High rev, intermittent P0171 code

Tiny
CGRIFF85
  • MEMBER
  • 2003 MITSUBISHI LANCER
  • 2.0L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 208,000 MILES
The car revs really high for a couple of seconds if I am cruising through town at 25 miles an hour it will jump up to about 3,500 RPM for a few seconds before going back down to about 1,500. It also stays at 3,500 RPM at 65 miles an hour on the freeway I am wondering if this could be the transmission slipping, idle air control valve, or the mass air flow sensor. I have replaced upstream O2 sensor, spark plugs, tube seals, and the valve cover gasket as well. I have checked all the hoses and hard lines but do not see anything wrong with those. Every now and then the car will throw a P0171 code which then goes away on it's own after a couple hours and will return briefly a month later.
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Wednesday, March 6th, 2019 AT 7:58 AM

5 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Welcome to 2CarPros.

What you describe sounds like the transmission is slipping. Is the fluid clean and full? Other that the P0171, have you gotten other codes?

The code you have indicates a lean fuel mixture which can be caused by several things. Things such as low fuel pressure, faulty or dirty injectors, and engine vacuum leaks can cause it, but that shouldn't cause the increase in RPM's you described. Also, the IAC or MAF wouldn't cause that. Since it is happening when the vehicle is in gear, the transmission must be slipping or losing pressure and disengaging a gear.

Let me know if there are any other codes and the condition of the transmission fluid.

Take care,
Joe
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Wednesday, March 6th, 2019 AT 2:46 PM
Tiny
CGRIFF85
  • MEMBER
Transmission fluid is red with a slight brown tinge to it. The car has never thrown any other code but the one. It went away after replacing the O2 sensors but really can't understand why it keeps intermittently returning. Injector test came back fine as did the fuel pressure test. Any idea about that code? Lol, I really don't feel like dealing with a transmission. I am getting surgery for both rotator cuffs so I will get another vehicle and donate this one. But out of curiosity I would like to know!
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Wednesday, March 6th, 2019 AT 3:34 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
The code deals with a lean mixture. If it comes back at times, I suspect there is a vacuum leak at some point.

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

As far as the transmission, the only suggestion I have for that is to check line pressure. If it is low, it could allow the transmission to jump in and out of gear.

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Thursday, March 7th, 2019 AT 5:04 PM
Tiny
CGRIFF85
  • MEMBER
Threw it into reverse last night backing out of my garage. A lot of chattering and shaking and very slow reverse speed. Decided to pull it back in, didn't feel like being stranded. I am guessing at this point the transmission is just about done.
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Thursday, March 7th, 2019 AT 6:06 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Welcome back:

If you feel it is transmission related and there are no trouble codes, you may have to have it driven by a person who could recognize the feel and symptoms. I hate to say that, but other than checking line pressure to see if that is an issue, we are at a stand still.

Let me know if you have other questions.

Joe
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Thursday, March 7th, 2019 AT 7:03 PM

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