Still in the process of installing the engine in my 1988 Sunbird GT. I've come across a problem with the cooling system. I have an Actron scan tool, and on cold engine start-up, CTS reads near ambient temperature. On startup, the temperature climbs normally until it gets to 122 degrees F and instantly shoots to 257 degrees, causing the cooling fan to come on. I've repeated this process several times and the temp readings are consistent every time. The problem is there is no way the engine is at that temperature - it's only been running for about two minutes. (BTW, I have only water in the cooling system until I finish exterminating the gremlins.) Yesterday, I tested the CTS sensors (the old one and a new one) on the stove in a pot of water using a thermometer and DVM and both sensors climbed smoothly in temperature and dropped in resistance accordingly, so I know they're good. Readings were very close to temp/resistance chart in factory service manual. I then used a potentiometer to "manually" send resistance readings to the ECM through the CTS sensor harness connector, and observed the same temperature spike behavior. Has anyone seen symptoms like this before and is it indicative of a faulty ECM?