What is the most likely cause of loss of compression in the No. 3 cylinder of a 2000 Civic Si (EM1) with 194,000 miles? It was running fine (as always) and then, while driving on the highway, it developed a misfire and the Check Engine light came on. We were hoping it was ignition related, but when replacing the wire set, spark plugs, rotor, and distributor cap did not help, we checked the compression. No. 1 cylinder is okay, No. 2 and 4 are slightly lower, and No. 3 is almost nothing. I'm inclined to sell the car as-is, but if there's something I'm overlooking, I'd appreciate hearing about it. Thanks in advance.

 

You could do a leakdown test and see if it's leaking by the rings or a valve or somewhere else.

 

Could have smoked a valve.

With a leak down test, you MAY be able to hear air leaking by a valve if you take the breather hose off and listen through an open throttle body, and remove the exhaust manifold and listen through the exhaust ports. If a valve is bad, most likely it's an exhaust valve.

 

Sounds like a burnt valve. #3 normally runs hotter so if you were using bad gas or low octane and have high load on the engine you could burn a valve. When tuning engines most tuners add more fuel to #3 cylinder.

 

Thanks for the replies. That makes sense. Even if it is just a burnt valve, it's more that I want to undertake (too many other ongoing projects), so the car will be for sale as soon as my probation period on this forum is over.

 

Just received this picture from the new owner. It was indeed a burnt valve in the #3 cylinder.

 

Thanks for the follow up info.