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1G ISC motor issue, idle is completely random

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Josh E

Probationary Member
12
0
Oct 28, 2015
Pensacola, Florida
Hey,

I'm Josh, and I'm new to the forums, at least posting here. (Been reading posts for a couple years)
I think I'm posting in the right area... :coy:

Vehicle - 1990 Talon Tsi
Engine - 4g63 from a 93
ECU - 1991+ into a 1990 chassis
MAF - 2g


Coil Pack, PTU, CAS, TB, ect have all been swapped to the 1991+ electronics

Alright, so let's begin. Simple as it may be, I'm out of answers. My ISC is failing to do it's job at keeping the engine idling. The only time the engine will have idle surge is if I purposely make a vacuum leak. It will "idle" anywhere from 500-1600 rpms for about 15 seconds and then eventually stalling.

What I know/Tried

The TPS is correctly adjusted and verified with a multi-meter.

The Idle Switch passes a continuity test.

The ISC tests at 40 ohms, I've read that's what new ISCs read instead of the 30 ohms.
I have also tried a known working ISC from my friend and tried mine on his Eclipse and mine worked fine on his.
I figured this would be wiring harness related, I pulled a donor ISC harness and 4 ECU pins and ran the ISC directly to the ECU plug with no change.
The stepper moves very slightly from its resting place when the ignition is turned on. However, if I bump the starter with the ISC out, the stepper moves in and out generously before going back to its resting place.

Out of curiosity I connected the MAF to the throttle body to be 100% sure it wasn't a vacuum leak in the inter cooler piping, no difference.

Also, normal driving and WOT are unaffected with this issue I'm having, and AFRs are where they need to be.


What I don't know

Is the ISC dependent on other parameters? I'm guessing the obvious would be the Idle Switch, but is there anything else needed to make the ISC work?

The ECU worked flawlessly in my Laser, but I have not tested another 1991+ ECU in the chassis yet.

I promise I have searched, haha, maybe I'm just completely missing something.

I have a data logging cable coming in on Saturday, but until then I figured I'd go ahead and see if anyone can point me at something to look it. :heystupid:
 
Probably to switch the pin #6 and #14 would solve the idle issue, since you are using a 91+ ECU on a 90.

I did do the pin swap... but I better double check to make sure it's right.. :pray:
 
Does it make a difference if the car is warm or cold? I'm assuming you still have your FIAV and that you hooked it up after you switched throttle bodys since you didn't mention it was bypassed.
 

I have multiple times... I think I wired it correctly, but I will double check for sure.

Does it make a difference if the car is warm or cold? I'm assuming you still have your FIAV and that you hooked it up after you switched throttle bodys since you didn't mention it was bypassed.

It struggles so badly I can't tell a difference. :cry: And the FIAV has not been bypassed.

Does the car idle well with a 90 ECU? and maybe should try another 91+ ECU also.

I have tried the 1990 ECU once before but it didn't affect the idle issue. Once my friend gets back in town Ill use his 1991+ ECU to eliminate that possibility.


What I'm getting from your responses is that this isn't complicated and should be a simple mistake, so that a good thing right haha

Thank you guys, Ill be back shortly with some updates and pictures.
 
ECU Pin 6 and 14 are correctly switched.

For the ISC I have pin 4 and pin 6 switched as well. The ISC came out of my 93.. just to verify this must been done?

I do have my dash removed and this idle problem is gonna make me rip out my whole wiring harness and attempt to make a new one, but there has to be a better solution and I just don't see it. :banghead:

I took some pictures of both ECUs, I'm not sure what I need to be looking for on the board :coy:

Also, how does the ISC receive power? Are pin 2 and pin 5 just mpi 12v?

Thanks again for any input...
 

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If you are looking for the ISC drivers, they are the black rectangular components squared in red in the attached picture of your ECU. They will have a bulge on them or have some dis colorization if they are having issues. Also, the capacitors circled in red are the ones that typically leak. The one closest to the harness plugs (top in photo) can leak onto a trace that goes to the ISC drivers, and that can cause issues. Judging by the colors of the capacitors, I'd say they were replaced on your 90 ECU and probably not on the 91 ECU.
 

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Are you running all the mods that that 91+ ECU is modified for?
Yessir, injectors, fuel pump, mas ect.
Have you ever tried to adjust your biss? It sound to me like its falling off after it comes off of the fiav.
I have, but I didn't have a way to log the isc steps but I will have a data logging cable in tonight :hmm:
 
Alright! So I got some values from the logger.

Using MMcd

ISC - Key Switch On 102 and when the engine is running it goes to 32
Cool -
reads -59.9C so that's definitely a problem
TPS - 3.9-4.2% and WOT reads 97% the SAFC reads 0% and WOT 100%

I attempted to adjust the BISS screw and set some sort of a idle, however it affected the idle in no way shape or form. I followed this thread idle adjustment .

I didn't save the log file... I apologize in advance.

Thanks for any input
 
Try unplugging the isc and then adjust the biss screw.

Will do next time. I don't want to be that guy that says well everything worked perfectly before. :rolleyes: But Im gonna see about the coolant temp sensor reading first, the engine is a b**** to start up at first.
 
I just have to chime in here. I've had IACs test fine but still act squirrelIy I had an idle issue with one of my tsis and even though I swapped in a good working iac it still acted up but only in that car. Mind you i always make sure to upgrade to the black IAC (used OEM; ive had the standard intermotor fail out of the box) whenever I get a DSM but still have the occasional issue on the one. I've noticed the old tan IACs can act very bizarre even though they test within spec. If you still have the old type you should definitely upgrade before you fry your ecu.
Just for future reference, I get my IACs from this place (the black IAC was available for 3kgts before DSMs). Can't beat a lifetime warranty on a used part!
https://m.ebay.com/itm/Mitsubishi-3000GT-IAC-91-99-IDLE-AIR-CONTROLLER-Control-Valve-Stealth-MD628053/162714925573?epid=1224952746&hash=item25e290ba05:g:~ecAAOSwlQlZ5S3A&_trkparms=pageci%3A83c098fb-bd75-11e7-b335-74dbd1800431%7Cparentrq%3A6d73834315f0aca4ad8b374afffb8ec1%7Ciid%3A25
 
Checking in, not much of any progress has been made. But I'm going to continue to follow up on this until I get this issue sorted out.

I just have to chime in here. I've had IACs test fine but still act squirrelIy I had an idle issue with one of my tsis and even though I swapped in a good working iac it still acted up but only in that car. Mind you i always make sure to upgrade to the black IAC (used OEM; ive had the standard intermotor fail out of the box) whenever I get a DSM but still have the occasional issue on the one. I've noticed the old tan IACs can act very bizarre even though they test within spec. If you still have the old type you should definitely upgrade before you fry your ecu.
Just for future reference, I get my IACs from this place (the black IAC was available for 3kgts before DSMs). Can't beat a lifetime warranty on a used part!
https://m.ebay.com/itm/Mitsubishi-3000GT-IAC-91-99-IDLE-AIR-CONTROLLER-Control-Valve-Stealth-MD628053/162714925573?epid=1224952746&hash=item25e290ba05:g:~ecAAOSwlQlZ5S3A&_trkparms=pageci%3A83c098fb-bd75-11e7-b335-74dbd1800431%7Cparentrq%3A6d73834315f0aca4ad8b374afffb8ec1%7Ciid%3A25
Thanks for the link

Ive read multiple threads pointing in different directions about the TPS and how much it should be open when closed. So the search will continue. Supposedly the reading on MMcd for the TPS is supposed to read 10% but mine reads about 4%. I cant figure out if the reading is in relation to the sensor adjustment or the actual butterfly adjustment. I'm probably gonna do the worst thing and throw money at the problem until its fixed :rolleyes:. But yeah, I think its time to buy ecmlink, since Im not planning on getting rid of the car anytime soon..
 
Last edited by a moderator:
According to the factory, a 1G the TPS should output 0.5v when the throttle is closed. That's the electrical adjustment and works out to be 10% of the possible 5v max output.

The TPS adjustment procedure starts with a mechanical adjustment to make sure that the throttle butterfly is closed against the Idle Position Switch and that the IPS is correctly set since it acts as the throttle stop.

If you don't already have them you should get a copy of the factory manual for your year and a copy of the 1990 technical manual that explains how many of the systems work.
 
Update, got the engine to idle.

Ok, so I've follow the steps for the TPS adjustments and for the BISS adjustment. This resulted in no change.

So out of curiosity, I switched back to the 1990 ecu, pin swap of course, and no change. I thought, well this won't really work at all because of the injectors, afrp, ect. I took away -30% fuel on the safc cause I figure it is running rich. And that seems to have fixed it, the word fixed being used very loosely haha. The idle sticks around 700 and 850.

Evoscan was giving me these readings while idling

TPS - 10%
TargetRPM- 780
ISC Steps - 28-35
CoolTemp - 80C

So far it hasn't stalled from things such as Power Steering, Alternator pulling (like headlights), or braking.

Now I need to figure out whats up with the 1991 ecu since it has the eeprom chip with the tune on it. Too many mysteries for a conclusion just yet, but my guess for now is too much fuel.
 
It seems like an ecu issue, I know when I had an idle issue it turned out to be the isc and my o2 sensor wire had been partially burned through and that caused my car to rev high and surge which ultimately made me delete the FIAV and isc. let us know if you figure it out.
 
Update!

Been driving the Talon around every other day, but not hard just to keep it moving. I put the 1991 ecu in and got it to idle better but still not great.
I was driving and a cylinder started to miss and then it felt like two cylinders missing one minute later and then boom no power. The engine starting braking and I pulled off to find nothing at first glance in the engine bay. For sh*ts and giggles I plugged in the 1990 ecu, took some fuel away from idle, and fired right up. I guess the 1991 ecu is/was bad. Im going to send my 1990 ecu off to get it socketed so I can use the old eeprom or get ecmlink. I feel like I should find some stock cams, injectors and frp to set the car back to a "stock" platform and rule out all the unknowns.

thanks again for all the input
 
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