I just bought a 2002 base Miata with 90,000 miles and have noticed a miss intermittantly when on the highway. Idle is fine and its not smoking. No check engine light. One of the belts chirps when the miss occurs. Previous posts seem to all mention idle being effected. Any ideas?
Engine missing intermittantly only at driving speed on 2002 Mazda Miata MX-5
by aarondaddy in Valrico, FL on December 16, 2010
ANSWER by Superbob on December 17, 2010
In a Miata, a miss like you describe is usually a sparkplug boot rubber failing and allowing the spark energy to arc in the sparkplug wells to the cylinder head. Pull the plugs wires and coils to see if there's water or corrosion in the wells. If so, you need to clean them out, compressed air and goggles are the best way. If the plug wells are OK, you need to check which plug is arcing. You'd need to pull up on the plug wire or coil-on-plug assembly to see the arcing, with the engine under load. This is easy with an auto trans but harder with a manual. If you remove all the plug wirtes and coils, you may be able to see where the arcing is by finding a black spot or hole burned in the rubber boot. There's always the chance that the plugs need replacement, remove and inspect the plugs for wear or cracked insulators, too. The reason the belt squeaks is that the engine decelerates when it misses and makes the belt slip briefly. You need a new belt, too, I suspect.