So when you say 'it will not close' do you mean it won't stay latched or does it 'bang' as you attempt to close it and won't go into the opening all the way?
The reason i ask as it could simply be the 'claw' is stuck in the 'closed' position IF the door bangs and doesn't want to go all the way into the cab opening. IF that's the case then you can use a screwdriver and while holding the exterior button like you're trying to open the door and use the screwdriver to 'flip the claw' of the latch to the open position.
IF it closes into the opening but simply doesn't want to latch it MAY just need some lubrication. Spray it good with your favorite lube (WD40, etc) then work the 'claw' while activating the outside (or inside) button. If it still doesn't want to latch then chances are the spring inside of the latch mechanism is broken and the latch assembly will need to be replaced.
I know you have a '91, which SHOULD possibly be the same as the old '73-'87 regular pickup. IF it's the 'new' body style (1988-1998) then the videos aren't going to be of help at all.
These videos MAY help explain how things work and how the linkages clip onto the actual latch assembly;
Here is the firing order diagram for all Chevrolet V-8 engines for that year (note distributor latch positions between terminals 3 and 6 and 1 and 2) and let me know if you need any help to understand this diagram, or if you require any further assistance.
you still have to check fuel pressure making sure your filter is good and pressure regulator and return fuel line clear from clogging also making sure your tps sensor commending advance timing and fuel at the same time
Need more info....this is a 3 quarter ton so is this a 7.3 diesel or a gas engine? There can be several problems with the Truck. There's 3 things that make the Truck run which is Spark, Compression & Gas. Sounds like you have Spark & Compression. I believe you have a fuel problem. Check both fuel filters & replace them. If you have a diesel its more than likely the filter as well!
Is there an Engine light on? Check the code if it is!
Since it starts I'd guess that the ignition system is OK. Maybe not great, but it's probably not the primary problem.
- That leaves fuel as the major suspect. I'd try a dose of spry type carb cleaner and make sure that the fuel filter is not obstructed.
The vehicle is over 25 years old so maintenance history is important, and at that age (and mileage) you can expect that components will be getting to the end of their service life.
One consolation of owning an older vehicle is that you can always find a project to work on.
save yourself some time and look under the truck and trace the fuel lines from the fuel pump. The line with the gas filter in it will be the pressure line and the other will be the return line. Not sure if that helps but it is better than wasting your time looking for a diagram for two fuel lines.
Have a complete change engine oil including change oil filter. and use unlead gas for the meantime,if your using a premium gas empty your tank and change it to unlead . run you vehicle for at least 10 miles before you have your emission test. after the test you can go back using you preferred gas
my best guess would be a short in the circuit. probably in the steering colam.
(if it's blows the fuse for the left and right turn signal.)
but it could also be just a wrong bulb in one of the brake light sockets.
but i would go with my first guess.