1999 Chevy Blazer backfires inside the engine and blows off a vacuum line on the intake manifold.

Codes indicate engine is running lean (bank 1 and bank 2). This is usually an indication of a vacuum leak - usually sizeable. Check to ensure that you have reconnected all vacuum lines, especially the power brake booster. The next suspect if all vacuum lines are properly connected is the intake manifold. As you have just replaced the gasket, you may want to double check that all fasteners are in and properly torqued.

most likely a vacuum issue with the air mix door...check for vac supply

Your blazer has central fuel injection (CFI). You said you changed injectors, but your injector in inside the intake manifold, and only has one injector and a "spider" of nylon tubes from the central injector to each intake port. It is common for these spiders to break, causing fuel to leak into the inside of the manifold, and cause problems like you described. The spiders are available as reconditioned and are quite expensive,

1999 Chevy Blazer 4.3L engine has an electronic EGR valve its in front of and below the intake manifold a cylinder looking part connected to a small wiring harness. It is bolted on with 2 10mm bolts alot of times a piece of carbon will get stuck in the pintle at the bottom of the EGR valve and will keep it open causing a large vacuum leak making it hard or impossible to start. Good luck.

That sounds like a fuel pump problem and you can test that by pulling a vacuum line off from the intake manifold and spray a lot of carburetor spray or brake cleaner in a spray can into the intake manifold through the open vacuum line port (it has to say "extremely flammable" on the can to work and the non-flammable will not work for this test) and then replace the vacuum line and see if the engine will start and run for a few seconds on the spray.

If the engine does start and run then the problem will most likely be the fuel pump, however a faulty ignition module inside of the distributor could keep the fuel pump from running or the fuel injectors from firing and if there is good fuel pressure in the fuel rail, (there should be a schrader valve in the fuel rail for testing) and there is spark out to the spark plugs but the engine still will not start, then the ignition module inside of the distributor would be the likely suspect of the problem.

If the engine does not start then you will need to determine why there is no spark to the spark plugs, and most likely the problem will be the ignition coil or the ignition module inside of the distributor, or there could also be a problem with the ECM (Engine Control Module).

there are many vacuum hoses,i dont know if this is a s10 or full size blazer, heres a diagram for the s 10 blazer with the 4.3 if this is not your engine let me know, but heres a diagram, just look over all the lines, hope this helps.

Do a compression test on all of your cylinders when it is cold. Backfiring is a sign of a worn valve seat allowing mixed fuel and air (lit by spark) to be forced back into the intake. You will probably see compression loss on one or more cylinders.

Also... there is a chance that the EGR valve is not totally seated when the engine is cold with just a small blow by. The EGR valve seats coke up over time with black gunk.

here are a few things to check, egr system not working properly, vacuum leak at intake or manifold, damaged valve springs or sticking valves. all of the above will cause a backfire in intake.

Check for vacuum leaks. Spray some carb cleaner or WD40 around the intake manifold, trottle body and any vacuum lines. Check for loose connections on engine wiring harness (Maybe you disconnected a sensor, and forgot to re-attach it?).

If the air blows from the derfost and the footwells, then their may be a vacuum leak. Check under the hood there should be a vacuum line that goes from the firewall to a vacuum port on the intake manifold. Alot of times the line get corroded and breaks off.(If you find the pcv hose you are in the right direction)