Low Temperature only 250 degrees

You don't want you engine to continue to run cold. 50 degrees is normal running temperature offering peak performance.

The normal Volvo Penta D13 engine temperature operating range is 194*F+/-5 to 221*F +/- 5 degrees. The engine coolant is overheating if it's around 240 to 250 degrees.

that is the opening temperature for the thermostat
it opens at 160 degrees which is good for hot weather operation or another with 180 degrees which is good for cold weather operations
The differences is 160 opens sooner to start coolant circulation to keep operating temperature low where the 180 keeps shut and allows the operating temperature to rise up as soon as it open the cold air will quickly cool the coolant.

If you replaced the SENSOR and not the gauge SENDER, there's your answer.

The SENSOR sends the computer (ECM, PCM, ECU, etc.) the engine temp so it can make the adjustments to the fuel, timing etc.

The gauge SENDER makes the gauge read what temperature the engine coolant is.

Hope the helps.

It should fall between 200 and 250 degrees F, I take it when you say 90, that means 90 degrees Celsius, that equates to 194 degrees F.

You are within range, actually running little bit cooler, which is GREAT!

The water expands up to more than 9% when it freeze below 0 degrees Celsius. Due to this phenomena, during the extreme cold weather when the ambient temperature falls below 0 degrees the cylinder blocks of the automotive engines often burst or crack badly.
This damage can be prevented by using the antifreeze coolant in the cooling system.

The thermostat opens at 195 degrees, the cooling fan comes on around 220 degrees, and the hot light would come on around 250 degrees.

The high side should read a little more than 100 pounds plus the ambient temperature. On a 100 degree day the high side will be over 200 pounds. A high side reading of 250 pounds is no problem. The important reading is the low side, and it should always be about 30 pounds. Higher than 30 will produce warmer air, and under 30 will cause the evaporator to freeze up.

250 DEGREES ENGINE GETTING TOO HOT.SOUND LIKE CLUTCH FAN FREEWHEEL.IF CLUTCH FAN DONT LOCK IN OR HAVE RESISTANCE TURNING BY HAND WHEN ENGINE HOT.FAN CLUTCH IS BAD.IF FAN CLUTCH OKAY COOLANT LEVEL CORRECT.I WOULD CHANGE THERMOSTAT FLUSH OUT ENGINE AND RADIATOR.IF ENGINE TEMPERATURE STILL READS 250 DEGREES OVER FLOW JUG OR RESERVOIR NOT BOILING OVER THE COOLANT TEMPERATURE SENSOR FAULTY.

sounds like its an air door or vacume problem could even be a defective temp setting switch.