I really need a Moderator to make this one a sticky:
When the Boost Display reads 0, but you still seem to have normal boost command:
If the ECM detects a slight variation between anticipated MAP pressure and observed MAP pressure:
1. The ECM assumes there is no mechanical failure in the air charge system (IE: Loose clamp, crack in the IC, etc.)
2. Therefore the ECM believes there may be a failing sensor.
3. Our cars have a three sensor system: MAF, TMAP, MAP. The ECM is capable of bypassing the TMAP and running the car on the MAF and MAP only. It will do this first, and run the car in this mode for a few key cycles. It does this as a test, to see if by running the car on the MAF and MAP only; the slight discrepancy between anticipated MAP pressure and observed MAP pressure goes away.
4. While in this "Test Mode" during a few key cycles, your Boost Display will not function. Because the Boost Display gets it's data directly from the TMAP, which has been bypassed.
5. If the ECM determines that the TMAP is faulty, via this test: It will throw a CEL and code for bad TMAP. (This almost never happens, and even if the ECM does decide it is a bad TMAP; it probably really isn't the problem anyway)
6. After this "Test Mode" is run for a few key cycles and the ECM is still detecting the discrepancy between anticipated MAP pressure and observed MAP pressure, even while running on just the two sensors: Then the ECM will throw a code for the air charge system. Most likely P0101.
7. Eventually, the ECM will return to three sensor mode and your Boost Display will magically start working again.
8. As the detected air charge discrepancy gets worse: Eventually the ECM will throw the car into "Limp Mode Light": Electronic Boost Command will be disabled, so you will only be able to pull about 4-5 PSI of boost. All other functions will be normal. There may be no CEL, but there will be a stored code. (commonly known as a "silent code")
9. Eventually the detected air charge discrepancy will get bad enough that the ECM throw the car into "Full Limp Mode": All Boost Command is disabled and the WG is locked open, you will pull no boost. Throttle command will be reduced to about 25%. You will feel like you are driving in wet cement. There will be a CEL and stored codes.
Be aware that many GM Techs have no idea that this is how our ECM works. I literally had a GM Tech replace my entire ECM because "It has to be bad because it won't show any boost command". I tried to explain to him that the ECM was currently running in "two sensor mode" so therefore it was not reading the TMAP and so no boost gauge was being displayed. Of course he didn't want to believe me or bother to learn for himself and changed the damn ECM anyway. And of course I was back in a couple weeks later with the same fricken problem, that he failed to properly diagnose originally.
Based upon your observation of "A faint gulping sound when you let off the throttle": You most likely have an air leak in the air charge system somewhere.
Most likely air leaks in air charge system:
1. If you are still running the OEM clamps on the air charge system: Then the clamp on the Throttle Body has probably failed. Replace with a good quality T-Clamp. If you have a leak there, it will gulp air big time.
2. If you are still running the OEM Intercooler:
2.1. Possible crack in the end tank seam on the passenger side, of the IC.
2.2. Possible "Mushroomed" IC, in the center.
This thread pretty much covers anything that can go wrong with a Kappa, knock yourself out: GMPP Tune and P0101 code. - Saturn Sky Forums: Saturn Sky Forum