Temp guage not working e36 325i

I recently had an engine replaced in my 95 325i, and the temp gauge was working before the replacement. I have changed the thermostat, temp sensor located on the head, completed a cluster test and the gauge works during test. What could be preventing the gauge from working?

How isn't it working?

Not moving at all?

The hand does not move at all

Does the heater blow hot air?

The heater does not blow hot air. While bleeding the system, the top radiator hose gets hot, but the bottom hose is warm at best.

Then your gauge is likely fine as you still have air trapped in your system.

Will air trapped in the system prevent the gauge from working?

If it isn't warming up, then definitely.

try to elevate the front of the car on jack stands and remove the cap from the coolant tank and allow it to sit over night. gravity should be able to force the air back to the top. other than that, turn the heat all the way up, fans to high and let it run while keeping an eye on things. if the thermostat doesn't open it's in backwards, defective, or there's air stuck still. I used to do coolant flushes all the time at work and i invested in a coolant flush kit which consists of a big bowl with an adapter that screws onto various coolant jugs, so you can fill it up and then let the car run and allow the air to work itself out without having to constant add more coolant. a temp gun, or temp leads for a multimeter help too, you can check the water temp and compare it against what the gauge is telling you.