Low Idle

[ksassy]

I have an 1996 I30 with 65K miles. The idle is approx. 500 rpm in Drive with no accessories on and warmed up. At a stop light it feels like it wants to stall. In Park (no accessories) or in Drive with the A/C on the idle is approx. 700rpm. Within the past 10K miles I have replaced the spark plugs, air filter, fuel filter, pcv valve, cleaned the throttle body, run a can of BG44K and had the transmission and coolant flushed. This problem is still unresolved after a trip to the dealer (only one in the area). I noticed that the idle stop screw on the throttle body (not the idle adjustment screw) has been tampered with. Per my Chiltons and Haynes manuals this screw is set at the factory and should not be adjusted. Is there a procedure to check the setting of this screwand adjust it back to the factory settings? Could this be causing the low idle condition? Any help would be appreciated.

[Mayhem_J30]

i would imagine regardless of the screw position the ECU would make sure the motor idles at the proper speed. Really though, 500-600 sounds about right for idling in gear. Sounds like you've already done a lot of the proper things to fix an idling problem. If the idle actually is off(i'll go check what mine is tonight) you can probably still check the MAF, knock sensor and O2 sensor to make sure they're properly functgioning. I would think the CEL would light up if there was a problem with any of these.

[Q45tech]

The idle air ecu controlled bypass circuitry can only do so much to add or cut air [especially the latter] as it is designed to operate at 10% extra in park, no ac load no power steering load . When the AC is turned on it should go to a 25-30% duty cycle AND the injector duty cycle will go up by 10-12% to supply the extra power needed to turn the compressor and MAINTAIN THE SAME idle speed!Additionally the ecu adds/subtract degrees of intial timing to ease the fine tuning of this process.When you shift to the severe load of DRIVE it takes around 40% IAC, 0.3 msecs additional injector opening and a 5 degree EXTRA timing advanace to allow the engine to run will only a 100 rpm drop [normal amount 750 idle in Park drops to 650 rpm [+- 25]. The above is very coolant temp dependent!

You could reverse engineer the proper TB setting IF you are sure the TB, Plenum, injectors , IAC, and all hoses are perfectly clean by disconnecting the IAC and setting the manual screws so that the idle is 675 rpm in Park then when you reconnect the IAC its duty cycle should be 10% lots of trial and error iterations here.

Fairly common in Shade Tree Mechanic Land to fiddle with screws they shouldn't cause they don't understand the technology or don't have the proper test equipment.....when nothing happens [because the ecu is fighting the adjustment] they keep turning things.

It took me 2 days [because somethings must be adjusted cold and others hot] to get a Q45 TB readjusted after an owner fiddled with it in hind sight we should have just replaced it!......as no way can you get it right in a 30F variation when it is designed to control...0F to 120F ambient.

The service manual has a few pages on resetting it close!