A stalling problem such as you described is usually caused by a sensor, or the wiring to it could be rubbed through and grounded out. To suddenly stall and then be able to be started again in a few minutes is not a generator or battery problem.
First of all, if the fellow who removed the battery cable was my employee, I would have fired him on the spot for doing that to your car! No professional today is that ignorant. That was a stupid man's trick that was done decades ago to tell if the generator was working. If the engine kept running, it was assumed it was okay. There are all kinds of things that can still be wrong with the generator even though it can keep the engine running. The voltage might be too low, or, there is a common problem that causes it to only put out one third of the current it is supposed to be capable of. More importantly, part of the battery's job is to smooth out the "ripple" that all generators produce. That ripple, without the battery, can cause system voltage to go too high. Had he increased engine speed with the battery cable removed, the voltage could very easily have reached over 30 volts. I did that as a demonstration every year for my students to prove what can happen. 30 volts will instantly burn out any light bulbs that are turned on, it can damage the generator itself, and it can destroy every computer module on the car. Fortunately your car doesn't have many computers. Most newer cars since the mid to late '90s and newer can have up to 47 computers. If only a quarter of them are destroyed, the car might be not worth repair.
That important issue aside, it sounds like you're relying on too many "friends" who are sending you in a bunch of different directions. Not one of them has done any testing to have a hint of a proper diagnosis. You need to have people all pulling in the one right direction. A dozen people can tell you why you're in pain, but you have to agree, unless it's a hang nail, sunburn, or you cut your foot off with a chain saw, without testing, all anyone can do is guess. How many "fixes" are you going to try before you give up? Also, every time someone puts in a new part or makes some adjustment, they are introducing a new variable that your Engine Computer might have to relearn. That relearning process occurs after a specific set of conditions are met. Until then, the engine could very likely run worse.
The best thing you can do is leave the car with a mechanic and provide as many clues as possible. You have already noted way more helpful things than most people. The high fuel consumption is likely related but that means the fuel pump is working just fine. GM cars have a history of pumps failing while driving. That is not the way they usually fail on other brands of cars. As I mentioned earlier, it is very common for some engine sensors to fail by becoming heat-sensitive, and they will work again after they cool down. It is probable that a diagnostic fault code has been stored in the Engine Computer that will send the mechanic to the correct circuit or system that needs further testing. On a few rare occasions they can lead straight to the defective part. It is very important to not disconnect the battery until someone has read those codes. Disconnecting the battery will remove the power from the computer's memory and that valuable information will be lost.
Whether or not there are any codes, the mechanic will connect a hand-held computer called a scanner that can display live sensor data while the engine is running. That may show him which sensor signal is dropping out when the engine stalls.
I know no one likes to spend money unnecessarily, but visiting a mechanic is less expensive than listening to everyone's ideas and getting nothing solved. If you can be without the car for a while, you might look for a nearby community college with an automotive program. They are always looking for live work for their students, but they will only take in cars that fit what they're currently teaching.
In the meantime, check under the car for signs of fuel leakage. The Engine Computer can't command so much fuel as to only get 4 mpg. A leak can cause low fuel pressure which can cause stalling.
Wednesday, April 20th, 2011 AT 4:54 AM