So, it seems like any time people talk about a loss in power, or engine hesitation, someone mentions that it could be the infamous "MAP sensor issue."

I'm just wondering if someone can explain how the ECU actually uses the input from the MAP sensor. Is it used to determine air/fuel mixture? Does it impact timing? How is it used, exactly?
Also, if the MAP sensor is giving false readings, should the impact to performance be consistent from car to car. In other words, if the MAP sensor is messed up, should we always expect to see a drop in power at exactly 5000 RPM, for example, or would some cars see a drop in power across the board, and other cars see entirely different results.

I ask this because I recently had a disappointing dyno session, where I lost a significant amount of power compared to my previous dyno session, with the biggest losses being above 7K RPM (15hp loss of peak power, and over 20hp loss near redline). At the time, I attributed the losses to heat soak, since my "poor" dyno had been performed during very hot & humid conditions (near 100F ambient temps, and 80% humidity). I assumed the ECU was seeing the elevated IATs and dumping in a lot of fuel and pulling timing to protect the engine (the AF chart on the dyno, definitely shows the car running richer above 7K).
However, the other night I took the car out for a spin, and ambient air temps were in the 70s... but still, the car felt sluggish. I knew that this shouldn't be attributable to heat soak, so my first thought was the MAP sensor issue (since it's such a popular topic here). So, I brought the car back home, performed the intricate and technical "MAP Sensor whack" procedure, and took the car back out. Maybe it was the placebo effect, but the car definitely seemed stronger after this.

So, I'm wondering if the results that I saw on the dyno are consistent with MAP sensor issues. Any input will be appreciated. For reference, here is the before/after dyno comparison:

 

Have you seen this? .................
http://forums.s2ki.com/forums/showthread.p...6&highlight=map

 

Oh, yeah... It had been a while since I had perused that. Thanks for pointing it out again.

While reading through that thread, it seems like the symptoms were much more severe than mine. In addition to the ones that noticed a rough idle, and even occasional stall, most people reported a hesitation or stuttering. To me, "stuttering" would be the feeling similar to what you get with a misfire, or some sort of semi-violent hiccup. My car did not feel like that. Mine was simply a consistent, smooth drop-off in power. In fact, the first time I noticed it, I was running up from about 3500RPM in 2nd gear to redline. 4000 -> 6000 seemed a little weak, but not terribly noticeable... however it seemed to take forever to get from 6K to red-line. At the time, I was driving into a relatively stiff head-wind, and I tried to tell myself that the slow accelleration was due to the S2000's poor aerodynamics, but in retrospect I don't think that was the case.

I hadn't read it previously, but this document, referenced in that thread, gives a pretty good explanation of how the sensor is used by the ECU:
http://www.autoshop101.com/forms/h35.pdf

That document seems to indicate that the impact of a bad MAP sensor can vary from car to car, but still I'd like to hear if anyone has experienced the same symptoms that I have.

 

My car often stalled at red light and feel like the fuel had been cut off because of no air coming through the intake. replace the Map sensor and it feel like brand new car (cost=$65.11). I tried the whack the sensor thing and end up destroying it. apparently the map sensor is sensitive. I could had saved myself a lot of headache had i just go out and buy a brand new sensor rather than whacking/breaking it and had to go without car for 2-3 days. luckily for me Honda came through and I am able to pick it up on the second day.