help

ok i have a little problem, when i am sittin at an idle, every once in awhile my rpm's will start fluxuating between 1,500 and back down to where it almost kills the car, whats up with this? i have a 98 eclipse gs by the way:sun:

Check for vacum leaks, check you plugs and wires, check the isc, could be lots of stuff.

ok how do i see if i have a vacuum leak without paying out the ass at a shop, and how do i know if its a misfire? and i am dumb so whats the isc?

whats the isc?

:confused: :confused: im lost 2

maybe ignition sensor control???

:confused:

ISC Idle Speed Control motor, it attaches to your throttle body & "Controls" your idle fluctuations. Basically when you're driving & come up to a light, this prevents the motor from staying at high rpm & holds idle at 750 +/-

Rob

Its late and I'm bored, so heres a little tidbit on throttle body functions and idle speed. The ECU has a target idle speed. It 750 rpm for 5 spds. It uses the ISC and ignition timing to control it. ISC makes broad adjustments, while timing makes slight changes. The ISC also raises idle when you put the AC on, as you come to a stop and the PS pump is using up some power, etc, as mentioned above. The BISS, or base idle speed set screw isused to get airflow correct, not adjust idle speed contrary to popular belief. The correct way to set it is to hold the ISC stepper motor at the center of its travel, or step 0, via a scantool, and set the BISS until idle speed is 750. It may also help to ground the timing adjustment pin so it doesnt fluctuate idle speed on you. There is also the FIAV, or fast idle air valve, which uses the temperature of the coolant to raise idle when the car is cold. The throttle closed switch has to be closed also, or the ECU wont even go into closed loop control of idle speed at all. :wink: On a 1g thats the one wire switch right behind the throttle plate, and on a 2g its incorporated into the TPS, or throttle position sensor (hence the extra 4th wire), so its important to have the TPS set properly on a 2g.

With that little primer on idle function, its easier to track isues down. First and foremost, get rid of any and all vacuum/boost leaks. See www.vfaq.com for a vfaq on how to build an intake leak tester. Its only like 8 bucks. There is no point in trying anything else unless you have no vac leaks. And good luck bringing it to a shop to find them. :wink: Also, clean the back of the TB plate as well. If thats all gummed up and can hang up and not close all the way. Once that has been taken care of, the most common causes for high idle are throttle cable tension, bad BISS, and bad ISC. Happy hunting :biggrin: Hope that helps.

Also check your coolant temperature sensor

Good point. That sensor tells the ECU when to come out of the choke maps, and is different from the one that operates the guage :wink: