QuestionMy cousin has a 2000 Chev 1/2 ton, 4.8L, 4WD and it will only start if the temperature is above 20 degrees F. Can you give me any information why this is and what can be done to remedy this problem?
Answer How often does it get that cold where you live? Here, not so much, but it does happen. There are several things that could cause this kind of problem. One is the quality of the fuel. If he's in the habit of buying his fuel in one place all of the time, he might want to switch when winter comes again and see if it makes a difference. And switch brands, not just location; all gasoline is the same given the same octane rating, so he won't be giving up any advantage which one brand might have over the other(i.e., none). Then, I would get a proper engine scanner (not a code reader) and check the voltages and resistance values on the coolant temp sensor and the intake air temp sensor. If they are out of spec, the computer might think it was warmer than it is and not compensate for it. If you live in a cold environment, this factor becomes more important, and should be checked every couple of years.
Now, what the computer has to do to get the engine to fire off at low air temperatures is to enrich the mixture being delivered to the cylinders by the fuel injectors. When the engine is warmed up, this is controlled by the oxygen sensor in what is known as "closed loop" operation. But, when the engine and the oxygen sensor are cold, the engine has to depend on a fixed program that gives an approximation of what is the right mixture for the circumstances. It does this with the input from the MAP sensor(vacuum), TPS(throttle position, i.e. how much you are pushing on the gas pedal), and the temperature sensors I mentioned above. If any of these are not working properly, then the mixture will be incorrect. If the computer believes it to be warm, then the mixture delivered will be too lean to start combustion, as though you didn't use the choke on a carburetted engine in an old car. Other things that could affect cold starting and cold running, although less likely, are a bad Idle Speed Control(I can't remember whether or not this year has a drive by wire throttle; if it does then this doesn't come into play) and dirty fuel injectors. If the injectors are dirty, they will not deliver enough fuel in open loop operation or at idle. The computer can't compensate, because it hasn't the right sort of input to make the decision. So, the first thing I'd do, before running to the repair shop for a scan, is put a large bottle of high quality fuel injection cleaner(the one made by Redline is the best I've found) in a mostly empty tank of gas and run it for a while. Then check the other stuff, along with a general tune-up(plugs, air filter, clean the throttle body). I'm sure that something along here will cure the problem. If not, well, I used to leave a light bulb burning under the hood of my '65 Chevy truck in the winter, and it always started.
