scotch
+1y
Finding trucks with the thermostat removed is a very common theme. If a truck has a bad (frozen shut or open) thermostat, then it should be replaced. But I think 9 times out of 10, an owner has taken it out as a band aid for another problem with the cooling system. After all, it's easier and cheaper to take something off, than to go and buy a new radiator or water pump and install them.
I would run the truck with the cap off and the heater full on for a bit in an effort to purge any air out of the system. Be sure to rev it a bit as well at it takes some good pressure to purge the air from the heater core. If you have air in the core, that usually just impedes your cabin heating. If you have air in the rest of the system, it should find it's way to the radiator where you will see the coolant is low.
Chances are you have a problem with either the fan clutch, radiator, or water pump.
Fan Clutch
Since it sounds like you have an electric fan in addition to the clutch fan (did I understand you correctly?), I doubt that is your problem unless you have some other accessories on the bumper or anything that might impede the air flow through the grill and across the radiator. However, a simple test it to take in on the highway (if you haven't already) and see what kind of performance you get at highway speeds. If the rest of the system is in good shape and only the clutch is bad, then it should cool pretty well on the highway as the fan clutch is hardly working at highway speeds.
Radiator
I would be suspect of the radiator in your situation - especially since you have already found some "goo." If two fans can't keep your truck running cool, then there is a good chance the radiator is the culprit. They tend to clog up naturally. So whenever the previous owner used the "goo" gasket, it probably didn't help the situation.
Water Pump
The water pump is probably the least likely problem. But they can and do erode away - especially if the truck has been run with mostly water and not much if any coolant. So I would look at the other items above first and then consider the water pump. In the states, you can get a cheap one for around $35 or a premium Gates brand for around $70. Not a lot of money considering it's the heart of the cooling system.
I think your 98 is the 2.6 G6 engine. If so, it's easy to change. You could also pull it and inspect it. If it looks good, then you cold always reinstall it with a new gasket.
Fan Shroud
It seems that fan shrouds (cowls) usually disappear on these trucks over the years. If your shroud is missing, I would definitely get a new one. They can make a big difference at idle and low speed.
Also, keep this in mind. Neither the fan clutch, the radiator, nor the water pump wear out/clog/erode overnight. So there is a good chance that one of them is the primary cause of your truck overheating, but you could also have two or even all three of them contributing to it running hot.
Hope this all helps!
Oh yeah, let us know what your fan setup is? Clutch fan plus electric? Or only electric? One large electric fan or two smaller ones? Pusher or puller?