The Nissan Path

87 SE engine revs up & down when idling. Revs ~ 300 RPM higher & lower ~ every 1.5 seconds. Happens ~ 30 seconds after cold engine start. Worse in cold weather. My dog raises & lowers her ears when she hears this rev cycling. What's the cause of this irritating problem?

My 87 did the same, I discovered that the wax thermo unit was out of adjustment.
Hope that helps.

For us non-gearhead types, what is the wax thermo unit?
Appreciate you taking the time to respond to this problem.
Now that it's in the 95 degree summertime, the engine idle speed doesn't rev up & down so much.

The wax thermo unit controls the fast idle, via a cam that acts on the throttle plate. When the wax inside the unit gets gets hot (it's controlled by the coolant temp) it pushes on the cam which then rotates allowing the the throttle to close.
So whats happening here is a combination of events with several components, throttle position sensor,
Idle-up solenoid, and the thermo element.
Basically if the themo element doesn't rotate the cam, when its hot the computer thinks its cold, and opens the throttle up solenoid.
Its becomes an endless loop.

Thanks for the detailed info.
Just read up on the idle air control system in the service manual.
Plan to work this weekend on the idle air control valve and idle air adjusting units.