2019 AT 11:00 PM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
Sorry, it has been awhile since I wrote. I was experimenting. Before that I tried making videos a few times on how the car sounds when idles when warm and at start up, none of them sounded like the car in real life. I can tell you my brother and his friend that has a body shop said it sounds healthy and beefy. Both at idle and running down the road. I did make a few discovery and that is why I'm writing to seek your advice. I heard that not a 5 volt reading from the 5 volt vref wire on the TPS can cause surging. I tested mine and it reads 4.92 volts - 4.93 volts. I'm not sure what they said causes the low volt reading. A broken wire or a bad signal from where the wire comes from. My question is, would the low vref reading cause surging? If so how do I find the cause?
My sending a picture or 2 of Penelope the pretty purple pony your helping me with. She is actually more purple than it looks in the picture.
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Monday, November 4th, 2019 AT 1:06 AM
Tiny
ASEMASTER6371
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That voltage to the TPS is fine. You are good.

You need to look in another direction. The TPS is not the issue at all.

Roy
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Monday, November 4th, 2019 AT 1:43 AM
Tiny
AKANYR
  • MEMBER
  • 52 POSTS
Where do you suggest I look?
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Thursday, November 7th, 2019 AT 1:02 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
This could be a vacuum leak in the engine, mass air flow sensor, PCV valve fuel injectors being dirty or fuel pressure.

Roy
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Thursday, November 7th, 2019 AT 1:17 AM

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