Very Rough Idle on 92 Q45

[yeasir01]

I have a 92 infiniti Q45 and the issue that I have with the car is a rough Idle I have done a FUEL INJECTION CLEANING, TUNE UP, CLEANED THE MASS AIR FLOW SENSOR, and checked the compression on three of the cylinders on the passenger side of the car one of the cylinders was 50 psi below the other two, I know that would make the car idel uneven but the question is what would make this cylinder 50 Psi below the other two cylinders Could it be the chain, maybe it jumped one tooth, could that cause the car to shake like a missfire I have not cheked the other cylinders but I have the feeling its the chain. see the thing is I just bought the car recently so I dont know mush about the service history maybe one of the chain guides are broken and that is my promblem anyways I just wanted to get some profesional opinon before I pull apart the engine to see whats goning on. ohh yeah the PSI on the other two cylinders were 150 isnt that a low number for this engine and also the uneven Idel turns into a vibration at 2000Rpms and goes away at 2500Rpms if that helps.

Thanks in advance

[DAEDALUS]

A previous fuel injector leak (worn rings/bore) is a possible cause of a low cylinder, or there could be something wrong with one of the 4 valves (or rockers). Did you do repeated tests? Just making sure it isn't tech error, as I have done and seen it a number of times. It cannot be a jumped chain, since all 4 cylinders on that bank run off the same pair of camshafts and same chains. Chains/camshafts would affect at least 4 cylinders, maybe 8, & not just 1. 150psi is a bit low, but still within spec. If you're doing the test cold that could be a factor. Spec values are when warmed up. Have you tried pouring a teaspoon of oil in the bad cylinder and retesting?

[3Q Jay]

also, the vibe at 2000-2500 is likely a bad transmission mount as i have recently learned and fixed. almost certainly completely de-coupled from your idle vibe, although perhaps the idle vibe hastened the demise of the transmission mount and is also working away at the motor mounts.

[yeasir01]

I have poured some oil into the low commpression cylinder and it went up like 25psi. so that would mean its the piston rings right. I know Im a mechanic I just hoped it would be somthing else.