I would bet my money on the problem being an defective idle switch assembly which is mounted
on the throttle body/shaft.
My 86 5000CS T and my more recent 89 200TQ both had this same intermittent idle speed
problem. It drove me nuts for some time because it would never do it long enough for me to get
out and check underneath the hood.
I finally found after doing an ohm meter check on the switch that the idle contacts would have
anywhere between 2-5K ohms of resistance and sometimes the resistance measured even
higher. The computer gets confused when it sees this sort of high resistance when the resistance
is normally supposed to be around 1 Ohm.
I cut the plastic cover off the switch and found that the solder joints connecting the
external connector pins to the internal micro-switch were cracked and would intermittently
make connection and then sometimes go open circuit. Re-soldering the connection eliminated
my problem idle condition for good. I would check this out before sinking money into any
sort of ECU repair or replacement.
From: Robert Myers <email@example.com>
Date: Thu, 29 Feb 1996 16:33:23 -0500 (EST)
Subject: Surging idle