I have a 2001 jeep grand cherokee larado with the 4.0 6cyl. When I start the vehicle the rpm's are really low. But when I drive and don't have to stop there is no hesitation or fluctuation in rpm's as they climb. But when I brake and let off the accelerator the rpm's drop so far that they vehicle will stall but then rise just enough not to die. Untill I come to a complete stop then it dies and I have to restart it and leave my foot on the gas and break. What is going on? No check engine light is on and i've been told its not the thottle position sensor. Please help!
Was the battery recently disconnected or run dead?
August, 11, 2012 AT 11:54 PM
Neither the battery disconnected or run dead.
August, 12, 2012 AT 2:10 AM
It still sounds like the Engine Computer lost its memory and has to relearn "minimum throttle" before it will know when it must be in control of idle speed. It also might not give you the normal "idle flare-up" to 1500 rpm when you start the engine. To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the brake or gas pedals.
August, 31, 2012 AT 6:14 AM
I will definately try that the next time it happens. B4 I got to ur reply it happened to just fix itself like magic. Until a few days ago and then the same thing started to happen again. I had thought that since it had rained that a sensor had gotton wet, because as of today it runs fine, with the idle flare up. And since you had said something about that and u being the only ONE I trust your advise.I will definately let you know if that works in my case if it happens again. Do you have any other advise that it may be since it has happened on more that one occasion?
August, 31, 2012 AT 7:38 PM
When the problem is intermittent it can be pretty hard to diagnose without a scanner. I use the Chrysler DRB3 to show the idle "steps" the idle speed motor has been placed at by the Engine Computer. There are 256 steps corresponding to totally closed to completely open for the air passage around the throttle blade. For a properly running engine step 32 is typical for maintaining the idle speed. A single cylinder misfire will result in the computer bumping it up to around step 45 to 50 to keep the speed up. The step number will give you an idea of how the system is working. If it stays on step "0", either the computer hasn't relearned minimum throttle, (which doesn't sound like the case here), or there's a vacuum leak causing the idle speed to be too high and the computer is closing off the air passage in an attempt to get the speed back down to where it should be. At the same time it shortens the injectors' pulse times to inject less fuel. A vacuum leak will result in more air but the less fuel from the computer's attempt to lower speed results in a very lean condition which is conducive to stalling.
An intermittent misfire can also cause stalling from the idle speed being too low and the computer can't respond fast enough.
The scanner will also allow you to increase engine speed up to 2000 rpm in 200 rpm increments to verify the idle speed motor is working, the computer has control of it, and the air passage isn't plugged with carbon. Even when that works, you may still want to remove the motor and clean any carbon out of the passage. The computer is programmed to operate that valve according to the known size of the passage and if carbon buildup changes the size, it isn't going to respond as expected.
You can also watch the idle speed steps on the scanner when you slow down to a stop. At slow speeds, those steps should start to increase from "0" when your foot is off the gas pedal.