JM wrote:
> Ok, so I'm still trying to figure out my hesitation off idle in my 1999
> Altima.
>
> I've checked for vacuum leaks and I'm relatively sure there are none.
> I've checked the fuel pressure, and it's within spec. The regulator behaves
> as described in the service manual when I disconnect the vacuum line.
> I tested the MAF sensor and its output voltage rises smoothly with any
> increase in throttle opening.
> I tested the TPS and it operates smoothly and is within spec.
> The coolant temp sensor and intake air temp sensor both are about right
> according to the service manual.
> It has clean plugs, newish rotor, cap, and wires, and is not burning oil or
> coolant.
>
> There are no trouble codes stored, and aside from this problem and having a
> slightly high idle, the car runs smoothly and has good power at all other
> revs.
>
> The problem manifests itself as a drop in revs/stumble when I tap the
> throttle off idle. It's most obvious with small abrupt throttle openings,
> the kind you would use to take off from a stop (it's a manual trans).
> It's more severe when the car has been started between 5 and 10 minutes of
> having been shut of and is still warm. With a shorter or longer stop, it
> doesn't tend to be any worse than normal.
>
> I tested the O2 sensor as well as I could with the tools I have; my analog
> meter's lowest range is 10V, so it's hard to accurately monitor the sensor
> with it, and my digital meter I suspect is a bit slow to show the voltage
> changes from the sensor. But, with the digtal meter, the voltage seemed to
> settle around .2 volts at idle, and would spike up to around .7 if I revved
> the engine. I measured the resistance of pins 1 and 3 (the heater element I
> suspect) of the sensor, as per the service manual's instruction, and it
> reads almost 8 ohms hot, and 4.6 ohms or so at around 5 degrees C.
>
> The service manual says it should fall between 2.3 and 4.3 ohms at 25 deg.
> C, so I assume the idea is the resistance increases with temperature. If
> that's the case, then at 25C, I would expect the resistance to be more than
> 4.6 ohms as I recorded at 5 degrees, so that is somewhat suspect.
>
> I priced a new sensor at around $100 CDN today from the dealer, so I'm half
> tempted to install one. If it doesn't fix the problem at least it will be
> new and might help my fuel economy a little although it isn't poor to begin
> with.
>
> Any thoughts/suggestions as to what else I might look at?

"Hesitation off idle" is all I need to see to point you towards the
most likely cause of your problem.

Your 2.4L has an EGR valve that feeds exaust gas to all four cylinders
individually via cast runners inside the intake manifold. When some
but not all of these runners clog up, the remaining open runner gives
all of the exaust gas to it's cylinder. This results in a small
misfire or stumble only during EGR operation. The EGR valve doesn't
open at idle; it starts to operate off idle from approx 1500 RPMs and
higher during light to moderate load conditions. If those conditions
match your car's hesitation enabling conditions, then we're on to
something.

To see if this is your problem, simply remove the vacuum hose from the
EGR valve and test drive the car to see if the problem has disappeared.

To repair this condition, you will have to remove the intake manifold
and find and clean these runners with some sort of flexible pipe
cleaner and carburetor cleaner. If your manifold is a two piece design
you may be able to remove just the plenum and get to the passages that
way.

As far as the high idle goes, it's most likely a vacuum leak such as
the PCV hose, brake booster, etc.

Toyota MDT in MO