Fast Idle
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New User
Joined: Nov 2002
Posts: 11
Likes: 0
From: Henderson USA
Fast Idle
I have a 94 with a 4.0. when i bought it the idle was high like around 1600 rpm's +or- a couple hundred it changed often. I replaced all the sensor's in the intake portion and fixed a couple of vacuum leaks. now with no leaks and an extra return spring it won't idle any lower than a 1000 rpm's and this is with the idle adjust screw all the way out.
I apreciate any help. it has a 5 speed in it so the high idle isn't good for the constant necessity to shift.
when i let off the pedal the idle drops to about 1200 or 1300 and then tkes a couple seconds to drop to a grand, i think that part is normal but just in case.
I apreciate any help. it has a 5 speed in it so the high idle isn't good for the constant necessity to shift.
when i let off the pedal the idle drops to about 1200 or 1300 and then tkes a couple seconds to drop to a grand, i think that part is normal but just in case.
Post Fiend
Joined: Dec 2001
Posts: 10,562
Likes: 4
From: South Jersey
Fast Idle
They don't always die with the IAC disconnected but you should see an rpm drop. Have you checked the base idle speed which is usually a few hundred below the speed with the IAC. I would also double check for vacuum leaks, check for codes, clean the MAF and test the ECT.
Logistics Pro
Joined: Sep 2002
Posts: 4,410
Likes: 1,015
From: Central KY
Fast Idle
My truck does this too. I truly believe it's one reason why my gas mileage sucks. But too bad it can't be adjusted...because the entire freaking truck is computer controlled now. 
Post Fiend
Joined: Dec 2001
Posts: 10,562
Likes: 4
From: South Jersey
Fast Idle
>My truck does this too. I truly believe it's one reason why
>my gas mileage sucks. But too bad it can't be
>adjusted...because the entire freaking truck is computer
>controlled now.
I don't think the IAC would cause a gas mileage problem, I would suspect an O2, dirty MAF or bad ECT problem might be the cause. What kind of mileage are you getting and what are you driving habits. I get bad mileage since I drive only a few miles each trip and it never goes into closed loop.
>my gas mileage sucks. But too bad it can't be
>adjusted...because the entire freaking truck is computer
>controlled now.
I don't think the IAC would cause a gas mileage problem, I would suspect an O2, dirty MAF or bad ECT problem might be the cause. What kind of mileage are you getting and what are you driving habits. I get bad mileage since I drive only a few miles each trip and it never goes into closed loop.
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Hotshot
Joined: Aug 2002
Posts: 12,679
Likes: 15
From: BC, Canada
Fast Idle
What vacuum leaks did you fix? It is very common for the fuel rail gaskets to leak. Both the gasket on top and the one under it.
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Senior User
Joined: Oct 2002
Posts: 420
Likes: 0
From: Harrisburg USA
Fast Idle
This may sound a little overly simplistic ... it's been discussed in many posts & replies ... have you checked your TPS (throttle position sensor)?? When you said it drops to about 1200 or 1300, then to 1000 a couple seconds later, that sounds like the behavior of the TPS & related equipment. If you make sure all the connectors & stuff are clean & functioning properly, that would be a good starting point. By the way, I don't know how much Ford has changed the TPS over the years. My '85 has a TPS, and any manuals I read say that idle speed is "computer controlled" and "non adjustable", when what they really mean is that you AREN'T SUPPOSED TO ADJUST THEM BY LAW (emmissions regulations). Mine is indeed FULLY ADJUSTABLE, and I have adjusted it to my liking. Are newer ones adjustable??
New User
Joined: Sep 2004
Posts: 7
Likes: 0
From: MN
unplug the mass air flow sensor if it has one, not sure if that year had one or not, and start it up and let idle. remove the rubber air inlet tube from the throttle body and close off the throttlebody opening with a piece of stiff cardboard. motor should die instantly and pull hard on card board, if not you got a vacuum leak. Many times they can be a bugger to find,exspecally if not an external leak. Leaks from stuff like purge solenoids and brake boosters can let air in past the iac and throttle plate.
