Typically on any vehicle with a cooling system, if the engine doesn't heat up, it's the thermostat (provided the coolant level is correct). Did you put the right temp thermostat in? You could have an air pocket or a blockage in the system somewhere before the coolant gets to the temp sending unit/ heater core...remote, but possible not knowing the condition of the vehicle or how well it's been maintained.
To give you an example, my Chevy V8 came with a 180 degree stat. I replaced it with a 160 degree stat so that when I was towing, it would still be in closed loop, but at the same time keep the temps lower. In winter, the 160 stat also keeps the engine cool. Still in closed loop, but heat from the core can be less. So instead of swapping the thermostat to the correct size which would fix the issue, I disconnect the fan belt that drives the fan only and place cardboard in front of the radiator. I gained 10 degrees of heat doing this, getting my engine temps to 170 rather than 160. In spring, I take out the cardboard and put the fan belt on.
My guess is it's either still the thermostat or possibly the wrong temp size put in.
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