re. 5kcstq engine problem when cold

[Swann, Benjamin R. (BSWANN)]

I may be haveing the same or similar problem.  Engine slightly warm, cold
outside ~ 30-45F.

Upon letting off the gas, the engine drops to about 300 RPM, like it is
going to stall, then recovers.

I had corrected a fast idle/poor fuel mileage problem about 2 months ago
caused by the throttle adjust screw vibrating/backing itself out.  Put some
of my favorite black goo - aviation gasket seal, and seated the screw all
the way closed.

Then the car was running and idleing perfectly until about a week ago.

The problem is as you describe, intermittant.  I  check my vac. lines about
every month, almost all have been replaced and everything is kept tight.

Most vac leaks I've dealt with cause the car to idle fast.  Where would the
opposite occur?

<On two occasions, I've had a engine problem in my 5kcstq: the engine
idles extremely low or tries to die. It might idle at 400 rpm,
increasing a little as the engine warms up. It's not only an idle
problem, though. While driving, if I lift my foot, the deceleration in
gear is more sudden than usual and the stock boost gauge (I know,
inaccurate) read 0.0 bar. (It usually reads 0.2 bar or so near sea level
when decelerating in gear.)

This happened on two chilly evenings, a few weeks apart, but hasn't
happened otherwise.

I would think it was a vacuum leak except for the deceleration behavior;
my previous experience with vacuum leaks has been that you only notice
them at idle. The intermittent nature of the problem makes it hard to
diagnose, too. Any suggestions?

- Wallace
  '87 5kcstq 167k>