Temp guage issue......1995 Riv 3800 series 1 SC

Upon start the temp guage will appear to rise as normal. After a few minutes it will swing all the way into max hot area. In a few moments, it will drop all the way back to start position and the check engine light then comes on. The car does not overheat and seems to be running at normal temperature. I replaced the coolant temp sensor and it had no effect. I just replaced the engine with a remanufctured one and it continues the same issue with the guage as it did with the old engine. The mechanic who installed the engine noticed that the cooling fan runs as soon as the car starts (I don't know if this is normal)

Anyone have any ideas what is going on here? Relay? Sensor? Computer? Ghost? Any help would be appreciated.

Ghost.I have the same issue.It was suggested to me that I take apart the dash.I have not done so yet.You be the first.

Since you've obviously eliminated any issues related to the engine itself or sensors on the engine, I would look to the wiring between the sensor (I guess you can probably start wherever the harness connects to the new motor) and the gauge, looking for anywhere the wire could be stripped or corroded or otherwise grounding to something else. This very well may involve pulling the dash apart. I had an oil pressure gauge on a Toyota do something similar and it turned out to be a corroded connector at a harness link (after replacing the sending unit and checking fuses and relays of course).