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If your car has two fuel pumps and two fuel pump relays the rear fuel pump relay also powers the heater elements on the WUR/CPR and the cold start aux air valve.
If the rear fuel pump relay stopped working the car will usually still run on the front pump only but then the heater elemenrt on the bimetallic spring thermostat in the WUR cools off and it lowers the control pressure which richens the AFR.
Just one of many possiblities with CIS... and it happened to mine while driving once.
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