ISC or ECU issue??

[DJ23GSX]

Today my car started idle surging for some reason so i pull out my pocketlogger and noticed that my isc is pulsating from 5 all the up to 77 at idle when its surging. My car ran and idled great since i replaced my old isc a year ago. It was surging so bad i decided to just unplug the isc and not risk frying my ecu and when i did it idled great again. Nice and smooth no stalling but i noticed that on the pocketlogger the isc was still pulsating from 5 to 77. Now the pocketlogger reads what the ecu is trying to do with the isc correct? If that is true what could be making the ecu want to go that crazy with the isc? Its a motoguys rebuilt ecu i purchased a few months ago so i am hoping its not fried.

 

[vc_wannabe]

try cleaning out the isc

 

[DJ23GSX]

I reset the ecu last night and left the isc unplugged and for some reason the ecu was still getting stuck trying to open it 77 steps but after grounding the plug on the firewall and adjusting the biss screw to idle at 750 it started acting normal again and still seems to be fine today but i'm still not sure if i should hook the isc back up. The car runs and idles very smooth without it.

 

[doggo]

I reset the ecu last night and left the isc unplugged and for some reason the ecu was still getting stuck trying to open it 77 steps but after grounding the plug on the firewall and adjusting the biss screw to idle at 750 it started acting normal again and still seems to be fine today but i'm still not sure if i should hook the isc back up. The car runs and idles very smooth without it.

Then positions you are seeing are just what the ECU is TELLING the ISC to go to, not where it actually is. The DSM ISC does not provide feedback to the ECU, so the ECU has no confirmation of where the ISC is actually positioned. It just sends a postion signal to the ISC, and expects the ISC to make that position.

The fact you are seeing bouncing ISC position via the ECU indicates to me the ECU is trying to compensate for some problem it is seeing.

 

[DJ23GSX]

can someone please tell me what things would cause the ecu to go crazy with the isc?

So far i've:
boost leak tested with probly as close to no leaks i've ever seen on a dsm
compression test 160+psi on all four
adjusted timing (tried 7* 5* and 3* none helped at all)
swapped out isc and tested both and both checked out good
cleaned tb spotless
reset ecu numerous times
rebuilt ecu


the one thing i've noticed that seems to happen around the same time is at idle i start to lose vacuum and the car almost stalls. Some days it idles great vac at 20hg no problems then other days while the car is warming up the vacuum drops from 20 down to 14 and the car barely runs. Usually it goes away once the car is warmed up also if i push the gas while the vacuum is dropped it stumbles a bit and backfires before revving up normal.

Both of these issues just popped up one day for no reason. I haven't modded or tuned the car in quite some time so there is nothing i did or changed that could of caused this.

 

[NN4S]

Sounds like you need to look for a vacuum leak; a hose could be cracked and leaking when the conditions are right. That would definitely give the ECU a reason to try & compensate for it.

 

[silvermitsu10]

Sounds like you need to look for a vacuum leak; a hose could be cracked and leaking when the conditions are right. That would definitely give the ECU a reason to try & compensate for it.

My money is here as well. Start with all the lines at the back of the intake manifold and go from there.

 

[Artago]

My money is here as well. Start with all the lines at the back of the intake manifold and go from there.

I have the same problem as the OP (Although I have this problem with a new (rebuilt) ISC). I've seen several procedures on testing for vac leaks. What do you all suggest is the best/easiest way of going about this. Thanks, Tom.

 

[the rage]

I have used wd-40 spray a little in the area if the idle changes then do a visual check of the lines.

 

[Artago]

I have used wd-40 spray a little in the area if the idle changes then do a visual check of the lines.

Not a bad idea. I heard that people also used starter fluid to do this. Which would be better / safer to do?

 

[DJ23GSX]

Sounds like you need to look for a vacuum leak; a hose could be cracked and leaking when the conditions are right. That would definitely give the ECU a reason to try & compensate for it.

I thought all the vacuum lines were pressurized during a boost leak test?? I visually checked almost all the lines while i had the tb off.

 

[NN4S]

The lines may be cracked in a way that seals when forced outward (boost) but leaks when drawing in. Also, differing weather conditions will affect the amount of swell in the lines (as does the age of the rubber).

The WD-40 technique mentioned above is great for this test; it makes the leak easy to pinpoint and is much less volatile than using starter fluid (ether). An added advantage is that you can wipe things down with a rag as you go & it will clean things up (mmm... pretty vacuum lines! ).