2006 M35 Deceleration "boom"

[ambradley]

Purchased a bone-stock 2006 M35 with 78,500 miles (now 80,500) about two months ago. Thought someone with a loud subwoofer was always behind me when I slowed down, but no, it's the car. When the engine slows to around 1,000 RPM, it does a little "blip" up to maybe 1,200 RPM, then drops again. The blip seems to produce a small "boom" that I can hear from the driver's seat, even with the radio on to a normal volume level. I can reproduce it every time I decelerate. Is this normal?

[FlawleZ]

Does it happen when decelerating in nuetral?

[ambradley]

It doesn't happen when the car is not moving. I will shift to neutral while the car is in motion and see if it happens.

[atlM35]

If you haven't already, I would change out the diff oil and check the condition of your transmission fluid to start with.

[fat3oy]

Where is the sound coming from? Exhaust? Engine? Is it a mechanical boom, or like a little explosion? Sounds like a rich to lean burp...

[thm8d]

It's probably the car downshifting lol

[ambradley]

Sorry to revive a dormant thread, but I was rude and didn't reply. My M35 was hit on 1/29/14 and declared a total loss. I only owned it for three months. On the plus side, the insurance company gave >$1,000 more for it than I paid. Still runs, and hopefully will be rebuilt, but will be a salvage title. I just watched it sell on Copart this afternoon.

I test drove another '06 M35 and did not hear it. Last week, I bought an '06 M45 Sport and do not hear it. Might have been something wrong with that particular car, or I might have just been overly sensitive - I don't know.

[TXT]

Post pics of new car. Glad you got into another M! Best bang for your buck.

[svard75]

Hey welcome back. Now you're riding with the big boys with the v8. None of the wannabes like us sporting the v6! Lol. Jk all.

Hmm. Typically an exhaust boom is a condition of backfire where the engine is running too rich and unburnt fuel travels down into the hot exhaust pipes. The boom is that fuel exploding. Either your cats were removed or are seriously gutted. That used to happen to a car I converted into a race car in the 90s. I had a 4 into 1 header down to a 2.5" free flow cat and out the back via a dual dtm tip muffler. If I gave it and decelerated the gurgling could be heard (also interpreted as popping to some). I'll bet one or more of the plugs in the v6 were either gapped too closely or were broken. Did the engine feel rough in general while idling?