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DJCharlie 11-06-2019 04:39 PM

RPM jumps up and down when starting
When i start my 96 SC2 the RPM starts at like 2000 and then drops very low for a second before it starts idling normally. If i dont light hold the gas down when I start it half of the time the rpm will drop to 0 and the car will stall. Does anyone know what problem i'm having. I know I have an exhaust leak somewhere idk if thats related to this.

leviklump1133 11-06-2019 07:10 PM

Re: RPM jumps up and down when starting
I would clean the Throttle body, if that doesn’t work maybe replace the TPS sensor?

My Saturn had a similar problem, but would rev to 2000rpm and not go down at all ( when first turned on). I replaced the TPS sensor, pulling one from the junkyard, and it has worked well for over a year.

fdryer 11-07-2019 07:25 PM

Re: RPM jumps up and down when starting
DJCharlie, what's normal idle rpm after the engine warms up? Did anyone meddle with the (factory adjusted) throttle stop screw? An exhaust leak doesn't affect idle rpm in the way you described unusual rpm drops.

DJCharlie 11-07-2019 07:55 PM

Re: RPM jumps up and down when starting
After the car warms up I think it idles at 800 rpm which seems normal. Its only the first second of starting the car that the rpm drops. I dont think anyone adjusted anything but i have a sticky gas pedal so ill try cleaning the throttle body

fdryer 11-07-2019 10:08 PM

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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