Fluctuating Idle, Help!

[Jim Bush]

Hi all,

I have a '89 200TQ with 165,000 miles, I have had it for about 6 months
and got all (well, all critical) bugs worked out. It has been running
fine for 2 months now.
I changed the plugs, cap and rotor and the 4 large Rad. hose clamps an
evening last week. Drove to work the next day (about 35 miles), around
at noon and about 5 miles back home when the trouble started. The idle
would go down to about 500 RPM and then pop back up to about 1200 RPM
and cycle like this until it would not be able to "catch" it and stall,
but that was only when my foot was off the pedal. It ran fine as long as
I was touching the pedal: acceleration, boost, drivability. It seemed to
happen all of a sudden.

I got home, cleaned the ISV (again) replaced it, no change. Then I got
to looking around and found that the large coolant sensor under the
upper rad. hose (near the motor) the rubber boot was filled with coolant
so I drained that dried it out replaced it, no change. In doing this I
found that it would idle fine until it warmed up. I then looked at the
idle switch (front of the intake) it would idle poorly then I unplugged
it and it ran fine for a while and then the problem started again,
plugged it back in. I then unplugged the O2 sensor and it ran fine for a
while and then started to idle poorly again (the same way) on the test
drive. I also replaced the ISV with an assumed good one. They both
seemed to be working correctly, and no difference in the idle problem.

One other thing that was found is that there is a vacuum leak, not sure
if this plays into it or not.

I will pull codes tonight and post them, but does anyone have an idea??


Sorry for the long post but Thanks in advance!

Jim Bush
Holland, MI