How old is the gas in the tank??? Old gas can turn in to varnish causing the problems you have mentioned. You can trip cleaning out the fuel system with Acetone, Add in tiny amounts from about one part per 5000 to one part per 3000, depending on the vehicle -- just a few ounces per ten gallons of gas. This comes to between 0.0003 % to 0.0025 % acetone maximum or approximately 1/15th of one-percent. Note that is around .78 cc per liter or one ounce per 10 gallons. Not more than three oz. per 10 gallons. In addition to increased mileage acetone added to fuel boasts other benefits such as increased power, engine life, and performance. Less unburned fuel going past the rings keeps the rings and engine oil in far better condition. The pure acetone label is the only additive suggested and is easily available from most drug stores in 16-ounce plastic bottles and in one-gallon containers from some large fleet farm supply stores. But any acetone source is better than none. Containers labeled acetone from a hardware store are usually okay and pure enough to put in your fuel. I prefer cans or bottles that say 100-percent pure. The bottles from Walgreen say 100 % pure, and Never use solvents such as paint thinners or unknown stuff in your gas.
You need 3 things.
Compression
Good hot blue spark to all plugs.
Good gasoline and proper fuel pressure. 63-64 lbs.
If you have all 3 then move on to injector pulse to all injectors.
I think each injector is on a separate driver on that one.
Check for pulse to all of them.
You may be weak on fuel pressure . even a couple of lbs low on a cold start will cause a no start.
Double check cold start cranking fuel pressure. If under 63-62 lbs you have problems.
one test is to give it a squirt of carb cleaner not starting fluid. If it starts then you have a fuel pressure problem.
Run the full fuel pressure test and post back pressures.
Here is the guide lines on testing.
Hard to start cold.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.
The 75-85 is only a check on the capable full pressure of fuel pump.
If you can not find the rubber return line check full pressure by plugging return line or just or the true full pressure can be taken direct at the end fuel filter.
What are the pressure readings that you have done.
What are your cold start pressure readings?
Cleaning your MAF sensor with MAF cleaner and then turn your ignition to the on position without starting the engine and pull the fuse to your ECM for 10 seconds and place back in. Turn off the ignition and back on for 30 seconds and then start. This will reset the ECM to default and clear any DTC codes, the engine may start but don't touch anything. Just start the engine again and go for a test drive after idling for 3 min or RPM drops under 1000 and it would not hurt to replace the thermostat. The 4.3 runs best at 190" unless the ECM was recalibrate to run colder.
Keep me posted, be glad to help you get your Jimmy running 100% again soon and don't forget to replace your fuel filter.
