1994 Chevrolet Silverado mystery problem please help!

You can't adjust ignition timing. That is set by the crankshaft position sensor. It has some serious timing advance built in, then the Engine Computer calculates the correct amount of delay to achieve the desired actual timing advance. By turning the distributor you just changed when the injectors fire. If they fire too soon, the fuel will just puddle in the intake manifold until the intake valve opens. That gives the fuel a little time to condense back to a liquid. Liquid gasoline doesn't burn. It has to be a vapor.

For the symptoms you described, the first suspect should be the fresh air tube between the mass air flow sensor and the throttle body. Only Chrysler has never needed a mass air flow sensor to make their engines run right. For all other manufacturers that sensor has the biggest say in how much fuel goes into the engine. There can't be any cracks or other leaks in the tube. If any air sneaks into the engine that the computer doesn't know about, it wont command the fuel to go with it.

Mass air flow sensors work on temperature. They heat up a temperature-dependent resistor, (thermistor), then watch how much the air flow, by mass, cools it off. The intake air temperature sensor is used to know the starting point. Those sensors have an extremely low failure rate because they contain just one component, but if there's a less-than-perfect connection on one of the terminals, that will alter its reading. The sensor is fed from a 5.0 volt circuit, but its signal voltage must be between 0.5 and 4.5 volts, (approximately). Anything outside that range will trigger a diagnostic fault code. The problem though, with a poor connection, is the signal voltage could be within that range, but wrong. No code will be set but the Engine Computer will base its fuel metering calculations, in part, on air temperature, and on the readings from the mass air flow sensor.

If there's an aftermarket cold air system on the truck, get rid of that. The goal of the intake system is to warm the air so the fuel will vaporize better and burn more completely. The goal of cold air systems is to condense the air so more can be packed into the cylinders. You need more fuel to go with that air, otherwise you'll be running lean which causes stumbles and hesitation. Once the engine gets to around 160 to 180 degrees, the computer adds the readings from the oxygen sensors to its fuel metering calculations. At that point a lean condition will begin being detected and the computer will add more fuel to get the correct mixture. Cold air intakes are a serious waste of money. They only provide value at wide-open-throttle when you're at your limit of how much air can be squeezed into the engine. At any other speed, if you want to go faster, you just push the accelerator pedal further.

Yeah, I'm smart all right. I have a lot of people fooled. Actually, I don't specialize in GM products, and they confuse me because they use so many different systems and variations that I can't remember them all.

As for that tube of great importance, it connects to the throttle body. If you don't know what that is, it has the throttle cable attached to it with a quarter-round cable guide. We had a '98 lemon-law truck donated to our school that I think is the same as your truck. There is no air cleaner assembly on top of the throttle body like there was years ago when they still used carburetors. Instead, I think you'll have a black plastic rounded cover, about six or eight inches in diameter, as I recall, and a couple of inches tall, in its place. If you pull that assembly off while the engine is running, it will either start to misfire badly, or will likely stall completely. That is because air is going in that is not being measured. The Engine Computer doesn't know about that air so it doesn't provide enough fuel.

The hose I'm referring to is attached to or is part of that round cover. On our school truck it ran to the passenger side right in front of the firewall. The intake air temperature sensor is plugged into a hole in that tube very near the right hood hinge, then the tube curved forward to the air filter box on the right inner fender. Somewhere along that tube is the mass air flow sensor. The tube will be fastened to the sensor with a large hose clamp. You'll see the electronics part of the sensor on top with about six wires connected to it. That is the tube that can't have any air leaks.

If my description doesn't make sense, try to post a photo of the top of your engine, then I'll see if I can point out the tube and sensors.

The photo didn't come through. This happens to a lot of them. If it will help, I copy my drawings into MS Paint, then save them as a. Jpg file. Those seem to work better.