Thread: low compression on number 5

So i just bought an 07gxp, far my first ls based vehicle but i figured id see if there were any nuances with the ls4's like breaking valve springs, lifter issues, etc. I know this is not a dod cylinder and its roughly 30psi lower than both adjacent cylinders. Oil pressure is great. Any good news? Lol btw hi everyone glad to be a member here, longtimer on clubgp and ls1tech.

Perform a leakdown test on number 5 cylinder and listen for air escaping either into the exhaust, the intake, or the crankcase to narrow down to either one of the valves or the cylinder itself, assuming the head gasket is still good.

Im gonna pull the valve cover tomorrow on lunch and check the valve springs and measure lift off the valve see if the cams wiped out. From there i figure do a leakdown test and hope its not rings. Its my own fault for buying a car with a miss i guess but the ls family in general is pretty damn durable so i didnt think much of it.

Btw i dont think its a headgasket/head bleeding compression since i have totally different numbers on both adjacent cylinders, thats y i checked those 3 on lunch today.

so let's really throw a wrench into things. Drive the car in this morning runs like glass no check engine light and good compression on number 5. Any takers? Lol

Within 3 psi its the same as the new reading...CEL is gone missfire counter reads 0 for current and history, runs great...maybe a lifter wasnt pumping up? Or carbon keeping a valve open? Only things i can think of, it was a city driven grandma car for the last 20,000 miles roughly

I did all that when it was missing less the seafoam, even a motorvac but its running great now.

totally off topic. Im gonna start a new thread if no one chimes in, does anyone know where the damn fog light relay is in our cars, i didnt see it in either fuse box. I have no power at the fuse and according to alldata the relay supplies it, if i jump power to the fuse the lights work fine.