1994 Ford F-150 '94 F-150 4.9 inline 6 engine high idle

Tiny
HOSS87
  • MEMBER
  • 1994 FORD F-150
Engine problem
1994 Ford F150 Four Wheel Drive Manual 153000 miles

I've got 153,000 miles on my truck, its 4wheel dirve with a manuel tranny. When she starts up she sounds great idle perfect just like she just came off the lot, but when she gets hot she idle very high and doesn't stop until she cools down again. She'll still start up great when she's hot but still idles very high. I took some carb and choke cleaner to the vacume lines to try and find a leak and when I sprayed it over the steel line coming from the exhast to the egr valve the idle got higher. I replaced 3 vacume lines that was so soft they could just colaps, and I replaced the O2 sensor just becuase it had the original factory sensor on it. I was thinking that this might be some sort of temp sensor since it idles high when its hot, but I didn't want to just go around replacing sensors so I thought I'd ask the pros. What do y'all think?
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Thursday, December 6th, 2007 AT 10:49 AM

9 Replies

Tiny
INDYUKE
  • MEMBER
How new is the coolant temp sensor? Sometimes a bad one will see the engine as never warming up, so it will tell the computer to keep the cold idle and mixture settings. Which causes the engine to idle even higher when warmer.
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Thursday, December 6th, 2007 AT 5:59 PM
Tiny
HOSS87
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Would the check engine light be on is it was bad?
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Monday, December 10th, 2007 AT 10:30 AM
Tiny
INDYUKE
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Not necessarily. The check engine light won't come on unless it doesn't even see the sensor due to a broken wire or if it's so out of range it might as well not even be there. The sensor could still be in range, just not able to give an accurate indication of true engine parameters. The computer just believes what it's told.
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Monday, December 10th, 2007 AT 11:02 PM
Tiny
HOSS87
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Ok thanks, I'll have to give that a try when I get a chance to head to the parts shop
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Tuesday, December 11th, 2007 AT 1:53 AM
Tiny
HOSS87
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When I replace that sensor is there anything I need to know or do I just plug it in and go? I know with something like a cam sensor the computer has to learn a new one.
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Tuesday, December 11th, 2007 AT 10:21 AM
Tiny
INDYUKE
  • MEMBER
Yes, you need to disconnect the battery for at least 15 minutes. Then take it for at least a 30 mile drive with a good portion of that being steady throttle. Ie cruise.
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Tuesday, December 11th, 2007 AT 3:06 PM
Tiny
HOSS87
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Alright I changed the temp sensor and it needed to be changed because the contacs were corroded real bad. But it did not fix the problem. Is there anything else I can check?
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Wednesday, December 12th, 2007 AT 5:56 PM
Tiny
INDYUKE
  • MEMBER
Yes, there are some more things you can check.

The IAC valve acts on the computer's command to adjust the idle speed at the throttle body. Sometimes they clog up. The best way to take care of that is to take it off and blast the internals of the valve body with carb cleaner until it runs out clear. Don't get any on the motor though.

And you can check your MAF sensor. It's on the firewall right above and to the right of the heater. When they go bad or become disconnected, the idle tends to be EXTREMELY high. 1700 - 2000rpm or so. Let me know.
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Friday, December 14th, 2007 AT 9:43 PM
Tiny
HOSS87
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Yeah I was sittin thinkin about the proccess and thought I should check the IAC but didn't want to spend tho money to replace it but thanks for tellin me how to clean it out. I'll check that
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Monday, December 17th, 2007 AT 6:12 PM

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