Head gaskets aren't a common problem on truck engines, and they often leak coolant into the combustion chamber or combustion gases can go into the cooling system. When that happens, it is possible to see air bubbling in the overflow reservoir and confuse that with boiling coolant. The secret is there will be no steam associated with the bubbling. Leaking head gaskets also usually involve loss of coolant.
Another secret is if you're watching the temperature gauge, does the overheating occur at highway speeds? If it does not, suspect a problem with the thermostatic fan clutch. I've done this, and seen it done, but I don't recommend this for the squeemish: after you're sure the fan spins freely, and the engine is cold, hold onto the fan while a helper starts the engine. It won't be very hard to hold. As the engine warms up, you will feel the clutch starting to tug harder and harder on the fan. Do this at your own risk. I don't want you hunting me down on Halloween night with a roll of toilet paper! It's important to not let go of the fan slowly. It will bang your fingers. Let go real fast or just have your helper stop the engine. I've seen people use a rope too, just don't wind it around your wrist in case of the unlikely event the clutch seizes. Don't use a coat hanger either as it can do damage or injury if it gets launched.
A more sane approach is to listen to the fan. When started from cold, the engine will not overheat for at least a few minutes. As the engine warms up, you should hear the fan noise diminish as the clutch slippage decreases and the fan speeds up.
Another thing to be aware of is overheating can occur if the plastic radiator shroud is missing. Air will bypass the radiator or circulate around so hot air keeps going through it. If the overheating seems worse at highway speeds or in higher outside temperatures, look for rotted fins between the radiator's tubes. That happened to my '88 Grand Caravan many years ago thanks to living in the salt-use capital of the nation, Wisconsin. Anything over 60 mph and 60 degrees caused the temperature gauge to creep up higher than normal. A year later the same thing happened down to 35 degrees. It was due for a new radiator anyhow. I had patched six different leaks due to corrosion!
Another clue is to feel the air from the heater set on high when the overheating occurs. If the heated air is cooler than normal, circulation of the coolant has stopped or slowed. That COULD be a sign of a head gasket, or a broken water pump impeller. That won't be your problem with a new water pump, but it is fairly common on Volkswagens. If the air is really hot, and running the fan on high brings the temperature down, that would also point to cooling fins in the radiator. When my van ran hot, I ran the front and rear heaters. That brought the tempeature down real quickly.
Friday, February 5th, 2010 AT 9:56 PM