Way Outta Control
Join Date: Sep 1999
Location: The Palouse
Thanked 3 Times in 3 Posts
Re: 4.0 L or MPI idle speed control
There are a few factors that take a major role in TJ's idle.
1. Air Volume, this is a given per RPM step.
2. Knowing the Air Volume Per RPM step is a factor in controling fuel mix per RPM step.
3. Now the fuel becomes a constant per RPM step.
Now if the engines' warm the O2 sensor senses a rich condition and of course then tries to trim the mix by adding more air.
This is done by moving a pintle in the Idle Air Control Motor. More air is introduced to lean out the mixture...
At Cold idle the mixture is not really "controlled" it is a pre-determined mix set by the pre-programmed RPM steps in the EPROM. Increase the rest on the throttle plate and increase the RPM step, increase the RPM step and a new algorithim takes effect enriching the mixture and raising the idle.
If the Idle Air Contol motor becomes defective the the idle may lope, as it's reason for bieng is to control the
Liken the IAC to a cruise control. Floor the gas pedal and you go too darned fast, but the Cruise control can do the same thing with small ammounts of pressure to the pedal to have the vehicle reamian at a constant speed. It's a matter of the sensitivity of the cruise control. Little steps, little correction preception. Big steps and big notice!
Couple the TPS to the IAC as inputs to the comuter and you've got a match made in heaven. The TPS monotiors throttle plate position. So it knows when the throttle plate is at rest, or idle. It sends this info to the computer and the computer then says " OK, I'm at Idle, what's the IAC telling me?" Once this information is found then the trim process starts in those little steps to keep idle constant.
That's why on a Cold efi engine you see the RPM idle jump up and then down. At first the engne's warming up... the once it warms up the idle drops. The preprogrammed rich fuel mixture is now trimmed in response to the engine warmth.
OK, keep the foot on the brake and drop in drive. The idle should remain constant because the MAP sensor is providing engine load input to the computer. When it senses the load has been applied it tells the computer... the computer still knows the RPM and the throttle plate position and enrches the mixture.
See... clear as mud and easy as eatin' pie!