nycstev
Full Access Member
these trucks? I know the set screw does but does the PCM do this also?
How does it work?
How does it work?
SERPENT said:Have exactly the same problem as Barry. Idle slow to come down from 1500 rpm. Have replaced the TPS and the Idle Air Controller and have brought the fuel pressure to 50 psi with vac off at idle. No fix there. Spoke to Justin yesterday--he had similar problem and took off throttle body and readjusted blade. Problem solved apparently.
We brought it to my shop and fix it, the tps was out of adjustment... The position of it sets the idlebigike said:Venom Powers did the same thing.
WOT said:Typically, the PCM commands a specific number of "steps" in the IAC as it's initial guess - meaning that when you first fire up the motor or when coming down from cruise rpm, etc., it may command say, 75 steps. That will correspond to a certain position of the IAC. This is the PCM's best guess on where the IAC's pintle position should be for correct idle speed (it also prevents backfire on sudden throttle chop, and helps reduce certain types of emissions, but let's ignore all that). Once the the PCM has had a chance to watch the motor rpm it will make an adjustment to get to the proper idle speed. If the throttle body is opened more via the set screw (or if the TB is bigger and flows more air at idle), the engine will hang at a higher rpm for a while until the PCM decides to lower the "steps" in the IAC.
The fix for this is to either back off the set screw or to chance the program's default steps vs. vehicle speed table to compensate for this.
Ever notice how when cruising down the highway, sometimes it feels like you are just barely having to push the accelerator pedal when going from cruising to costing to back to cruising? That is because at highway speed, the PCM holds the IAC open a little to make the throttle on-off transition seem smoother. It makes driving the truck easier, such as with a manual transmission you don't have to push in the clutch immediately when backing off the throttle at speed to prevent jerking, or in the case of an automatic, it doesn't have to unlock the converter as quick to prevent jerking a bit on throttle lift at speed (on sudden throttle lift, it holds the IAC open for a specified number of miliseconds until the converter has a chance to unlock, and then it backs down).
Anyway, that's how the IAC vs. engine speed vs vehicle speed works.
WOT said:Typically, the PCM commands a specific number of "steps" in the IAC as it's initial guess - meaning that when you first fire up the motor or when coming down from cruise rpm, etc., it may command say, 75 steps. That will correspond to a certain position of the IAC. This is the PCM's best guess on where the IAC's pintle position should be for correct idle speed (it also prevents backfire on sudden throttle chop, and helps reduce certain types of emissions, but let's ignore all that). Once the the PCM has had a chance to watch the motor rpm it will make an adjustment to get to the proper idle speed. If the throttle body is opened more via the set screw (or if the TB is bigger and flows more air at idle), the engine will hang at a higher rpm for a while until the PCM decides to lower the "steps" in the IAC.
The fix for this is to either back off the set screw or to chance the program's default steps vs. vehicle speed table to compensate for this.
Ever notice how when cruising down the highway, sometimes it feels like you are just barely having to push the accelerator pedal when going from cruising to costing to back to cruising? That is because at highway speed, the PCM holds the IAC open a little to make the throttle on-off transition seem smoother. It makes driving the truck easier, such as with a manual transmission you don't have to push in the clutch immediately when backing off the throttle at speed to prevent jerking, or in the case of an automatic, it doesn't have to unlock the converter as quick to prevent jerking a bit on throttle lift at speed (on sudden throttle lift, it holds the IAC open for a specified number of miliseconds until the converter has a chance to unlock, and then it backs down).
Anyway, that's how the IAC vs. engine speed vs vehicle speed works.