Hey Guys,

I have a 00 Si. The motor is stock aside from an AEM intake. Ive been having these problems with the motor stuttering. It happens when Im at mid rpm range, like 3k-5k (for the most part), either accelerating or cruising on the highway. Suddenly the engine will start decelerating, kinda like as if the car has run out of gas, or like one of the cylinders went out. When this happens, the engine is not making enough power to accelerate, even if I really give it throttle. It lasts either for a few seconds happening quickly on and off, or just the occasional stutter/hickup. One time it was particularly bad though. I started the car up in the morning, and the car was shaking like some cylinders werent working. I saw smoke behind the car, but i wasnt sure what color it was since the windows were fogged. The engine was running so badly, and i gave it gas and even at full throttle it was struggling to increase revs, and slowly made it to about 2500rpm in neutral before i decided to stop torturing it. I just left, as I was in a rush, and about a minute after starting it the engine returned to normal.

Ive tried a bunch of things already. The car has 89,000 miles on it, but the problem has been there for easily the last 30-35k miles. It comes and goes, so its hard to pinpoint a cause. I have changed the plugs many times, i have changed the distributor cap and rotor, the plug wires, and the fuel filter twice. At one point during an oil change, i asked mechanics to look into it themselves, as i was stumped, and they couldnt replicate the problem so they couldnt fix it, but they said that the plugs showed the engine had been running incredibly rich at some point, which i presume is when all this stuttering occurs.

Today I had some thoughts about the possibility that all these things were ok, and that maybe some sensor was going bad and so it was forcing the ecu to do silly things every now and then, but im just not sure. Its a strange problem, and what makes it stranger is the randomness with which it occurs. Its happened a few times to me on the highway with cruise control on - suddenly the car will just start slowing down and i can hear the cruise trying to maintain the speed. As far as gasoline brands, I have tried to stick to a "good" or rather expensive brand such as chevron or shell, so I doubt im running into 2 years of bad gasoline. Im really stuck otherwise though - the problem is so rare that it cant be replicated for anyone really, so Im wondering what you guys on here think it could be?

Thanks for reading my post!