Expert: Sean Halliday - 3/14/2009

Question
Hi,
I have a 1992 Mazda 323 Astina. It has about 270,000kms, I think it has dropped a cylinder, it idles ok but when I accelerate it loses power and has a faint popping sound. When I ease off on the throttle it seems to run ok. It only has trouble when under load. If this sounds like a dropped cylinder, what are the most common causes?

Thanks

Ray


Answer
I have never heard of a dropped cylinder.Dropped valve yes, but this sounds more like a misfire.First,do a compression test on all cylinders then replace the plugs and wires.Replace the cap and rotor as well.Thats if the compression is within range,100 psi approx across all cylinders or more.High mileage cars may have a burned valve which will lose compression and cause a misfire.So,the compression test is crucial before you go buying parts.If the compression is bad the head will need removed and valve/s will need replaced.The popping sound may mean the cylinder is firing with a valve open,possibly the intake valve is burnt on that cylinder.