Yeah, I really wouldn't expect you to have a bad head gasket but I had to throw that out there. The 4.3L isn't known for head gasket issues. As a side note, it IS possible for engines to overheat, or at least run hotter than normal without a thermostat due to the hot coolant not staying in the radiator long enough to cool off. Doesn't happen on every vehicle but it is something to keep in the back of your mind.
Let's get back to the basics. If the fan is blowing the right way, the water pump is pumping the right way too. There can only be three possibilities; insufficient coolant flow, insufficient air flow, or insufficient heat removal.
1. Carefully feel both radiator hoses while the engine is running. If the bottom hose is cold and the top one is warm near the radiator and much hotter near the engine, there is insufficient flow of the coolant. The warm you're feeling at the radiator is just migrating over from the hot engine without really circulating. Suspect blockage someplace, a stuck thermostat, or broken water pump impeller. I know the water pump is new. The impeller is very common on Volkswagens, but worth mentioning. Something really embarrassing to not overlook is a plastic cap that might have been on one of the tubes of the new radiator. A fairly sharp mechanic got stung by this one a few years ago. The radiator shop installed the protective plugs, then painted the radiator. Since the bottom plug got painted too, he didn't see it.
Feel the radiator from top to bottom. If it gets progressively cooler the further down you go, you can assume coolant is flowing. You should also see the coolant flowing when you remove the radiator cap. The flow through each tube should be about the same as a weak drinking fountain. It must not be just a little trickle.
2. If both radiator hoses are hot, suspect insufficient air flow through the radiator. The most common cause is running without the shroud. Air will bypass the radiator or come back around and go through again. The air will continue to heat up instead of removing heat from the coolant. This can be misleading because you will feel the heat and mistakenly assume the heat is leaving the coolant. Also look for a rubber strip under the front of the hood. On some vehicles it is clipped on over the core support from fender to fender. Its job is to seal the gap between the hood and core support to prevent air from bypassing the radiator. This will usually not cause it to run hot at idle because it is necessary at times to run the engine with the hood open, and you don't expect it to overheat then.
3. If the lower hose is still hot but not as much as the top one, not enough cooling is taking place within the radiator. The things to look at are physical size and the number of rows of tubes. Radiators are referred to as "single core", "double core", or "triple core". That has to do with one, two, or three rows of tubes from front to rear. You might be able to see and count them through the radiator cap opening, otherwise you'll have to look at the top or bottom tanks where the tubes go in. Your new radiator might be for a four cylinder engine. I can't say for certain, but I would expect your truck to use a two-core radiator at a minimum, but probably a three-core.
One last note. A 50 / 50 water / antifreeze mixture is best. The antifreeze has anti-corrosion and water pump lubricant additives that last about two years, but it does a poor job of moving heat from the engine to the radiator. Because of its low ability to absorb heat, its temperature goes up real fast so it takes very little BTUs over to the radiator where it gives up that little bit of heat very quickly. Water can absorb a lot of heat and carry it to the radiator but obviously it freezes and doesn't have any of the additives. If the coolant has too high of a concentration of antifreeze and not enough water, it won't be able to move enough heat from the engine and overheating will result. The clinker is if you use an antifreeze hydrometer to measure the freeze point, you will get a false reading. You can prove this to yourself if you have this tool. Use it to measure straight water and you'll see it shows a freeze point of 32 degrees. Use it on straight antifreeze and you'll see it shows a freeze point of around minus 80 degrees, but in reality it freezes closer to minus 5 or 10 degrees. It's when you mix them that the freeze point goes lower. The discrepancy between the reading and the freeze point of straight antifreeze is due to the fact the hydrometer doesn't actually freeze the liquid to test it, it compares the weight of the liquid to pure water. Antifreeze is much heavier than water, therefore the more antifreeze in the solution, the more balls will float in the hydrometer or the higher the pointer will go. So while too much antifreeze in the mixture makes the freeze point appear to be lower, in reality it is going back up AND the ability to carry heat away from the engine is reduced. About the best you can hope for is to hit around minus 40 to 45 degrees. Once the hydrometer shows minus 50 or colder, there is too much antifreeze and not enough water.
The inexpensive hydrometers with floating balls or a pointer work fine but for real accuracy, there is a refractometer. You place a drop of coolant on the tool which must be perfectly clean, then close a cover and look through a sight glass. Part of the viewing screen will be light and part will be dark. The line between the two areas indicates the freeze point, and there's a scale on the side. These are REAL accurate because it doesn't measure according to the weight of the mixture, but most mechanics have never even seen one. I have no idea how they work, but they do.
Saturday, May 8th, 2010 AT 3:36 AM