Odd ? concerning RPM's
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Junior User
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Odd ? concerning RPM's
This may be hard to understand, but here it goes. 93 F-150 5.8 Idling in park, if I romp on it, the rpm's seem to decrease slowly after it's throttled. After it's throttled, rpm's seems to hang unusually long back down to idle. Has been like that since I've had the truck. The decline of rpm's seems a little sluggish as though somethings interfering. Does this even make sense?
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Junior User
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Here's a terrible, jerky video to demonstrate.
MVI_0666.flv - Video - Photobucket - Video and Image Hosting
MVI_0666.flv - Video - Photobucket - Video and Image Hosting
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
Could be a sticking idle bypass valve, remove it from the TB, disconnect the solenoid from the valve and thoroughly clean with TB cleaner until the valve moves freely. Could aslo be a sticking throttle cable, you can disconnect the cable from the TB linkage and operate it manually to determine if the cable is the problem or not. It could also be the throttle valves themselves, they should snap shut when the throttle is blipped.
Lead Driver
Joined: Aug 2006
Posts: 5,670
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From: Sand Lake, MI
I'd start by checking out this part,
Ford Fuel Injection Idle Air Bypass (IAB)
Ha! never mind, Conanski covered it.
Ford Fuel Injection Idle Air Bypass (IAB)
Ha! never mind, Conanski covered it.
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Junior User
Joined: Jun 2002
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Could be a sticking idle bypass valve, remove it from the TB, disconnect the solenoid from the valve and thoroughly clean with TB cleaner until the valve moves freely. Could aslo be a sticking throttle cable, you can disconnect the cable from the TB linkage and operate it manually to determine if the cable is the problem or not. It could also be the throttle valves themselves, they should snap shut when the throttle is blipped.
Lead Driver
Joined: Aug 2006
Posts: 5,670
Likes: 13
From: Sand Lake, MI
Looked pretty normal to me.
The IAC may be a little slow to react sometimes, that may be what you notice when <i>The decline of rpm's seems a little sluggish as though somethings interfering.</i> no big deal though. If it gets worse clean or replace it when you think it needs it.
I have to clean the one on my wife's explorer about every 4 months, replaced it twice even but I have yet to have a problem with one on ether one of my trucks.
The IAC may be a little slow to react sometimes, that may be what you notice when <i>The decline of rpm's seems a little sluggish as though somethings interfering.</i> no big deal though. If it gets worse clean or replace it when you think it needs it.
I have to clean the one on my wife's explorer about every 4 months, replaced it twice even but I have yet to have a problem with one on ether one of my trucks.
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From: Iowa
I know with the programming in Mustang AT ECU's they are set to bring the idle down slowly (I believe that it is to help elimimate stalling on decel). It may just be normal stuff. Check the standard stuff....but it may just be nature of the beast when revved at idle under no load.
Postmaster
Joined: Oct 2008
Posts: 3,026
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From: Renton, WA
Unless it's this lag is extreme or jerky is normal. The computer has whats called the idle air follower function. This function has the IAC follow the proggress of the throttle as you drive. And yes it's to prevent a stall then the throttle is snapped closed.
I personally hate this function. It makes going from full go to full whooh unpredictable and dangerus cause it effects the brake balance.
I personally hate this function. It makes going from full go to full whooh unpredictable and dangerus cause it effects the brake balance.
Thread Starter
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Junior User
Joined: Jun 2002
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Unless it's this lag is extreme or jerky is normal. The computer has whats called the idle air follower function. This function has the IAC follow the proggress of the throttle as you drive. And yes it's to prevent a stall then the throttle is snapped closed.
I personally hate this function. It makes going from full go to full whooh unpredictable and dangerus cause it effects the brake balance.
I personally hate this function. It makes going from full go to full whooh unpredictable and dangerus cause it effects the brake balance.
Postmaster
Joined: Oct 2008
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From: Renton, WA
Of all the vehicles I've owned my vortec 7.4 was the worst for this. Even with the hydroboost brakes it took way to much brake pedal pressure to hold the truck still for about 30sec after a full throttle romp. I even had situation on slick serfaces were it would powerbrake at this high idle, even with the locker. It was sometimes scary cause it would play havoc with the ABS and the front brakes would release.