ok just went 2200 miles in 48 hours from vancouver to wyoming and back mostly on I90 with the 1100 mile return trip towing a boat with a truck and trailer combined weight of 9878 lbs and i think i can comment. I crossed something like 7 mountain passes each way from sea level to 6300 feet and many places in between.
it works. no matter what i did, it never went over 212. It seems to kick in around 206. it only went over 210 once, climbing out from the columbia river westbound on i-90 in 75 degree temps running over 3000 rpm for what seemed like a 5 mile 6% uphill grade ?? with the boat on the back I never dropped below 50 mph but at the top the temp touched 212.
with no boat on the back eastbound it really worked great. I had the experience of climbing up snoqualmie pass eastbound (a long 6%ish uphill grade) running at 3200 rpm and all of a sudden at about 209 on the temp gauge the fan really kicked in and the temp actually dropped down to 206 with no change in speed, rpm or grade. the truck got up to maybe 210 at the most on hills and it was quick to drop down to below 206. mileage thoughwas definitely lower than my last similar roadtrip by about 2-3 mpg. Part could be tire pressure at 38 instead of 44 psi and also running a little faster, but I'm pretty sure the fan is at least 1 mpg.
with the boat on the back it was also good. I found I could run the truck at 3-4000 rpm on a sustained basis if i wanted to without going over 210 and I was able to average close to 60 mph over 1100 miles on a very hilly interstate where I never went over 75 mph anywhere and rarely got to 70.
however, with that much boat on the back the truck is a dog with the fan clutch engaged and it seems to use about 200-400 rpm worth of power to turn it. the longer you run up and down a lot of hills the slower the fan is to cool and disengage between climbs goes and after a while you get a negative feedback loop where the fact the fan is engaged robs you of power so you have to run at higher rpm to maintain highway speeds on flats or even downhill which in turn keeps the engine hot and prevents the fan from ever cooling enough between climbs to disengage!
The first time this happened I ran about 40 miles thinking there was some kind of optical illusion and when the road looked like it was going down it was really going slightly uphill. the only thing i noticed was that the temp was stuck at about 208 and would not go down. then i stopped for gas and food and this was long enough to cool the fan clutch and suddenly I could run on exactly the same grade at a comfy 2100 rpm at 60 that I had been running at 2400-3000 rpm at 55-60 before i stopped.
And yes I was very careful when I used the overdrive towing. I would turn it off if there was any sign of lugging, but the temp gauge shows that almost coasting with light throttle at about 2000 rpm downhill in overdrive is definitely the fastest way to cool down the engine.
The only way I think you could fix this is maybe with a lighter grade of oil (I am at 6500 cst) but then you would sacrifice an unknown amount of cooling effect. The real trick is not to try to tow 4000 lbs of boat over 7 mountain passes while trying to maintain 60 mph
I wasn't thinking about the blue clutch fan mod at the time. I didn't think about it until today when I was looking at the crank pully.
