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1G electrical load causes low/rough idle or stall

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dwb

Proven Member
813
560
Sep 9, 2021
Broomfield, Colorado
When I start turning on power hungry accessories - blower, headlights, rear defroster, etc. - my idle becomes a little erratic. It idles a little lower and the engine runs a little rough. Sometimes when coming to a stop the engine stalls or barely catches in time when all the accessories are on. The more electrical load, the worse the symptoms.

I have a 90amp alternator, a lifetime warranty unit from Advance Auto. Charges good at 13+ volts. My idle is 750rpm most the time, ISC is good, blocked FIAV, idle switch working, TPS set correctly, and no major boost leaks. I set curb idle per the vfaq by grounding the firewall plug and the pin on the diagnostic plug.

Obviously extra electrical load causes more load on the alternator which takes more mechanical power. But why doesn't the ISC account for the extra load and most importantly, why would it not always catch idle when coming to a stop? It never has a problem catching idle if the headlights are off.

Thoughts, ideas?

It's a 91 TSi AWD mostly stock.
 
do you know what your IAC counts are during a normal hot idle with all loads off? This may need to be adjusted slightly to account for the low idle when your electrical loads all on at the same time.

I have had this issue before, so I just adjusted my IAC counts with all my electrical loads on, I just made a slight turn to the screw to keep the idle low enough with the loads on without stalling the motor, this way the idle is too high with the loads off.
 
Is this a new problem or one you've always had? Did it occur after you modified some things? My first thought is the BISS screw is not set perfectly. You may need to reset the isc and and then set the BISS screw again. It could also be that you set reset the isc on accident and the car is still adjusting ( can take some time). It's difficult to say based on current info.
 
Thanks for all the replies!
I'll get my logger out when I get a chance and post the results.
OEM flywheel
It's been like this for several years and I've just dealt with it. Seemed like it started after my 70amp alternator died and I swapped to a 90amp. Although I can't say with confidence that was it. Very few modifications have been done. MBC, hard UICP, blocked FIAV, blocked EGR, coolant line bypass on the TB, rewired fuel pump, and probably a couple other small mods that I'm forgetting.
I've already reset the curb idle several times per the vfaq. The BISS seems like it is just about bottomed out to hit 750rpm. When I'm running the headlights and defroster a lot in the winter months I'll bump the curb idle up to about 850, but even then it still has a hard time catching idle.
A/C works and it does idle up when the compressor engages.
I've cleaned the TB multiple times with no improvement.
 
I was taking some logs yesterday and while blipping the throttle my idle switch stopped working intermittently. Since the engine wasn't up to temp yet I don't feel my logs are very accurate. I'll get the switch replaced and then log it again.

I did notice, though, my ISC steps at idle with no accessories was 0. I's that normal?
 
The isc has 0-120 steps with zero meaning the ecu is trying it's hardest to reduce the idle and 120 meaning it's trying to increase it. It's not normal to be sitting at 0 and at best it indicates your idle needs some work and at worst it means your isc driver is bad.
 
The isc has 0-120 steps with zero meaning the ecu is trying it's hardest to reduce the idle and 120 meaning it's trying to increase it. It's not normal to be sitting at 0 and at best it indicates your idle needs some work and at worst it means your isc driver is bad.
If 0 steps is not normal, what should I be seeing? I thought that grounding the firewall plug and diagnostic port pin was to zero out the ISC?
What would be a normal ISC step range on a properly functioning 1g? My log yesterday, not really a cold start but around 100deg coolant, it started at 67 steps and down to 6 steps at around 165deg. It does hit 0 steps when hot.
 
The isc should be reading roughly 30 when the car is fully warmed and no accessories are on. Grounding the timing connector keeps your timing at 5 degrees so you that the idle ignition doesn’t bounce around allowing you to set your idle speed accurately.
 
Have you done any electrical maintenance? Possibly clean all grounds and verify grounds are present where needed, OEM ground points are all specified in the manual.
 
I was taking some logs yesterday and while blipping the throttle my idle switch stopped working intermittently.

Do you have the ground strap on your TB between it and the bolt on the Throttle Body Elbow?
 
Have you done any electrical maintenance? Possibly clean all grounds and verify grounds are present where needed, OEM ground points are all specified in the manual.
I did go through the grounding points but it didn't make a lick of difference. All of them are there, clean, and in good shape.
Do you have the ground strap on your TB between it and the bolt on the Throttle Body Elbow?
Yes, I even removed the switch and bench tested it with an ohm meter. I could only get it to work if I wiggled the tip around while depressing it. Hard to believe those suckers are so expensive ($130+). I put up a wanted ad for the switch if anyone has one laying around: https://www.dsmtuners.com/marketplace/1g-idle-switch.1226/

Once I get that switch replaced I'm going to take another look at my FIAV block off and do another good boost leak check.
 
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