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Re: '87 5KCSTQ: What sensor is this ?

> The $64,000 question of the day is, what is that small threaded
> sensor on the top right front of my '87 5KCSTQ engine?  It goes into
> the aluminum block, is upright and has one blue-green wire going to
> a single brass connection terminal on top of it.  It is above what
> (I think) is the coolant temp sensor.  That one has two wires going
> to it. Is the single wire sensor the ISV temp sensor?

Yes, the top single wire sensor is used by the ISV control relay. The
same type sensor is mounted below CYL #1 and is used by the ECU. The two
wire sensor is for the after-run fan, turbo cooling pump operation. My
web site has some more info on testing these sensors. Check out:


Your idle problem could also be caused by an intermittent throttle
switch (idle portion), check out the throttle information on the same
page listed above.

Scott Mo.
1989 200TQ