Hi group,

I've got a 1999 Altima with 100,000 kms.  Ever since I got it almost 2
years ago, it's had a slightly too high idle (varies, but around 850-900
rpm) and it also has a stumble off idle.  The idle itself is more or
less smooth, although if I listen at the end of the exhaust it does seem
slightly irregular, but not by too much.

If I punch the throttle while it's idling, especially when it's just
been started (warm or cold) or before it's warmed up first thing in the
morning, it'll stumble for a second or so (the whole engine shakes), and
then rev up as expected.

When I do this, as soon as I open the throttle I can hear a sucking
sound from the end of the intake, and I can hear the injectors speed up
quite a bit, but it's not until almost a second has gone by that the
engine actually revs.

If I am less abrupt with the throttle, it'll rev just fine, and if I
punch the gas when it's up around 1500-2000 RPM, there's no noticeable
hesitation.

After the car's warmed up, the severity and delay decrease a fair bit,
but they're still noticeable.

I've always suspected a vacuum leak, but today I went out and
disconnected the intake tube from the MAF sensor, sealed it off with a
plastic bag, and then tried blowing into the tube that runs off to the
idle control valve.  My plastic bag inflated slightly, but there was no
obvious leakage from anywhere except a very slight and very slow hissing
from around the EGR valve area that I wouldn't expect would affect anything.

I then tried unplugging the coolant temp sensor.  This caused the idle
to be even higher, and amplified the hesitation off idle quite a bit.  I
know this is no way to make a diagnosis, but it seems suspicious to me.

I had it to the dealer last fall, and everything checked out okay -- the
TPS was within spec and the idle control motor was at 30% which
apparently is normal.

In the past I've checked the throttle position sensor and MAF sensor
ground, and both were fine, and I've replaced the plugs, wires, dist.
cap and rotor within the last few years.  I have tested the coolant temp
sensor a few times and it did seem to change from hot to cold, but it's
hard to get at with my multimeter leads and I wonder if it may be out of
spec just enough to cause my particular problem but nothing more severe
than that.

Can anyone suggest what else I should look at?  I hear the coolant
sensors are pretty common to go bad, and not too expensive, maybe I'll
price one and swap it out just to cross one more thing off the list.

Thanks in advance for any help.1