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Rough Idle and RPM fluctuation

Tiny
SCHEETZ21
  • MEMBER
  • 2008 MERCURY MOUNTAINEER
  • 4.0L
  • 4 CYL
  • AWD
  • AUTOMATIC
  • 130,000 MILES
At intermittent times it will idle rough, shake, and the rpm's will fluctuate. It throws a misfire code for cylinder two, but a new coil and injector were installed and it still does this. I just had a new engine (re-manufactured) placed into it and it still has this same issue going on. The engine runs great under power, but Idle is rough, but only intermittently. Any help would be appreciated before I throw more money at this vehicle.
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Thursday, June 8th, 2017 AT 8:30 AM

1 Reply

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,643 POSTS
Hello,

Two things come to mind is the MAF. When this sensor starts to go bad it will not trigger a check engine light which will make the engine idle rough.

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

With that may miles I would not try to clean it, I would replace it.

Next, is a vacuum leak which will cause the same problem.

Here is a guide so you can check for that as well.

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

Please run some tests and get back to us so we can continue helping you.

Cheers, Ken

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Saturday, June 10th, 2017 AT 11:09 AM

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