really low idle

[mtndew]

so ive known that i have a low idle for a while, the motor shakes a little and on the gauge it looks to be around 400, but i didnt think it was actually that low. i know a lot of stock tachs dont read right, but today i picked up a DVOM with an inductive pickup and tach option. i hooked it up and it was jumping around between 340 to about 420. idle should be 800 correct? i just have a little trouble believing this is correct. i know in my miata which is supposed to idle at 850 it will shake like crazy and seem like its going to die if it goes below 700, and my tracker is reading about 400? i would think the whole car would be shaking violently.
so my first question would be is it normal for the rpms to jump around like that? and second what would be causes of this? i just recently cleaned my isc and it is working properly, i hooked it up to 12v source and watched it open and close.

also my DVOM can measure dwell. what is that?

[jtgh]

welcome to low idle hell

a good motor , will idle low, the lower the better if compression is really good , it can. but shouldn't.
if you take a good motor , and lower the bleed screw from 800 and lower it.
a good motor may go really low before stall.
"lots of power in each cylinder " and yes , it will shake.


forget that, it is not REGULATING idle.

idle will not regulate if there is something really serious wrong.

ISC , sure it could be stuck, but yours is not.
TPS is out of calibration or just plane bad. "TPS SWITCH SIDE"
EGR stuck open. (must not leak at idle or idle will suck)

stripped crank pulley key way. (low compression ) ign timing varies with freeze timng jumper in place , crank pulley wiggling all over.


dwell is for setting points in points ignition.
and be used for setting duty cycle too. on iSC. but only when it works , yours dont. The ISC dutycycle is 50% if set perfect, FYI.

also if car entered in to limp home , it can sometimes drop idle controls.
95 and older.

clean the EGR, makes sure it snaps shut fully every time you flex it.
EGR MAIN VALVE.

Originally Posted by mtndew

so ive known that i have a low idle for a while, the motor shakes a little and on the gauge it looks to be around 400, but i didnt think it was actually that low. i know a lot of stock tachs dont read right, but today i picked up a DVOM with an inductive pickup and tach option. i hooked it up and it was jumping around between 340 to about 420. idle should be 800 correct? i just have a little trouble believing this is correct. i know in my miata which is supposed to idle at 850 it will shake like crazy and seem like its going to die if it goes below 700, and my tracker is reading about 400? i would think the whole car would be shaking violently.
so my first question would be is it normal for the rpms to jump around like that? and second what would be causes of this? i just recently cleaned my isc and it is working properly, i hooked it up to 12v source and watched it open and close.

also my DVOM can measure dwell. what is that?

[jtgh]

hi, you are studying cars.
ask your instructor what command authority is . ?
and how it applies to ISC. (and other sensors and actuators.)

EFI cars don't idle for 100 reasons.

bad motor. 80 reasons here. huge list.
bad ISC
air leaks (on 16v, car goes lean and will NEVER IDLE) 8v motor races.
egr leaks,
any way for an 8v to be lean (fuel pressure bad only at idle)


let me explain.
the ISC has limited range, (it can only open so far and shuts 100%) see.
its has finite flow rates of control.
the isc on your car my be wide open and idling at 400 , if the egr is stuck open.
or (a gross air leak)
if the induction has a leak and your isc closes fully to compensate, it can never hold 800,
it will race.
some ECU will hunt when this happens. resetting ISC each time.
1000 , 1500, 1000, 1500 rpm over and over.
some just stick high. depend on year, and if calif or fed ECU.

but yours is the opposite. it is stuck low..
at that moment apply 12v to the ISC coil and if motor dont speed up
then the iSC is stuck or the EGR is leaking.
if motor speeds up , then ECU has stopped controlling ISC for some reason.

the latter can be hard to sort out.
cheers.




the motor could be sick.
low compression, one cylinder very weak others ok.
cam timing set wrong.
ignition timing set wrong..

I had the same problem as you, 2 times.
1 time it was intermittant TPS switch , switch didnt close and when that happens, no idle controls will ever be active.
the idle will be random , every time that happens.

2nd time , i had this infernal EGR main. it kept sticking about every 10th red light. cleaned it 5 times, now its working.
it looked perfect in side, but just loved to stick open.


I bet:
1 leaking EGR
2: cam or ign timing jumping around due to bad belt , keyway or ?


I repeat the FSM ,
it says , Idle is the last thing to set and to work correctly , all else must be perfect.
true it is.