Why Backfiring

[Garland]

For the third time today my V6 backfired on start up "popping" the plenum off. Several weeks ago after the last time, I pulled off the valve covers and checked all of the timing variables. Cams, distributor, etc. Everything is correct. Today when it blew, the plenum cracked the plexi cover as mine is a Maratona. Any ideas as to why this would be happening? Car runs fine otherwise. Is not difficult to start etc. Again, logic would say it has to be timing but it's not. I'm puzzled. Ideas please.

[alfaromeo1green]

There have been many discussions on this here, so do a search, and see what comes up. My two cents: The ignition system is most likely at fault, but not to worry, it is usually an easily replaced component like cap, rotor, or the most likely culprit, the wires. Unless this stuff is new, this is probably the problem. If the ignition is old, sometimes you can see grounding wires while running the engine while it is dark, with the hood open. Not unless things are really bad. Sorry about the plexi..........

[AR4me]

Typical causes:
1. Driver! If throttle is open (even just a little) while starting you run the risk of blowing plenum off. Foot off the gas.
2. Air leaks, resulting in similar condition as above. Perhaps the rubber couplers between intake plenum and runners are cracked or don't seal properly.
Jes

[Garland]

Thanks
I'll look for grounding in the wires. It's not an open throttle from my foot as I keep it off while starting. I'll also check for air leaks though would be surprised to find any.

Any one have a hood insert?

[conedriver]

I'll chime in here,even though we no longer have any V6 Alfas. Did you press the gas pedal before cranking?. In my case, and also written many other places, this will promote plenum blowing. My old GTV-6 started much better with a little gas pedal, but I was sure to only do this after beginning to crank the engine.

Quote:

Originally Posted by Garland For the third time today my V6 backfired on start up "popping" the plenum off. Several weeks ago after the last time, I pulled off the valve covers and checked all of the timing variables. Cams, distributor, etc. Everything is correct. Today when it blew, the plenum cracked the plexi cover as mine is a Maratona. Any ideas as to why this would be happening? Car runs fine otherwise. Is not difficult to start etc. Again, logic would say it has to be timing but it's not. I'm puzzled. Ideas please.

[alfaromeo1green]

Just a clarification: You probably won't SEE anything bad looking about you plug wires. An old cap/rotor can be a fault too. BTW, I never have to touch the gas while starting, I love bosch injection!!
Dan