4.0 L or MPI idle speed control - Off-Road Forums & Discussion Groups
Jeep-Short Wheelbase All discussion of short wheelbase Jeeps: CJ, TJ, YJ and JK

 
LinkBack Thread Tools Display Modes
post #1 of (permalink) Old 07-29-2002, 08:12 AM Thread Starter
 
Join Date: Nov 2000
Posts: 2,687
Thanks: 0
Thanked 0 Times in 0 Posts
 
4.0 L or MPI idle speed control

How is the idle speed controlled and enhanced on the mpi system for cold start?
TiminMb is offline  
Sponsored Links
Advertisement
 
post #2 of (permalink) Old 07-29-2002, 08:59 AM
Way Outta Control
 
Join Date: Sep 1999
Location: The Palouse
Posts: 13,432
Thanks: 1
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!
LEVE is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Off-Road Forums & Discussion Groups forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome