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Old 11-07-2019, 10:08 PM   #5
fdryer
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Join Date: Jan 2006
Location: NYC
Posts: 43,512
 

2003 L-Series 3.0L Sedan
Default Re: RPM jumps up and down when starting

800 rpm is about right and indicates an unmolested throttle stop screw. The idle air control valve (IACV) in the throttle body is commanded by the pcm to control all idle speeds. It may be sticking. Removing it for cleaning off crud on the shaft may help. When the valve is extended, bypass air is closed off. When retracted, bypass air raises idle rpm well above 2k. A quick way to test and verify iacv operation; with normal idle and intake air tube removed to have access to the throttle body, place a finger over the hole in front of the throttle plate. That hole feeds bypass air around the almost closed throttle as supplemental air. This supplemental air is controlled by the iacv under pcm direction. When the hole is blocked at idle, the idle should drop to around 500-600 rpm where air is flowing thru the almost closed throttle plate. Remove your finger and idle should jump up immediately then lower back to normal idle. When the hole is blocked, the pcm detects lower rpm and retracts the iacv to allow more air thru the bypass air passage but the blocked hole is preventing air flow. When the hole is unblocked, a sudden rush of air raises rpm suddenly with the pcm detecting it and commanding the iacv to extend to choke off air flow to lower rpm back to idle. The drop in rpm with hole blocking and sudden jump in rpm with hole unblocked and a return to normal idle tests the iacv operation from commands coming from the pcm.

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