Question : 3.1L MPFI Backfire while Revving Down?

[c0re]

Hi everyone!

I've been browsing this site for a few weeks now and there really is some amazing content on here and it seems to be an awesome community! Hopefully somebody smarter than I can help me out with a little problem with my engine.

To start off with a little history, I got this car with about 120,000 kilometers on the clock and I've put about 15,000 on it since. Started out getting 220km on a tank and I'm up to nearly 450km per fill now. The things that I've done to the engine itself in no particular order include:

• New plugs
• New air filter
• Seafoamed the engine
• Two oil/filter changes (Mobil 1)
• New fuel filter
• New O2 sensor
• New PCV valve
• Cleaned MAF sensor
• New thermostat
• Full coolant flush

Now don't get me wrong, the engine is running really great for the most part, but it has always had a problem where it backfires pretty obviously if I rev the engine and then let the RPM fall off on its own.

I do wonder if the timing could possibly be very slightly off or if there could be something up with the valves causing this? For the most part the engine runs very smoothly although I do feel the idle could be just that little bit smoother too.

Any thoughts?

[86FieroSEv6]

Inspect the exhaust very carefully for leaks, sometimes they are not always obvious. Generally, a deceleration backfire will be caused by an exhaust leak a little closer to the engine.

[Superdave]

that's pretty normal, it could be any number of things but nothing to worry about.

[96lumina]

That's normal when you have no muffler...the mufflers job is to build up some back pressure so the catalytic converter can do its job to burn the excess fuel coming out the cylinders...once you re-install the muffler all should be good...without the cat you are flowing the exhaust too fast so some gets past the cat and expands and that is what you are hearing.