99 Chevy LS Blazer, 4 x 4
On a rare occasion, but still often enough I hear a load BOOM upon start up. It has to be really loud as I am a deaf & dumb apparently don't hear most things, but I hear this. I cannot tell where it is coming from or what it is. On the even more rare occasion that someone was in the vehicle with me when it happened other than saying that was loud, they could offer no help as to the source. The engine runs perfectly fine & the mechanic can find no problem. Any ideas.
Also, the airbag light comes on occasionally for just a few minutes while driving then goes away. I nor the mechanic can find any visible & obvious reason for it. Sometimes you can drive forever & it does not come on. At other times you can drive for just a few minutes & it comes on before eventually going away. Thank you

 

I having the same issue. It doesn't do it all the time. When I start it, it starts abruptly with a "boom" like a internal combustion boom that is louder than it should be.

 

Have your timing checked

 

If it was timing..wouldn't it effect how the engine runs after it starts? It runs perfectly after it starts.

 

It's fairly important to figure out if it's coming from exhaust or intake.

For exhaust "backfire" (technically afterfire), it comes from excessive unburned fuel. On this engine, at start up time, that can come from fuel leaking inside the manifold/plenum while the engine was stopped. A fuel pressure/leak down test can diagnose that issue.

For intake backfire, it's some combination of lean mixture and/or advanced timing.

Intake backfire on this engine isn't as common, but when it happens, it can crack the plastic intake plenum, leading to more unmetered air getting in, leaning the mixture further, causing more opportunity for intake backfire.

If you've got a timing light, checking the timing would be a quick first test. However, I suspect it's more likely to be a fuel leakage issue in the plenum/intake manifold. For that, you'll need to do a pressure and leak down test at the service port, and at the fuel filter.

If the timing is advanced, the effect would likely be "improved performance" beyond normal stock levels, and you might not notice it.

 

My boom seems to be coming from the passenger side of the engine. At times the sound will include a metal on metal "clank". I am going to pull my breather off in the next day or two to get a look in the plenum for leakage, clean it, and clean my Idle Air Control. From what I read I have one of the better injection systems with my vin X 4.3. With only 109K on it I am hoping for the best. As for timing... My cap an rotor look good and clean, no visable wear, they are about 2 years old. I often wonder about just replacing the whole distributor. Maybe the gear is worn to a point where it jump timing?