1999 gmc wont start 454 motor

Check fuel pump fuel pressure , if no pressure change fuel pump .

starting definately has a lot to do with the crank/cam position sensors, but it wouldnt have nothing to do with hot or cold weather, oxygen sensor will make the vehicle run too rich if its out, you will smell fuel very strong out the exhaust, that could make it run sluggish also, and usually if the mass air flow sensor goes out the vehicle wont start at all...

Hi, I see you have changed a bunch of items, but do we know if the engine is getting fuel pressure? (55 psi at test port), or getting good spark? Could the spark plugs be fouled?I think before you replace too many items, you should start with basics. Good luck, Thanks...

Did you replace the cam sensor??? A code in the computer won't stop it from running, it just tells you there is an issue. It will clear itself when the issue is fixed. (After some driving)
Back up and see what is missing, spark or fuel. (Note...a working fuel pump may still not get fuel, as the injectors need to be "fired" to allow fuel into the engine.)

i have a 06 sierra that done the same thing and it was the distributer gear stripped

ck the map sensor

Hello;
Have you checked your fuel pump relay. It is located on the panel with the fuses under the hood. If the relay is not working, the truck will crank but not start. This is a typical problem of vehicles that run fine and then suddenly quit. Hope this helps, rate me highly please.
Thanks;
Rich
RPM Northwest

ok... there are a couple of things you can do. i must have miss typed on the cap,rotor . in the back of the motor where the plug wires conect that is the cap that is bolted to the distributor with two torques bits that i believe are t15 in size. there may lie your problem. the posts inside are made of aluminum and they get coroded and dont alow the proper spark. the other commen problem is the exhaust the catalitic converter becomes pluged up. something you can try is remove one of the oxigen sensors and try to start it up if it runs the cat needs replaced. the o2 sensor is a 7/8 nut to remove it. give it a try

Engine Cranks But Will Not Start

For an engine to run, you need three things to happen inside the engine. Compression, fuel and ignition, without any one of these components the engine will not run.
Basic Checks:

A. Test Fuses. Test both under hood power distribution center and the fuse panel under dash. This is done with a test light tool. The test light should illuminate on both sides of the fuse, if not the fuse has failed and needs to be replaced.
B. Testing Fuel Pressure Test for proper fuel pressure with a test gauge on the fuel rail, most throttle body injection cars (TBI) are between 13 psi and 17 psi. and most (DPI) direct port inject systems are between 40 psi and 55 psi. If no or little fuel pressure is observed and there is 12 volt power present the fuel pump, the fuel pump needs replacing.
C. Checking Timing Belt Test for compression by removing a spark plug and insert a compression gauge. Crank the engine over about 5 seconds the gauge should read between 125 psi and 165 psi, if no compression is observed remove front engine cover to inspect timing belt or chain. Crank engine to observe cam rotation, if the cam does not rotate the timing belt or chain has failed. (note: if the timing belt drives the water pump it is best to replace the water pump at this time)
D. Test for Injector Pulse Remove injector connector to insert injector test light (node) or volt meter. Observe light flash or gauge pulse while cranking. If light/voltmeter doesn't respond, an ignition distributor/crank angle sensor (CKS) or an ignition module is most likely your problem.
E. Test for Ignition Spark Remove spark plug wire, insert an extra spark plug into end of wire, rest spark plug next to a ground source then crank engine. You should observe a blue spark between the spark plug gap and ground source. If no spark is observed, an ignition coil, ignition rotor or spark plug wires might have failed. (always keep hands away from engine when cranking)

Happens all the time with 70's 80's and 90's GMC trucks. Replace the control module in the distributor cap.