How does ECU decide how much air is needed during idle?

[freeonboard]

re 92 s13

In theory, ecu ignores o2 sensor readings so how does it decide on a good idle. My ACC is definitely working but it insists on being 50 percent open during idle which shoots my idle to 1000 rpm. If I unplug the acc, the idle is around 700 but the ecu cries for air by sending a message to acc to open up 80 percent. The ACC goes down to about 30 percent when I go into open loop. It stays at 50 percent open even when I go into timing mode. I have 4 computers and they all do this. I'm thinking I'm rich during idle but curious how the ecu determines this.

[wackawacka]

What is ACC? Cruise control?

Do you mean IAC?

[freeonboard]

Sorry, I meant auxillary air control valve. ie aac or iacv-acc. It is the valve controlled by solenoid to provide supplemental air.


Modified by freeonboard at 7:25 PM 5/8/2009

[wackawacka]

[Q45tech]

IAC duty cycle [10-15%] plus +- in ignition advance to hold a preprogrammed rpm. Ecu increases IAC in steps at some point it tries a 1 or 2 change in advance. The goal is to try to maintain rpm without exceeding say 50% IAC duty cycle.

5 grams per second of air needs 5/14.7 or 340 milligrams of fuei per second. 660 rpm = 1.1 revs per second so 2,3, or 4 [I4, V6, V8] fuel squirts per rev.

In most Nissan engines the cylinder volume = 500cc>600cc so every cylinder is within 20% so amount of idle air and fuel is within 20% at exactly the same rev. Add to that whether 2, 3, or 4 gulps per revolution.

Obviously a 2 liter I4 comsumes less air and fuel than a 4.5 liter V8 does at idle ....................1 liter vs 2.25 liters [air] per second, minute, hour is kinda significant.

One thing one must consider is that at idle the engine is restricted [throttle/IAC] to just about 20% volumetric efficiency so the cylinder only fills ~~ 100cc instead of its full volume of 500>600.

Newer 3.7 have greater than 600cc cylinders ~~616cc

[Q45tech]

The after hot warmed up proceedure is you set timing with a light, disconnect IAC connector, clean throttle body and plenum, then set IAC manual bypass screw out from bottom by 2-3 turns to precise set rpm per FSM. Then reconnect IAC connector.