Welcome to BF!
What happens in time of an intake backfire is that You have the open cylinder valve in time while combustion. The question to answer here is what is a nature of the combustion. If there's no spark, the injector will flood the combustion chamber with fuel, and will finally ignite from knocking at any time, even when the valve is open. But if the spark is fine, then it may be something to the valve that doesn't get shut properly.

A good place to start is to check for 1" spark at the plug terminal. If unsatisfactory take care of the cap/rotor and wires. If fine - follow the plugs inspection. I am crossing my fingers replacing the cap/rotor already solved Your problem.

 

Thank you for the information Mike.308, I was hoping the cap and rotor would fix the issue as well. It was starting fine for a couple days (as it backfires randomly), but it started backfiring again yesterday, so it didn't fully fix the issue. The good news is that after the MAF and throttle body clean and replacing the broken vacuum line, it idles very nicely and I have my 4x4 back up and running. My next steps will be checking timing once more to confirm it's on and hasn't skipped a tooth, and then I will be replacing the ignition coil, wires and plugs. While I change the plugs, I will also run a compression test to see if there is a stuck or damaged valve as well. Thanks again for the info and I'll keep this post updated as I continue my task.

 

Alrighty guys, I think I have figured it out, although it will be a little while before I get around to fixing it because the weather forecast is looking pretty darn cold for the next week or so. When I was cleaning the throttle body right after turning the truck off last week, I noticed vapor coming out of the intake manifold, and it hadn't backfired at all that day. After a little thought, I considered the idea that the fuel pressure regulator that is located inside of the upper intake manifold might be leaking. This would very much cause a backfire out of the intake in my mind, so I hooked up a fuel pressure gauge. This forum told me the pressure you're looking for at the hard line is about 65psi, which it was right on. But leakdown is supposed to be 55psi after 10 minutes. It only took about ten seconds, which is telling me fuel is dumping out of the regulator into the manifold. I know it's not the pump causing the rapid leakdown, as it was just replaced. This also makes sense to me because it only backfires after I have shut it off and turn it back on again shortly (IE refueling), causing the upper plenum to fill with fuel vapors inducing a rich intake condition.

Let me know if this sounds like it could be the problem. I'll order the parts ASAP (including new plenum gasket) and I'll update the thread upon installation. In the meantime let me know if my thought process seems flawed or if there are any other ideas floating around our collective minds.

 

 

GeorgeLG, thank you for the information. The fuel pump was definitely ACDelco, so I'm pretty confident in that. My hard line pressure builds up to 65psi immediately, which from what I read in captain hook's sticky is right on. He also said that the pressure must stay above 55psi for ten mins. Mine dropped lower than 55psi in less than ten seconds. Between the intake backfire (fuel vapors in plenum when I cleaned the throttle body), I'm fairly confident that the fuel pressure regulator or one of the spider lines is leaking. I'll check what's going on with a boroscope soon and see what the inside looks like.

 

Alright everyone. Issue has been solved. The weather has been crap and I've had to do a lot of unexpected work on my daily- a p80 volvo- but I finally got the chance to replace the whole fuel injection spider on the blazer, so i apolgize for the delay. It came with new injectors, fuel pressure regulator, and intake gaskets. The job was simple and quick if you stay organized. No more leaking fuel into the intake manifold, and instant startups with healthy fuel pressure readings in leakdown tests now. Thanks for all your help, and I hope this thread can help someone later who is having the same issue.

 

If you have a 4-button transfer case, you might find useful my thread on the replacement of all of my under-hood vacuum lines. (see below) They will rot and leak eventually. Funny thing, the vacuum lines in my 82 Honda are all still original and work fine.