> wrote in message
oups.com...
>
> JM wrote:
>
>> I tested the O2 sensor
>
> 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.
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.

> Look at the throttle position sensor, MAP sensor, and fuel delivery
> rate and pressures (pump and pressure regulator), and see how the
> pressures hold up when the pump is turned off, to check for leakage of
> check valves. Don't replace parts unless they test out bad, and don't

Already checked the TPS, and fuel pressure (not sure if that = delivery rate
or how to test rate), and the pressure does hold after shutdown. The MAP
sensor I haven't looked at yet, but I'll make a note to check it out if I
can.

> use a low-impedance analog meter (that is, one that doesn't need a
> battery to measure volts or amps (FET-VOMs are OK though), directly on
> an O2 sensor or you could damage it.

I know it's a bad idea to start replacing parts in an uneducated attempt to
fix a problem, but with 100,000 km on this sensor I figure it can't hurt.
I'm not about to start replacing every part under the hood... yet.

Thanks for your reply