Im having trouble getting my ISC to stay around 30 at idle. I rebuilt the TB with new gaskets, shaft seals, ISC valve, BISS screw and O-ring but on warm or cold idle the ISC position drops slowly to 0.
Ive gone through the idle troubleshooting pages I could find, but nothing seems to fix this. BISS is almost bottomed out, like 1/2 turn backed off.

ISC ohms at 39.3 38.5 38.5 38.5
Throttle cable is adjusted properly
TPS is .63v at 0 throttle
Idle switch is controlled by Link

I installed a FIAV block off plate that keeps ISC as a last resort but it didn't change anything. I feel like I'm either overlooking something simple or I need to scrap the whole TB and start over.

Thanks for any help.


History on the car:
It sat for 8 years,, decided to get it running again and have been going through and replacing everything. Cut out the old fuel system and put in new rewired pumps, lines, injectors, fuel rail, afpr. Should've seen the crap I scraped out of the tank! Replaced valve seals, all timing components and water pump, no balance shaft. Replaced the engine and dash wiring harnesses, previous owner had them all sorts of hacked up.

Profile is up to date

1). No boost, vac, or exhaust leaks.
Holds pressure well at 25#
Valve cover vent is a breather
2). Verify mechanical timing.
Timing is correct and all components are new.
3). Verify base timing
No Timing light, will get one if needed though.
4). Ignition system
New BPR7ES NGKs at 28
5). Motor health
Good engine, ~82k miles
6). Basic throttle body adjustments.

This includes properly adjusting and setting the idle switch, throttle cable tension, TPS (Non-V3 adjustment and V3 adjustment), and BISS (1g and 2g). There is a specific procedure for adjusting each of these, and they are critical to getting the engine to run right. If you don't know how to properly adjust these, learn. Don't guess at it or assume they are adjusted properly. If you are experiencing idle issues, please make sure to test your ISC, and check out this page.

7). Compression ratio
170-175 on all 4
8). Wiring and sensors
Replaced harness with uncut harness from Miller
9). No DTC/CEL codes
No codes
10). Electrical system
New Odyssey battery and 90amp alternator
11). Base fuel pressure and injector values
Idle with vacuum line off is 43psi
12). Properly calibrated and configured wideband sensor
LC1 wideband wired into rear O2 ecu pin, no front o2 simulation
13). Type of fuel
Sunoco 260GT 100 Octane

If the ecu is commanding an iscposition of zero then you have a big vacuum leak allowing in excess air. The ecu will demand less air through the isc hence the lower iscposition value. Also, your isc drivers in the ecu could be bad which will lead to this behavior too. Pull your ecu and check for burning/bubbles on the isc drivers. If they’re bad, send in for repair.

Vegas Smith said:

If the ecu is commanding an iscposition of zero then you have a big vacuum leak allowing in excess air. The ecu will demand less air through the isc hence the lower iscposition value. Also, your isc drivers in the ecu could be bad which will lead to this behavior too. Pull your ecu and check for burning/bubbles on the isc drivers. If they’re bad, send in for repair.

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Thanks for the quick response. I went ahead and did another boost leak test and found a small leak coming from my newly installed fiav plate. PSI dropped from 25-10 over 45 seconds or so. I'm going to track down some allen head screws for it and cut another gasket to try to fix that.

I pulled the ECU and it doesn't look terrible, but I think something definitely happened. The board is a little discolored around the ISC driver and the case looks like something popped in that area. I'll try to get ahold of them Monday and see about getting it checked out. Thanks again.

edit: I wonder if the driver blew in the past and was repaired, it would explain the board discoloring and jumper wire without visible damage no?

Yes, that ECU had blown a driver and burnt the circuit trace for the ISC back in 2007.