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CTS 2003 3.2 V6, Eldorado 8.2 v8 1974, SRX 2005 4.6 V8, STS 1993 4.6 V8, Chevy Van 1980 5.7 V8
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101 Posts
Discussion Starter #1
Hi all,

After really long search i decide to ask here about my problem, i have a 93 Seville 4.6 with the Northstar engine and i get a little issue with the idle that is slow to come down when i release the pedal, so when i release the pedal, RPM go down to about 1000 RPM, and just get up to 1200-1300 RPM and then slowly go down to normal (take about 3 to 4 sec.).

So, i check the ISC motor, because that was looking to be the problem, when i release the accelerator, i can clearly see the ISC get in action, he make the RPM go up to 1200-1300 RPM and i see it get slowly in to get the right idle speed.

I have a spare (but used) ISC motor, i install it, and that just do exactly the same....

The engine is fully rebuild from top to bottom with new piston 0.25mm over sized, all new valves, new valve seat, bearing, timing chain.....
I get this problem since i have the car, but 1st i was thinking that because the bad engine (was lot of vacuum leak, and really bad valve and seat, only get 75 PSI of 2 cylinder and other was not a lot better, that why i rebuilt the engine), but now that all is new, i get the same problem with the idle.
I already replace the PCM because of a really rough idle that i get too since i have the car, the idle was playing from 700 to 1500 RPM after 15 to 20min driving, so that problem was solved with the new PCM.

I check for vacuum leak, don't find any, i plug the EVAP system just to check if that the problem, and no, i change the ISC motor, and of course i adjust it according to the service manual and i do a IDLE learning like said in the service manual, to be sure, i reset the learning IDLE (override PS13 in diagnostic mode), so i get the P080 code to tell me to learn the idle and TPS, so i did it and the P080 go in history, but still exactly the same issue.
So i change the TPS and do a learning again, but no change.
And of course i check the pedal cable, that not the problem, he is moving freely and little loose when release, and seme for the cruise control, the cable is perfectly adjusted i little loose when ISC pin is retracted.

I check with the GM TEch all live data and all look good, don't see anything wrong, the switch in the ISC motor work nice too, all value of sensor is right, and all sensor are new.
When i play with the GM Tech IDLE testing, all is fine, the ISC motor can increase speed and go down fast with no issue, it is really that moment when the ISC get in action (when the switch is pressed on the ISC pin) that the moment all the problem happen...
I try to unplug the EGR valve, but no change, but maybe this EGR valve can be the problem ? i check it, and all is clean, of course the throttle body is clean like new too.
I try a power balance to see if there is not a bad injector, but all look perfect, and O2 sensor (the 2 are new) give the same value on the 2 bank.
The car run perfectly, i just finish the break-in and remove the break-in oil, all perfect, no idle variation, really stable when A/C is switch on, and go in P.... just slow down to idle issue.

Maybe is that normal on this car ? but i don't think.

I see there is a adjustment screw on the throttle body for the minimum air that should give a 450-500 rpm idle with ISC retracted, i don't think that the problem, because mine stall when the ISC motor is retracted manually and frozen bu unplug it, but i have no access to adjust it.

Thanks for help.
 

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Registered
CTS 2003 3.2 V6, Eldorado 8.2 v8 1974, SRX 2005 4.6 V8, STS 1993 4.6 V8, Chevy Van 1980 5.7 V8
Joined
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101 Posts
Discussion Starter #2
And i forget to say, this is a export model (i am in France), i don't think that really change anything, but who know.
Thanks
 

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Registered
CTS 2003 3.2 V6, Eldorado 8.2 v8 1974, SRX 2005 4.6 V8, STS 1993 4.6 V8, Chevy Van 1980 5.7 V8
Joined
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101 Posts
Discussion Starter #3
Oh, and a other try i did, i let the car tun at idle and unplug the ISC, and try that way, the problem is gone, but can't run that way, that just tell me the ISC get a wrong info or is bad (but as i say, look strange to have 2 bad ISC motor that do exactly the same), plus a check in internally (clean is, put new grease, check the gear and switch).
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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19,867 Posts
you don't simply bolt in a replacement ISC -

there IS a special procedure that you need to follow to SET-UP/ADJUST the ISC -

also - inspect/clean the TB - Throttle Body -
 

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Registered
CTS 2003 3.2 V6, Eldorado 8.2 v8 1974, SRX 2005 4.6 V8, STS 1993 4.6 V8, Chevy Van 1980 5.7 V8
Joined
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101 Posts
Discussion Starter #5 (Edited)
Thanks for help, but as i say, i did the learning procedure, no problem about that, and the engine is new (TB included).

I did the learning procedure (by put ignition on, go in diagnostic mode, ignition off for minimum 20sec, and that 3 time, then start engine en let IDLE to temperature, then go in D and let idle for 30sec, same with AC on and off.....) that all right, and to bu sure he did the learning, i first reset the previous learning (override PS13 with cooler button), so that set the code P080 and this code get in history when learning is done.

And 1st i did the ISC pin bolt adjustment like say in the service manual, by override to fully extend and retract it, and check difference of position in the live data (don't remember exactly the parameter number), difference must be about 10 to 11 count, so that done too and before the learning procedure.

At this point i order a new ISC motor, because i don't have any other idea, maybe the 2 i have are bad...
 

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Registered
02 Seville, 04 SRX (sold), 96 Eldorado (sold), 99 DeVille (sold)
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1,359 Posts
I ran into a problem with a replacement part. My RPMs were too high at idle. It was the ACDelco part being generic. When I found the actual Delphi manufactured version and used that, the problem went away.
 

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Registered
CTS 2003 3.2 V6, Eldorado 8.2 v8 1974, SRX 2005 4.6 V8, STS 1993 4.6 V8, Chevy Van 1980 5.7 V8
Joined
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101 Posts
Discussion Starter #7
Thanks for help, good to know, i order a new ISC, i will receive it today, will see if that better, i order the OEM but it is a ACDelco (not ACDelco professional).
 

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Registered
CTS 2003 3.2 V6, Eldorado 8.2 v8 1974, SRX 2005 4.6 V8, STS 1993 4.6 V8, Chevy Van 1980 5.7 V8
Joined
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101 Posts
Discussion Starter #8
Hi,

Receive the new ISC motor and TP sensor, but no luck, i install it and did the learning procedure, but still the exact same issue...

So i think i will check more on the EGR valve.

I can't really understand why the ISC motor to that !
 
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