I just found out i do not have any compression on one cylinder (Valve 6) on a 4.8L 2008 Chevy Silverado? any sugestions on what to check or how much to repair?
What is the measured compression on that cylinder? “No compression” would indicate “zero” psi and that is unlikely unless the piston has the top blown out or one of the valves is held open significantly.
Loosening the rockers to release the valves and then connecting air pressure to an adapter at the spark plug opening should isolate the cause of the leak. Escaping pressure will be evident at the oil fill, the intake or the exhaust.
The first thing I do is remove the valve cover for that cylinder and check if a valve spring has broken.
I would pull the valve cover on the bank of the cylinder with no compression and do a visual check first. You might find a broken valve spring or a pair of popped keepers. Jiggle the valve springs side to side to see if they feel loose. See if the valve is retracting to the full height. Just be sure the lifter is off the high spot on the cam lobe when you do this. If there is nothing visible then I would do as @Rod Knox says in his second paragraph.
Has the engine been over-reved recently? Are there any engine noises such as clicking?
I believe that the engine has aluminum heads with pressed in steel valve seats which can drop out and hold the valve open. But yes, as @my2cents mentioned inspecting the valve train would quickly give some insight into problems there.
thanks all, look at valve spring and was broken, replace but still have misfire and there is no knocking sound while engine is running. Done compression test again and “0” PSI for cylinder.
It sounds like you may have bent that valve and it is unable to close completely, or the valve punched a hole in the top of the piston.
I think you’ll need to pull the head and look at the valve at this point. The valve may have gotten damaged. A slight bending of the stem is enough to prevent the valve from closing and prevent compression.
Thanks, i will do that this evening, give update later
Make sure to check the pressed in valve seats as Rod Knox mentioned. I have fixed a slew of riding mowers with this problem. I corrected the problem by staking the seats. I’m not sure this would work in an automotive application but those B&S engines are still running to this day.
The 4.8 L V8 is really cheap at wrecking yards because of very low demand. You can get a complete engine for less than $1000 and swap it yourself or have a shop do it for you. Figure $600 or so for the swap. It is likely easier than pulling and fixing the head. Go to Car-parts.com for used parts nationwide.
If the head is somehow bad, I’d be inclined to just get a used or rebuilt head, versus a complete used motor . . . with unknown history. A rebuilt head is far cheaper than $1000
A head can be removed without special tools
Replacing the motor requires a cherry picker, and there’ll be a lot of parts to transfer
Figure $600 or so for the swap.
??? Where are you? Swapping an engine, figure new oil, filter, coolant, maybe a couple of hoses and a belt, spark plugs, that’s $100 right there. That leaves $500, or 5 hours, for the labor charge. If a guy is good enough to do a job like that properly in 5 hours he’d better be paid a lot more than $500 for his abilities.