If the idle speed is normal until you shift into a gear, the idle air control valve is probably raising the idle speed. Try unplugging the iac motor at idle then shift into drive.
If the IAC is raising the RPM the computer controls it, so for some reason the computer thinks the RPM needs to go up. I am assuming you do not have a check engine light on or you would have checked for trouble codes.
Just for the record, I am showing the same IAC for both transmissions on a 2.0 engine code 3.
Is it possible the battery was disconnected and the learn process needs to be done for the computer to know where normal idle speed is ?
SOURCE: buick lesabre nightmare
Need to get a scan tool and look at the data stream while it is running for alot more info.Also any trouble codes.Sounds like initially,the engine isn't getting any fuel.I would have a pressure gauge connected to the fuel rail on the engine,so i could monitor the fuel pressure while all this is going on.Items to look at on the data stream would be timing,what timing is the PCM commanding for the engine.Also,have you checked for continuos power feed to the coils during this no start condition,and fire at the plugs while cranking.You could possibly still have spark,but maybe it is extremely weak?I have seen bad ignition switches cause weird problems before too.Many things power up through the ignition switch,just a thought.Have also seen bad PCM's cause strange problems..38 volts is a little on the low side for GM.Usually see those at,at least .50 and up,and rise steadily while pressing the throttle,up to just under 5.0 volts.Potentiometer with 5 volt supply.Vacuum leak can be checked with engine running at a steady idle speed,spray brake cleaner all around the intake manifold and listen for engine speed change.
SOURCE: car runs rough at idle
clean out ur throttle body, just follow the hose from the air filter to the throttle body. Need 2 people to do this. Don't remove the hose completely from the throttle body. Just enuff to be able to spray ur intake cleaner into the throttle body. Do this especially if its worse when the motor is hot, as this would confirm the problem.
SOURCE: Service Engine Soon Light and Rough Idle (Below 1200 rpms)
do you get check engine codes?..have you done a leak down test and compression test to determine mechanical engine health?
SOURCE: PCV valve? IAC valve? Sparkplug wire? Throttle body? Vacuum hose?
the most common causes are plug wire, plug, vacuum leak at intake, fuel injector failure and last no/low cylinder compressio.
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