Driving style and backfires

mazda
exhaustsystems
miata

#1

My car often backfires on downshifts. It happens as I?m decelerating in gear, and then blip the throttle to match the higher rpm of the lower gear. The car runs well and is well maintained. The spark plugs, MAF sensor, O2 sensors, and cat are all in good order. I?m confident the air/fuel mixture is working as intended by Mazda (at least under normal driving), so I?m not looking for a mechanical culprit. I?m sure it?s my style and frequency of downshift/rev matching which causes the backfires. It happens anywhere from 1-2 times a week to 1-2 times a day depending on my driving, or on something like 2-5% of all downshifts. Personally, I rather like excitement of the backfires and the exhaust burble which accompanies them, it makes me feel like I’m in a race car.

My question is whether frequent backfiring is going to drastically reduce the life of my catalytic converter and O2 sensors. I don?t mind replacing the cat 5 years from now instead of say 10, but I don?t want to have to replace it within the next 10,000 miles or less.



My Miata is a 1999 model with 130,000 miles on it. The exhaust system uses stock manifold and muffler but with an AEM performance intake and monsterflow catalytic converter.


#2

My wild guess is that when you blip the throttle you’re pushing raw gas into a larger than designed for airflow (because of that AEM performance intake) and some of that volatile mix is getting ignited. When I say “larger flow than designed for”, I mean that the CFM capacity of the intake is substantially more than the capacity of the intake valves under that vacuum level. I’d be concerned about my valves. I’d take that performance intake out.

Mine is, I’ll admit, a wild guess.


#3

Nice wag! My wag is a timing problem, but then again aftermarket parts can cause aftermarket problems.


#4

Yeah, and those CAI add-ons can also mess up the MAF readings and throw the mix off. Even the ones with dry fliters, like the AEM.


#5

I believe there is a decelleration valve on the MX5? I have to check…and have seen them before, but this issue sound perfectly normal…I dont really see an issue to be honest. Unless you were having serious backfires like a large booom type then I dont think the occaisional pop type will hurt anything at all…Are these the huge “Old Style” backfires? Or just some popping on decell?


#6

Yeah, I was wondering if you’re getting a “BOOM” true backfire through the intake manifold (see above WAGS), or popping out the exhaust on decelleration (nothing to worry about), or something else. Loud explosions out the exaust? The your EFI isn’t shutting off correctly when you let off the throttle.


#7

Thank you for all your replies
To be honest it hadn?t even occurred to me that the engine could be backfiring into the air intake. That seems pretty severe and I would imagine I would know it if the backfire was traveling into my intake. Perhaps I used the wrong word to describe what?s happening. The sound I am referring to seems to come from the rear exhaust.
I drove a VW dune buggy once that had awful loud sluggish backfires and intermittent power. This is nothing like that.
What I hear is a single moderate to loud pop, (something on par with a small firecracker). It occurs immediately after bliping the throttle, right as i let off. The engine feels fine when this happens, just like it spit out some extra fuel it decided it didn?t need. There is also a subtle but noticeable pulsing/burble “papababaaaaba” deceleration noise, but I’m pretty sure that?s completely normal.