calpatriot

My car has always displayed a 'flat spot' in the 3600-4200 rpm range under heavy acceleration. Recently I got it to go (almost completely) away--while working on something entirely different.

Long story short, I solved a drivability problem by identifying and repairing an open in the O2 sensor circuit. But, with the car now operating in closed loop mode, the 3800 rpm hesitation was *worse*--a lot worse. In fact, it was 'throw you against the belts' worse.

I then started down the FSM troubleshooting guide for loss of power. I adjusted the idle speed, and idle mixture. Checked both side of the TPS with an ohmmeter, and then set it to exactly 1k ohm at closed throttle. I checked the timing (it was ok) and the operation of the advance (also ok).

Having done that much I decided to take it out for a test drive to see if there was any improvement. There was. The hesitation was almost completely gone. Most of the time it is not even noticeable; the others it is very quick and not rough at all, on the order of 1/4 of a second, and so light you might not feel it if you were not looking for it.

Verified the operation of the closed loop system with, remoted to the cabin, the 'green light' (rich-lean) test point, and with a voltmeter attached to the O2 sensor signal.

The hesitation that remains coincides with a very short dropout of the O2 signal from a norm of between .56 to .7 volts (anything abovec 450 mv is supposed to be rich) down to about .15 volts (very lean) but just for a small fraction of a second.

Bottom line, the car drives better than it ever has.

If you are having trouble with the hesitation, consider doing the checks: closed loop operational check, idle speed, mixture, and TPS.