1995 Ford F-150 constant high idle

Tiny
RXONFIRE
  • MEMBER
  • 1995 FORD F-150
  • 0.5L
  • V8
  • 2WD
  • AUTOMATIC
  • 80,000 MILES
Cannot get engine to reduce rpm's. When I put it in drive, it still goes 15-20 mph down the street! Replaced the TPS, IAC valve and 02 sensors without any changes to the idle runs like the butterfly flap remains in the cold position
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Friday, August 21st, 2015 AT 8:33 PM

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Tiny
CARADIODOC
  • EXPERT
This is commonly caused by a vacuum leak. Start by looking at the hard plastic and rubber vacuum hoses. You can pinch them off in various places while the engine is running to see if one brings the idle speed down. If you find on that does, follow it to its branches and pinch those off to work your way to the leak.

Intake manifold gaskets can leak too. You can often find that by spraying water over the engine while it's running and still cold. Watch for where it gets sucked in.
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Friday, August 21st, 2015 AT 8:43 PM
Tiny
RXONFIRE
  • MEMBER
I sprayed some starting fluid all around vacuum lines throttle body ect. And did not get a rise in engine rpm but will look a little harder for leaks. If no leaks are found, what would be the next step to check?
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Friday, August 21st, 2015 AT 8:47 PM
Tiny
CARADIODOC
  • EXPERT
The next thing would be to connect a scanner to view live data, in particular. "Idle air percent". Ford does almost everything differently than all other manufacturers, and how they control idle speed is no exception. They vary the average voltage to the solenoid to make it pull more or less against spring pressure to pull the air valve open. If you see that percentage being called for is high, the Engine Computer is requesting a high idle speed in response to something. Ford had a lot of trouble with their coolant temperature sensors in the early '90s, and high idle is one of the symptoms. If you see the computer is calling for a very low idle air percentage, it is trying to reduce idle speed but is not having success. That is almost always due to a vacuum leak.

Another approach is to borrow a smoke machine to inject a white, non-toxic smoke at 2psi into a vacuum port, then you watch for places it sneaks out. You might find that at an auto parts store that rents or borrows tools. We use those to find leaks in the vapor recovery systems on '96 and newer vehicles. Small leaks are virtually impossible to locate without that machine.
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Friday, August 21st, 2015 AT 9:23 PM

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