engine dips when rpm is low
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Posting Guru
Joined: Mar 2009
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From: Montague, NJ
engine dips when rpm is low
97 f150, auto, 4.6 260,xxx miles.
ive had this problem since i got the truck in February. it will idle beautifully. if i blip the throttle a little bit it will rev up and drop back down, but it will fall to almost 450rpm before it catches itself. ive actually stalled it a couple times. the other day when i was backing up a log splitter in my back yard and i had the wheel cranked all the way and it stalled twice. i dont think the plugs are bad, it runs like a top otherwise.
its really annoying. and id like to get it fixed. air and fuel filter are brand new. pump is new-er. i dont know what else is left to check.
ive had this problem since i got the truck in February. it will idle beautifully. if i blip the throttle a little bit it will rev up and drop back down, but it will fall to almost 450rpm before it catches itself. ive actually stalled it a couple times. the other day when i was backing up a log splitter in my back yard and i had the wheel cranked all the way and it stalled twice. i dont think the plugs are bad, it runs like a top otherwise.
its really annoying. and id like to get it fixed. air and fuel filter are brand new. pump is new-er. i dont know what else is left to check.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
Here is where a little knowledge is important.
What controls idle speed?
I device called an IAC (idle air controller).
It is pulsed by the PCM to keep the engine under control at all times near idle positions (closed throttle).
It is electro-mechanical.
Remove it and do a cleaning with special spray cleaner to be sure it's free working.
It's purpose it to meter air "around" the throttle plate so idle is controlled.
Be sure to clean the throttle body throttle bore and plate area, as well.
Good luck.
What controls idle speed?
I device called an IAC (idle air controller).
It is pulsed by the PCM to keep the engine under control at all times near idle positions (closed throttle).
It is electro-mechanical.
Remove it and do a cleaning with special spray cleaner to be sure it's free working.
It's purpose it to meter air "around" the throttle plate so idle is controlled.
Be sure to clean the throttle body throttle bore and plate area, as well.
Good luck.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
The IAC has a dashpot function built in so 'blipping' the throttle dosn't cause a stall.
A slow or faulty IAC pintle movement could cause such a reaction.
It is sort of interference spring loaded so the PCM signal has it to work against so the air can be controlled both up and down very quickly.
The old cars of the 60s and 70s had one seperate mounted on the carb such that it slowed the throttle return a bit to prevent stall especialy on an auto trans equipped motor.
A slow or faulty IAC pintle movement could cause such a reaction.
It is sort of interference spring loaded so the PCM signal has it to work against so the air can be controlled both up and down very quickly.
The old cars of the 60s and 70s had one seperate mounted on the carb such that it slowed the throttle return a bit to prevent stall especialy on an auto trans equipped motor.
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