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New Member. I have an idle problem with a 2003 with auto trans I just bought from dealer with 70k miles. Starts with normal idle rpm's when warm. When cold it fires fine and instantly revs to about 2500 rpm for 5 seconds, then slowly over 5 seconds slows down to about 800 rpm. I worry that sudden high rpm on dry engine is bad for it. Vacuum test is abnormal. At idle vacuum is about 10 inches of mercury and steady. Book says it should be 17 to 22. When throttle is blipped vacuum does not change at all--not to zero and not to 24 for a moment as books say they do. Smoke test shows no vacuum leaks. I cleaned MAF and throttle body plate, but that did not help. Could some kind of sensor effect this only when it is completely cold? I can't check codes because I foolishly left inside lights on and battery went dead 3 days ago, so no data there yet. Any ideas would be very helpful. Thanks, Lee True
Welcome to the Forum !!…a reading of 10 “inches of Hg” is good if you are using a scan tool…a pic of my MAP Data PID at idle…9.4 inHg….yes, 17-21 or 22 is good manifold vacuum…MAP/Baro is influenced by altitude and weather changes…my reading of 9.4 is good at my sea level reading….a normal key on engine off (KOEO) MAP reading at sea level is 29.9…you may have a bad MAP sensor.
Remember
…BARO-MAP= Intake Manifold Vacuum !!..in my case…
I realized I got vacuum at wrong point--off of hose on right valve cover. When I put vacuum quage on engine side of brake booster line the vacuum was about 20, on throttle blip it when way down, then shot up to about 23, then settled down to about 20. I did it again tonight after my post, and read vacuum properly, but still when cold the rpms shot up to about 2500 for at least 5 seconds. Sorry I did that first vacuum test wrong.
I am a novice at understanding manifold air pressure in this situation. I have never seen or used a scan tool of that sophistication. I just bought my first $20 dollar OBD tool!! I have to read about a MAP sensor on the 2003 Corvette and how it could effect my cold start rpms. Thank you for the picture and data. Randall
Last edited by Randtrue; Mar 12, 2023 at 12:29 AM.
Reason: to add more relevant information
Cold start idle problem
