Low RPM hesitation/jerk

Tiny
JONEST28
  • MEMBER
  • 2002 FORD MUSTANG
  • 4.6L
  • V8
  • 2WD
  • MANUAL
  • 127,000 MILES
My car is the Gt model. It has a low rpm hesitation on acceleration. Specifically in the 800 to 2,000 rpm range. Sometimes when I give it throttle everything works great and other times it sort of hesitates and almost jerks as it tries to accelerate. What I've also noticed it once I surpass the 2,000 mark is tends to smooth out and runs great. I can hold it at an idle and various rpm's I've gone up to about 3,500 and it doesn't do anything abnormal. It appears to happen only under load. It is especially bad in fifth gear if I give it gas. No check engine light and the only code thrown was a generic communication code.
Tuesday, February 12th, 2019 AT 9:18 PM

20 Replies

Tiny
JONEST28
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I will take a better video tomorrow.
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Tuesday, February 12th, 2019 AT 9:19 PM
Tiny
ASEMASTER6371
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Good morning,

Yes, the video tells us nothing.

As far as the generic code, what was it? That could be a clue.

My first thought is the idle air control which controls RPM at low speeds. This cannot be serviced and needs to be replaced if the idle speed is not correct.

Roy
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Wednesday, February 13th, 2019 AT 4:19 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello,

This could be a problem with the fuel filter. I have included a link down below that might be helpful.

https://www.2carpros.com/articles/how-to-change-a-fuel-filter

Also, in the diagrams below I have included a diagram of all the fuel components for your vehicle. Let us know what you find out.

Thanks,
Alex
2CarPros
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Wednesday, February 13th, 2019 AT 4:27 AM
Tiny
JONEST28
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In fifth gear doing about 30 mph just nudging the gas it started to feel like the car is trying to shake itself apart. As soon as I push in the clutch it stops. And if I downshift it still does it but not anywhere near as bad.
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Wednesday, February 13th, 2019 AT 9:15 AM
Tiny
ASEMASTER6371
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Again what was the code.

Roy
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Wednesday, February 13th, 2019 AT 10:55 AM
Tiny
JONEST28
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U1262.
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Wednesday, February 13th, 2019 AT 11:33 AM
Tiny
ASEMASTER6371
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Let me check.

Okay, this is a communication network issue between the PCM and the data link connector.

The serial data line is how the modules all communicate with each other. When there is an issue, information does not get there quick enough and the result is what you have.

It is not a generic code at all. It is an important code.

Note

This Pinpoint Test is intended to diagnose the following:

- Standard corporate protocol (SCP) communication bus harness circuits: BUS(+), BUS(-), between DLC and PCM
- Data link connector harness circuits: chassis ground, power ground (PWR GND), battery positive voltage (B+)
- Open VPWR to Powertrain control module (PCM) (12A650)
- Powertrain control module (PCM) (12A650)
Note:Concerns found that affect the entire network are referred to in the Module Communication Network, Information Bus.

Pinpoint Test Schematics and Connectors

You are going to need someone with an advanced scan tool check this and have them test the serial data lines to see if there is corrosion somewhere causing this issue.

Roy
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Wednesday, February 13th, 2019 AT 12:50 PM
Tiny
JONEST28
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Are there schematics to where I could meter the wires to test it? Or is a tool the only way?
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Wednesday, February 13th, 2019 AT 4:33 PM
Tiny
ASEMASTER6371
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You need an advanced scan tool that can read the serial data.

This is not a test with a voltmeter. This requires an advanced scan tool.

Roy
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Wednesday, February 13th, 2019 AT 4:48 PM
Tiny
JONEST28
  • MEMBER
  • 12 POSTS
So you think either my ECU or the wiring to the ECU is the issue?
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Wednesday, February 13th, 2019 AT 6:34 PM
Tiny
ASEMASTER6371
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Correct but it needs to be confirmed by someone first.

Roy
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Thursday, February 14th, 2019 AT 3:31 AM
Tiny
JONEST28
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I just found a vacuum line that is collapsed running from the passenger valve cover to the throttle body. What is that for and could it be causing my hesitation/bucking? Or is this just a coincidental find?
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Friday, February 15th, 2019 AT 2:45 PM
Tiny
ASEMASTER6371
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I would repair it and see if it runs better but it has no connection to the code you currently have.

Roy
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Friday, February 15th, 2019 AT 2:49 PM
Tiny
JONEST28
  • MEMBER
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Updates: throttle position sensor replaces, repaired the vacuum line from valve cover to throttle body. Plugs have been changed. Unplugged each plug/injector at an idle and could hear the noticeable difference. Fuel filter has also been replaced. No change in problem has been noticed. On acceleration I don't seem to notice it in first gear but from second up it's a mixture of hesitation and it feels like the car is moving forward like how an impact works. Hits let's off hits let's off as I accelerate typically until about 2,000 rpm and it seems to smooth out. I can rev it all the way to 5,000 with no noticeable defect. It only seems to act this way when the engine is in gear and under load. I also put it up on jack's and put it through the gears. Nothing seemed out of the ordinary. I consulted a mechanic who told me the code I had is a signal going to the cluster. My odometer sometimes randomly shuts off and comes back on so the assumption is that's what threw the code. Any other ideas?
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Monday, February 18th, 2019 AT 1:47 PM
Tiny
JONEST28
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Oh, as well if it's doing it as soon as I let off the gas it stops.
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Monday, February 18th, 2019 AT 1:49 PM
Tiny
ASEMASTER6371
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I would check the catalytic converter and see if there is any back pressure.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Roy
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Monday, February 18th, 2019 AT 2:12 PM
Tiny
JONEST28
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If it was the cat wouldn't it have issues all the time not just under load?
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Monday, February 18th, 2019 AT 2:48 PM
Tiny
ASEMASTER6371
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It would be most noticeable under load.

Roy
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Monday, February 18th, 2019 AT 2:49 PM
Tiny
JONEST28
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A bad coilpack was the problem.
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Monday, March 18th, 2019 AT 4:53 AM
Tiny
ASEMASTER6371
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That would do it 100% of the time.

Roy
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Monday, March 18th, 2019 AT 4:57 AM

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