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I bought this car used 3 yrs ago. Previous owners spun bearings in motor and replaced with 03. I had numerous issues with the cooling system and overheating since I purchased. I replaced water pump, thermostat, hoses, radiator, did flush, put in a sealant. Drove fine for a while. Recently, intake gasket leak led to me spinning bearings in motor and waterlocking. I completed motor swap. Now has 97 motor, new everything in cooling system, radiator, water pump, coolant temp sensor, air intake sensor, ambient air sensor. If I run it with a thermostat it overheats quickly, take it out and it overheats while stopped at an idle. Unplug sensor and fans come on. Before replacing coolant temp sensor fans would come on at 212 sometimes, not always, engine gets close to overheat. We have been tinkering with it for a month. Not driving it currently. A zillion codes reading, from abs sensors also all new, to engine new engine sensors and code reader shows temp of -39. I verfied by vin that all sensors and parts are indentical from my old 99 to the new 97, only thing different so far is alternator which isnt an issue since mounting brackets were changed. This car seems to be back at the exact point where I was when I originally purchased it. Could it be they never had computer reprogrammed? At my wits end, only thing I haven't changed is the computer. Could it be due to new motor not matching up with computer specs? Do I need to change computer or just have it reprogrammed to fit new motor? What is an ECU? Is ECM and PCM the same? Where is the PCM located on a 99 Grand Prix?

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Yes. Had a friend who changed motors. New motor had a slightly different cam otherwise exact same motor. He spent $4000.00 changing parts to get the car to run right. He finally gave up and sold it. He found out later that because of the cam change he needed a different model ECU. Good thing he works in a parts store. If you have a different engine that's a much bigger change than a cam, you need an expert tuner tech.

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An ECU is an engine control unit. A PCM is a power train control module. They are the same thing. Automakers love alphabet soup.

I would start my investigation at the coolant temperature sensor. A CTS normally reads -40 when it's unplugged or there is something wrong with the connector or wiring like an open or corroded/deformed pin. A simple way to check is to carefully (don't bend or deform the pins in the connector) short the two wires in the connector for the CTS together. When this happens the code reader should show a super high, something around 290 degrees.

If it does not change or does not read very close to 290 degrees I would look over all the wiring from the sensor to the engine computer.

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