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Old 12-14-2015, 10:05 PM   #4
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Join Date: Jan 2006
Location: NYC
Posts: 43,523

2003 L-Series 3.0L Sedan
Default Re: Sudden idle drop, cleaning EGR & throttle body did not correct.

Correct me if I'm wrong.

1- When shifting from neutral to a gear, the engine isn't engaged to the clutch/xmission so the engine is free revving (neutral) until the clutch is engaged (in gear). Whenever the clutch is disengaged, the engine is free to rev. When the xmission's in neutral the engine is free to rev. No gears are engaged so the entire engine/clutch/xmission input shaft is allowed to run freely.

2-If there's something wrong with hydraulics like a worn slave, master cylinder or leak, gear shifting will not be easy as the clutch is still semi engaged. This would mean the xmission is never fully disconnected from the engine since the clutch isn't completely disengaged. If the xmission is never fully disengaged from the engine, gear shifts will be difficult as the xmission is still turning at engine rpm. This is the classic worn out clutch or hydraulics where difficulty is encountered trying to shift into first gear. The clutch and hydraulics must be in good operating condition to avoid typical end of life situations when either the clutch or hydraulics are worn out.

The EFI system may not be well understood by many, especially when shifting at speed. Emissions control programs and practical engine running prohibits full throttle closure while shifting at any speed. If throttle were allowed to close completely while at any rpm, the engine would starve itself of air and immediately shut down. EFI systems cope with (semi) closed throttle by commanding the idle air control valve to retract and allow air thru and around the throttle plate, supplementing the air flow. This results in a high idle some have noticed when disengaging clutch and either holding clutch down or shifting into neutral and coasting. The high idle, approximately 1200-1500 rpm is programmed to ensure the catcon doesn't cool down. The high idle returns to normal when speed drops below 15 mph. There are circumstances when a worn out, faulty or sticking iacv causes unusual rpm changes.

The easiest way to test iacv function is with a warmed up engine idling at correct (800-900) rpm. With the throttle accessible, use a finger to block the small port in the throttle body (in front of the throttle plate). The rpm should drop immediately down to around 600 rpm. Blocking off the supplemental bypass port where the iacv controls the air flow thru the blocked port only allows air flow thru the semi closed throttle plate. This is the factory setting.

When the bypass air port is blocked, the pcm detected the lower idle and already commanded the iacv to retract completely. Since the port is blocked, nothing occurs until your finger is removed. The moment the port is unblocked the sudden in rush of air raises idle rpm. This sudden extra air flow from the bypass air port allows more air in until the pcm reacts to the sudden rise in rpm by extending the iacv. The drop is rpm back to normal idle is the pcm commanding the iacv to close off part of the passageway until normal rpm is once again established. The iacv reacting quickly is indication of a good one. A sluggish iacv will react slowly. A faulty/broken iacv will act differently from what's described; either stuck in one position with a fixed idle number or raise rpm and not return back to normal idle. The iacv must retract and extend quickly when manually testing its function as described. Simply blocking and unblocking the bypass air port should result in dropped rpm and sudden jump to a higher rpm before returning idle back to normal. The description of the iacv has no effect on clutch use since the xmission is presumed in neutral to allow idling. In normal circumstances, an idling engine doesn't change rpm when shifting in and out of gear while parked (clutch disengaged).


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