I concur about the O2 sensor...if it's not doing its job, then it can't tell the ECU to adjust the mixture to correct the situation.

I don't know if a check engine light comes on for the O2 sensor on the '85. Obviously, it's not sending any codes...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car