Overheating

[S13XERolinLo]

Ok man I have a 1990 240sx XE and I recently blew the motor somehow and put another one in. Before it blew the temp gauge always read hot and I asked the guy I bought it from and he said the gauge just stuck there but every once and a while it would read cold after sitting for a while. I didnt think much about it since it didnt really show symptoms of overheating but now that I got the new motor in its doing the same thing. The gauge reads hot and the engine is hot but there is no coolant bubbling or rising. Without having run any tests yet is there asnything that you would suggest?

[NISTECH]

the single wire black connector located at the front of the intake manifold is the sender for the gauge. With the car not running and the key in the on position. Dissconnect that connector and look at your gauge,it should show cold. if ok then short it to ground somewhere and look at your gauge again, it should read full hot. if those 2 checks come out ok you need to remove the sensor and place it in a pot of water that you can heat up. hook an ohm meter to its terminal and heat the water ,have a thermometer in the water to watch it as it rises. Whatch your ohm meter and its reading should decrease as the water temp goes up. The second probe of your ohm meter goes on the threaded section of the sensor. if that tests ok then you need to figure out why your car is running so hot.

The reason I suggest testing the wiring and the sensor first is you indicated there may simply be a problem with the circuit. As mentioned by the prior owner.