backfire through intake, major issue
 

this issue has just been getting worse and worse. here are the symptoms:

very boggy at start up
backfire through intake
seems to go away for the most part once you get up and going
only happens when you get snappy on the throttle, if you take it really slow its fine and its fine at idle

here are the parts i've replaced/checked:
fuel pump
tps
coil
wires/cap/rotor
spark plugs
intake manifold gasket
fuel filter
air filter
MAF sensor


i've tested, inspected and replaced to be sure its not a vacuum leak.

here's what i'm figuring..either its getting too much gas or not enough spark, when the injector pushes gas in, the spark doesn't ignite it all the way or at all, and when the valve opens up again it ignites causing the backfire through the intake, and since i've checked the coil and the spark portion of things that only leaves too much gas, which could be the dreaded spider issue, could my lines be leaking or could my poppets be faulty? the only confusing thing is i've NEVER smelled gas like people usually do.

any input on this? i'm eager to get this issue diagnosed and resolved, any help is much appreciated!

Have you checked the timing (cam & distributor)? Backfires are normally caused by timing problems.

have to check something with distributor but am confident thats not the problem because i've never messed with the timing, unless the distributor slipped somehow

It's obvious that something happened... Gear could be toast on the bottom of the distributor. The timing chain could have stretched too far for the computer to compensate, etc. Lots of possibilities. But hey, if you are confident that it isn't the problem... :icon_doh: Good luck.

Before dismissing the advice that you asked for, take 5 minutes and check the timing. It's simple and easy.

i'm confident thats its not the timing because i've already checked it. and i have also inspected the distributor, i should have added that i literally just changed my intake mani gaskets and had a chance to check some stuff. this is a stubborn problem, i think the diagnosis is going to be one that it tough to find i've been trouble shooting this for a while its not going to be a simple find, which i why i need help from fellow blazer owners. i appreciate your input, at least someone is replying :) i'm not the kind of guy who will post just any old problem, i only ask if i truely am stumped and have covered most of the bases.

thanks again/ thanks in advance for your input!

And we were supposed to know this how?? We can't read your mind. Simple as that.

So you have verified that the timing chain is good? Please be specific...

Quite literally, the most common cause of backfiring through the intake is a timing problem. If the cam were holding the valves open too long, it would allow the compression stroke & combustion front to push back through the intake.

Next would be the possibility of a bent valve which a compression and/or leak down test would highlight.

check maf again
 

I had very similar situation with my blazer only thing it is a 2000. changed all kinds of sensors no dice. The only one I had not changed was the MAF. No trouble codes this went on for almost a year farting through the intake, not wanting to rev when gas was pressed hard same thing you are describing. One day cruising ebay found a MAF for 30 bucks said what the hell nothing to lose. Well what do you know that was the problem all along!!! just a suggestion, maybe borrow another MAF from a buddy or something and try it out if it is your problem you will see an immediate change. Good luck!! :icon_playing:

Same here, on a 99. If I gave gas and then closed the throttle abruptly, blam. Took it to the dealer (still had the service contract on it then) and they said it was the MAF sensor. I still can't see what the heck the MAF sensor has to do with backfiring, but I told them to go ahead and change it anyway, and it fixed the problem.