I have a 2009 Miata w 3000 miles. About 2 mos ago, after having the car about 6 months, I noticed a sound like pre-ignition knocking, that occurs after the car has been run about 20 miles. Have been running 91 octane since I first noted the sound. Noise occurs in any gear, under slight acceleration load, between 1000-2500 rpm, and disappears when car cools off. Sound is like wood ***** in a rotating wood barrel. Dealer says it is normal Miata gear noise, but I don't recall it being present when car was new. It was so disquieting when I first noted it, that I was reluctant even to drive the car for fear of engine damage. No computer irregularities reported, according to the dealer. Dealer seems reluctant to pursue issue with Mazda HQ, but has the car now. Ideas?

 

You might be hearing a knock (aka. detonation) but you sure as heck aren't hearing preignition. How can I say that with such confidence? Simple, preignition is a condition where the fuel self-ignites prior to the spark event, and when said condition occurs, your engine will self destruct in a matter of a second or two.

Detonation on the other hand is where small (or not so small) pockets of "End-Gas" explode or detonate prior to the flame front reaching said pockets. The detonations are due to excessive heat/pressure in the combustion chamber following the spark event and well into the fuel burn, and this is typically due to a spark event that happened too early for the combination of fuel being used and the conditions under which the engine is operating at that given moment. Mild detonation is actually considered "good" in that keeping the spark plugs timed so that the engine runs on the ragged edge of incipient detonation will eek out as much power as possible from the fuel without damaging the engine. That said, there is a fine line between mild detonation and detonation that is severe enough to damage the internals of the engine. Moderate detonation, if left unchecked, can damage the engine enough that severe detonation will usually follow, and if still left unchecked, preignition and engine failure after that.

In this day and age of electronic engine controls, it is HIGHLY unlikely that your engine is detonating to any significant degree, if at all. Said another way, I'd look elsewhere for the source of the noise.

 

Right on! A second dealer (#2) located the problem as soon as he put it on a lift and ran the engine/wheels: Some sort of noise in the rear part of the transmission. Mazda USA said to remove the tranny and inspect. With only 3K miles on the car, I'm planning to insist on a new tranny unless I can be assured that there is no corollary damage to other parts of the tranny due to debris floating around the oil. It was over 700 miles since I first reported the problem to the first dealer. (Ironically, Dealer #1 just got a new Service Manager, hired away from Dealer #2! None too soon!)