1999 Ford F-150 Rough Idle

Tiny
SPECIALK17
  • MEMBER
  • 1999 FORD F-150
  • V8
  • 4WD
  • MANUAL
  • 120,000 MILES
For the past few weeks my truck has started having a rough idle. Everytime I come to a stop or slow roll, the idle drops way low and all lights dim and flicker, then it revs back up. Sometimes my truck has even stalled, but starts right back up. I was thinking alternator but do not know for sure. Thanks in advance!
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Thursday, October 29th, 2009 AT 12:48 PM

7 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
If the battery hasn't gone dead, the alt is fine. Chances are it is a bad sensor or a vacuum leak.

Has the check engine light come on?
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Friday, October 30th, 2009 AT 8:15 AM
Tiny
SPECIALK17
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Hi Jacobandnickolas,

It did come on once and went away right away. Has not come on since and Autozone couldn't find any codes. About three months ago it came on and said the o2 sensor was bad. I replaced one (driver side) not knowing there are others. I just bought one to replace the other side (Passenger).

Spcialk17
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Friday, October 30th, 2009 AT 10:28 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Let me know if it helps. That could be the problem.

Joe
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Friday, October 30th, 2009 AT 10:52 AM
Tiny
SPECIALK17
  • MEMBER
Hey Joe,

I did replace the other sensor this weekend. It was pretty old looking having a white substance all over. However, changing the sensor did not help the problem. Do you know how many O2 sensors are on my truck. I think I fopund one more that I am going to change as well. This one is just before the muffler. Any other thoughts or ideas as to what may be causing the problem?
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Monday, November 2nd, 2009 AT 10:25 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Based on where you live it could have 4. The one that you mentioned, right before the muffler, is called the down stream O2 sensor. It identifies if the catylatic converter is doing its job.
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Monday, November 2nd, 2009 AT 7:37 PM
Tiny
SPECIALK17
  • MEMBER
The O2 sensors did not affect the idle dropping at stops. I did notice that when the window is all the way down and I push the window button down again the idle drops low and the lights dim as well. Do you think this is related in any way?

Thanks in advance!

Specialk
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Tuesday, November 10th, 2009 AT 12:52 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
That is because the motor can't move any further and is putting a draw on the engine.
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Tuesday, November 10th, 2009 AT 2:17 PM

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