Voltage/Idle

Well i just got the car a week ago from a friend. Said this problem has been going on for alittle time now i am just trying to figure out what could be causing it. Anyway what is happening is when i am coming to a stop or sitting at a stop sign or light, the rpms dip low (400-500 range) as like if the car is going to stall but it doesnt. Also i notice at the same time my turbo timer the voltage is anywhere from 11-12.5 . This problem only occurs at night when i have my Headlights on. During the day with no lights it will not do this and the voltage is anywhere from 13-13.5 . Anyone know what is going on. Car has battery relocated to trunk could this be the cause by any chance??

 

It sounds to me like your ISC (Idle speed controller) isn't functioning. It should hold 800 rpm no matter what.
Setting the BISS
ISC Testing
You should also open the ECU and check for any damage to IC105 or IC107, the ISC drivers. Also, the 47uF cap in the ECU often leaks, eating an ISC trace that runs right under it.

 

i purchased a throttle positon sensor, because thats what i originally was told it could be...have not put it on yet...

get alot of mixed answers of what it could be

 

[ktmrider]

If the voltage is that low your alternator is dying.
Voltage should be 14v or so at idle and under load.
Same thing happened to me ( poor idle under high electrical load ), no BATT light ever came on even when the alt died completey.

 

[gtluke]

ever since i got my car like 10 years ago and NINE alternators ago my idle voltage has been POOP. if i have my crappy stereo on loud it actually turns itself off when the bass hits. i have to hold the idle over like 1300 to make it not do that.

 

[Lonewolf64]

Might be a stretch but are you using OE replacements or junk autozone/advance auto crap alternators?

 

the alternator is a mitsu part, not sure if it was ever replaced....

 

Well, if you look at the output curve of an alternator, it drops off steeply below 1000 rpm. If you can't keep the idle at 800 rpm, the alternator will never be able to crank out enough amps to maintain the right battery voltage, especially at night with heavy loads. 400-500 rpm just won't cut it. That's why I would crank the BISS screw out a couple turns first, getting the idle speed up, and then see what happens.

 

cranking the biss screw counter clockwise you mean? also dont i have to ground something to adjust the biss screw properly

 

Yes, "out" is counter-clockwise. To set it "properly", yes, I gave you the full procedure above. But as a simple test to see if the idle comes up and the charging voltage is better you can just turn the BISS screw 1-2 turns. I usually never followed the "proper" BISS setting procedure. I just watched the ISC step setting on my logger. With the engine fully warmed up, at idle, with no accessories running, I liked to see the ISC sitting at 8-10. That *assumes* that you have a functional ISC and ECU.

 

ok i will have to go give this a shot today and see what happens..

 

update: So i turned the biss screw back about 2 turns like KeyDiver suggested. Seems to be better now. With no lights and car fully warmed idling around 900-1000rpms and volts about 13.4-13.7
When lights are on car idling about 800rpms and volts at about 13
Does not seem to drop as low when coming to stops anymore with lights on either, used to drop to like 500 rpms and almost stall out now it is only dropping to about 650rpm or so.....
hopefully it will stay this way and thanks again keydiver.....

my only question is it ok to leave it like this where it idles about 900-1000rpm with no lights or should i tighten the biss just a tad to get it lower ...i know it normally supposed to idle at 850rpm.....but if this is fine ill just leave it be

 

Well, if you adjust it to 800 with all accessories turned off, but it doesn't hold at 800 when you turn things on, then you have an ISC issue that you need to troubleshoot. As I said in my first post, there's a good chance that either the ISC motor is bad and/or the ECU has issues. There are 2 main causes of premature ISC death:
1) Coolant leaks past the FAIV O-ring and shorts out the windings of the ISC, or
2) The 47uF cap in the ECU leaks and eats an ISC trace. many times this gets bad enough that the one ISC driver IC, IC105 or IC107, blows and burns up, which is easy to see when you open the ECU.
Its also easy enough to remove the ISC and check it for water/coolant damage, and ohm out the coils in it.

 

[fastasleep]

I have the exact same problem.

I've posted about it before, but nothing conclusive.

Post if your ISC motor is the problem, then I'll replace mine... /ubbthreads/images/graemlins/idea.gif

 

[Kenny_Kline]

13 volts is still to low. You should be charging at 14 volts -/+ .3 all the time.

Sounds like a slipping belt or a bad alternator

 

bought a throttle postion sensor...have not installed it yet though looks like a pain..the isc was just replaced about 2 years ago with a brand new one...does not have the idle problem as much, but i still have it only showing aron 13.5 volts with no lights and with lights like 13.0 volts...

 

belt was just replaced about 6 months ago on alternator.....

my mitsubishi gauge shows about 14 volts (3 gage kit from evo)....but my turbo timer shows 13-13.5 im not sure if it could just be my turbo timer showing a lower reading...???

 

i would grab a nice optima yellow top and run some 0-4 gauge wire right from the alternator to the battery and do the rewire. The oem wire from the alternator to the battery is crap!!! I have yet to do it until it warms up here, but a couple people have done it and they see no voltage drop with everything... if anything it would be a very small drop. I think the oem sh*t mitsu uses is like 10 gauge or smaller.

 

the battery is relocated to te trunk....i could try to pick up a optima battery though i hear good things