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1987 944 Hard start after changing O2 sensor

Engine

Vehicle Information: Model(924/944/968): 944; Year: 1987; Body Type: Coupe; Total Mileage: 140K miles;

My engine has been running fairly well for several years. There has always been a slight amount of surging under heavy acceleration, but this disappeared when I installed an Autothority chip last fall. Starting and driveaway have always been fine. The idle has a slight fluctuation, but not enough to cause problems. Compression and oil consumption are good. The injectors are probably the original units. I had them cleaned and flow tested a few years ago and the tech said the spray pattern looked good.This weekend, I changed the (probably original) O2 sensor for a new unit. The car immediately had problems with cold starting - long cranking, then vast clouds of unburnt HCs and rough running for the first 15 seconds, after which it returned to its normal smoothness. Warm starting is fine, as it always was before.I reinstalled the factory DME chip last night, but the behavior remains unchanged. I have also checked the engine temperature sensor (yes, I read the previous entries!) - at room temp the impedance was 2K Ohms, which sounds about right. I have unplugged the O2 sensor and am currently letting the engine cool down in order to see how it starts open-loop.

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