poor idle on 4kq

[Luke Rickert]

Problem with an 87 4kq:
poor initial idle

 I have a problem, which I suspect to be caused by a bad O2 sensor,
although at this point that is just conjecture. Any guidance before I
investigating further would be much appreciated.  Just to see what it
would do, I duct tapped up a rather large hole in a very deteriorated,
what I believe is called, “cold air inlet” a rubber ~3 inch tube that
makes a 90 degree bend below the air box.  Can anyone confirm that ID
and even give me a part number for that hose? I haven't been able to
make a positive ID with Bentley. Anyway the real problem is that now
that I have closed up the leak in the hose, the car idles very poorly
when initially started from cold. It starts ok and then immediately
begins to oscillate between ~1400 and 300 rpm with a period of a few
seconds.  At the bottom of the range it sounds like it is going to die
and I have to give it some gas to bring the engine speed up.  After it
has warmed up for a short time the oscillation goes away and everything
seems OK if perhaps a little rough. Does anyone have any ideas of what
to check?  I don't know for sure but would not be surprised if this car
were on its original O2 sensor.

Thanks for the help

Luke