Q: I've been having this problem for a long time now, and it's just getting worse. I have a 1991 Nissan 300ZX non-turbo auto. Got about 105K on it. Engine modifications as follows: K&N intake (not just filter,) Stillen high-flow headers, Stillen cat-back X-pipe exhaust, and a grounding kit. Otherwise good shape, it hesitates, and I can't seem to reproduce from any specific conditions. Happens every time it's driven. When I accelerate, it gets to about 2000 RPM, it starts to bog down. Like it will be a healthy acceleration until that point, then it will drop RPM a couple hundred, gain a few hundred more, start to drop again, gain some, back and forth until around 3000RPM then it accelerates hard. All with the throttle pressed down an even amount the whole time. I've even noticed it backfiring when I let off the throttle quickly. This is driving me nuts and makes it unpleasant to drive. Any help is very much appreciated, and I don't have much money for trial-and-error.
A: I have a 90 that did the same thing and it turn out to be my Crank Angle Sensor. You can run the ecu for codes also. Heres the link: http://www.ttzd.com/tech/diagnostictech.html