I had a similar problem with my Excursion.
First, the fan can run 24/7 and it will have a minimal impact on actual engine reaching operating temp so long as the thermostat is working properly. Since you said you swapped thermostats, let's assume it is ok. Next possibility is the wrong size water pump may have been installed. I know it doesn't sound likely, but it's possible. After that, air/gas getting into the system from somewhere is the next likely culprit.
To diagnose, revving the engine to see if you get heat won't work because that might give you heat no matter what the issue is. I would recommend attaching a pressure gauge into the cooling system (lots and lots of threads on that here) and seeing what the pressure is like. Get it up to temp, carefully release pressure on the degas bottle, now run it some more and watch what the gauge does. Also, watch it while doing a couple WOT
runs. If it gets anywhere near 16 PSI
, or if it jumps up quickly when going WOT
, he has either a head gasket issue or EGR
cooler issue. If the pressure remains and there are no fluctuations, I would suspect he may have installed a 90mm pump in a 100mm housing. Those are the only things which come to mind based on the symptoms you're describing.