i have got a 1991 sentra with a 1.6. on sunday morning, i started it up and immediately noticed a sputtering sound from the engine, like it was out of time or had a fouled plug. after changing the plugs, wires and cap, the missing feeling was still there and got worse the further i went. i took it to a mechanic today where they said that the compression on the #2 & 3 piston was at about 90 and the #1 &4 where at 120. they said that it looked like either a head gasket or a valve. now before i took it there, i pulled the plug wires while the car was running one at a time. #!, 3 &4 had a noticeable drop in engine performance while the #2 did not. the question is this: while this sounds like a classic compression problem, this engine has 145,000 miles on it and i am wondering if the o-rings on the fuel injectors can cause the same symtoms as a head gasket or a bad valve. i had heard that the o-rings on the nissans can be damaged by by products like injector cleaners and such. should i look at this before anything else or am i barking up the wrong tree?