Rough idle when in gear not driving and loss of MPG?

Tiny
DMBUIW
  • MEMBER
  • 2011 FORD F-150
  • 5.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 176,000 MILES
The truck idles rough when stopped and in gear. The truck has had the following recently replaced. All spark plugs, Mass Flow Value, EVAP Purge valve, and electric throttle body. All OEM parts. Have lost 3 to 4 MPG since all were replaced. Do I need to get the PCM flashed? Or ideas on what to do?
Tuesday, May 14th, 2024 AT 11:25 AM

3 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,477 POSTS
Hi:

A rough idle is usually caused by either a vacuum leak or a fuel pressure issue. Since you have dropped MPG, the first thing I suggest is checking for a vacuum leak as well as checking the fuel pump pressure. A faulty fuel pressure regulator can increase fuel pressure beyond the manufacture's specifications and force more fuel into the engine than wanted or needed.

On the other hand, an unmetered air leak will produce a lean fuel mixture causing the PCM to try to compensate by adding fuel to the engine.

Here are two links that explain how to check these two things.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Also, I attached the manufacturer's pressure specifications below. Try this and let me know what you find.

Joe

See pic below
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Tuesday, May 14th, 2024 AT 7:00 PM
Tiny
DMBUIW
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Joe

Thank you for the info and advice. I looked things over and did not find any vacuum leak and the fuel system pressure looked good it was in the correct range. It still seems sluggish when stepping on the gas (like it's taking a while to switch gears) and still has a shimmer while stopped and in gear.
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Monday, May 20th, 2024 AT 8:15 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,477 POSTS
Hi:

Thanks for the update. If it is in neutral, does it have a rough idle? As far as in gear, there is a load placed on the engine which causes it to torque one way or the other. I wonder if you may have a worn or faulty engine mount. If so, that can allow engine vibrations to be felt throughout the vehicle.

Take a look through this link. It explains what and how a mount works.

https://www.2carpros.com/articles/how-a-motor-mount-works

If possible, with the hood open, have a helper place the engine first in reverse and then drive. The engine will move a bit, but not too much. If you see more than an inch movement, this could be the problem at which point, we will need to inspect the mounts for damage.

Let me know.

Joe
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Monday, May 20th, 2024 AT 6:35 PM

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