JM wrote:
> > wrote in message
> oups.com...

> > The O2 sensor has nothing to do with fast changes like acceleration,
> > and a 1999 Nissan should have a function to check the duty cycle of the
> > O2 sensor while the engine runs.
>
> What if the ECU has richened the mixture enough (or leaned it enough) to
> affect off-idle performance? The test function shows about 10 crossovers
> (if that's the right term) in 10 seconds, but at idle, maybe 1 in 10
> seconds. I was doing some reading that indicated a weak heater could cause
> bad response at idle when the exhaust stream isn't heating the sensor
> optimally so I half wondered it this might be something worth considering.

I'd look at the O2 sensor directly, either with a digital meter that
includes a bar graph display (responds faster than the digits) or an
oscilloscope.

> This also would explain why the problem is more noticeable after a semi-warm
> restart since the heater might not work at idle speeds. A lot of
> supposition, yes, but I don't have much else to go on.

Most heaters are run all the time.

> Already checked the TPS,

How did you check it? It takes only one bad spot to cause poor
response. Turn it very slowly and check for smoothness of resistance
change. Better yet, inject a weak signal into it and look for noise on
the scope as you turn it.

> and fuel pressure (not sure if that = delivery rate or how to test rate),

Pressure and delivery are different. You measure pressure with a
guage, delivery rate by running the pump for 30 seconds to see how much
gas it pours into a coffee can.

> and the pressure does hold after shutdown.

How long? An hour?

I hope you have a factory or Mitchell book because for fuel system
problems they're far better than a Chilton's Hayne's. I'd also check
www.nissanforums.com and www.nicoclub.com. One of those has
factory-trained mechanics.