• The site migration is complete! Hopefully everything transferred properly from the multiple decades old software we were using before. If you notice any issues please let me know, thanks! Also, I'm still working on things like chatbox, etc so hopefully those will be working in the next week or two.

low compression on number 5

C

chrisferguson84

New member
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.
 
QUICKSILVER462 said:
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.
Click to expand...

This.

Nothing particular to the LS4 that I can think of. Do all the standard procedures to narrow down the problem and go from there.
 
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
 
Get another compression gauge and test with that. See how it compares to the one you're currently using.

I'd still do a leakdown test to make sure everything is sealing like it's supposed to.
 


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
 
Well I would just do routine maintenance on it, ie plugs, wires, fluids, filters and maybe give it a seafoam treatment. And just keep an eye on it.
 
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.
 
You must log in or register to reply here.
Back
Top