All of the GM cars/trucks I have owned exhibited the same problems with the fuel gauge. My Camaro was really bad. I never let it go below 1/4 because once that needle hit's the 1/4 mark you better be darn close to a gas station! The needly would also stay above full for about 1/3 of a tank. It's never been a serious issue for me because I know that that's just one of the things that GM vehicles do, and I compensate for it.
I have been told that if your temp was to spike really fast that the temp gauge in the dash wouldn't pick it up depending on how fast the spike is. one theory is that is an averaging situation so that the consumers don't freak out because their temps jump around so much. my temps sitting at a stoplight will fluxuate about 15 degrees (according to the scangauge). my situation with the domestics may be more linked to the cavalier. they are known for creaks and noises.
as far as the gas gauge. I was extatic when I first started keeping up with my mileage. I was at half a tank and had 300 miles on the trip odo. I just knew I could get 600 out of that tank. then the needle went down fast. got 400 out of that tank. now I am semi-regularly getting 500 or so out of a tank.
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Guys and gals. Overheating will not happen unless you have a cooling system problem. If you completely block the grille your cooling fan will run a lot more but still keep the coolant temp in a safe range. You may see temp rise if you are sitting in traffic. Completely blocking the grille (assuming it eliminates all airlfow through the radiator may actually be counterproductive since your cooling fan will need more power to replace the electrical energy you are using.
An easy test for your radiator, with the engine running at operating temp, touch the top hose with your finger to get an idea of how hot it is, then touch the bottom radiator hose. If the bottom hose is not significantly cooler than the top hose your radiator is not removing the heat. It could be a loose water pump belt, but probably is in the radiator. If your radiator is 10 years old it has some blockage, after than it can get worse depending on age.
Summertime is radiator time with ambient temps contributing to the loss of reserve capacity in the radiator.
I found that most blown head gaskets were due to cooling system problems, that reached the point where the result is a blown head gasket.
If you are going to restrict airflow to your radiator make darn sure you have a good radiator to begin with.
Also when you run the tank to practically zero the temp of the fuel in the tank gets much higher that when the tank is more full. The fuel temp in the tank rises as the heated fuel returns from the engine to the tank (fuel injection with fuel return line to tank). If the vehicle is not moving, which cools the tank, the temp gets a lot higher. I have measured a 15 degree increase in fuel tank temperature over 10 miles of driving, more so at low speeds.
I wouldnt want to swear this will shorten the life of an in tank electric fuel pump, but I would think over a period of time subjecting the pump to much higher temperatures would reduce the normal life expectancy. Cavitation from driving close to completely empty wont hep either.
The shape of the tank will also play a part in why the fuel seems to run out quicker after half a tank. Some tanks taper in on the bottom half of the tank. Some have and indentation for the spare, and some have a hump in the middle to straddle the exhaust pipe and or drive shaft. It causes less volume of fuel in the lower half.
Mine has the hump.