A take it is the 104 engine.
There is really not anything switching on/off, except the air pump
swiching off after a minute or so depending on the temperature.
Until the oxygen sensor warms up, the engine will run in open loop
mode (without feedback from oxygen sensor). I open loop the mixture is
controlled by inputs from throttle position sensor and temperature
sensors. And in idle the throttle is controlled by ECU.
So something is disturbing the control. It could be the throtte
position sensor (potentiometer), the temperature sensors or a vacuum
leak (intake manifold and vacuum lines).
Since it works in closed loop, I would suspect a small vacuum leak
(the ECU is still able to correct the mixture according to the oxygen
Might try pinching one of the vacuum line tubes or pulling off line
ends while the car is surging and you might find that one has no or
little effect which might point to the culprit. Sometimes the vacuum
tube ends get stretched and leak when cold where they connect to
engine. You can snip off stretched tube ends and reconnect. Keep us
posted if this worked....
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