Quote:
Originally Posted by dippasan
I performed a compression test on the engine this past Saturday.
7 cylinders showed 175 psi but #4 cylinder has 45 psi.
In talking to my mechanic, in the 90's for some reason, it was a common problem to "tulip" a valve. Probably an intake valve hence the backfiring through the throttle body. I removed the valve cover and ran the motor and saw nothing out of the ordinary with the spring, push rod or rocker (as you might expect because the problem is on the combustion chamber side).
So it looks like I have a valve job in my immediate future. I'm just a little stumped. I've worked on cars my whole life and I've never had a valve fail on a motor. Especially a Chevy 350 which are typically bullet proof. These motors have 360 hours and run as sweetly as can be with no sign or indication that a valve would fail. Strange!
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45 PSI could be a valve for sure, but it can also be a broken piston ring, cracked valve seat or a cracked head. Put a little oil in the affected cylinder and try again. If the number goes up, it's likely a ring issue. If you do a leak down test, you may be able to hear where the air is escaping by listening at the throttle body (intake valve), exhaust manifold (exhaust valve), thermostat housing (cracked head) or dipstick tube (rings). That said, the back firing sure points to the intake valve...
Valves tulip for two reasons, they are defective (not hardened enough when manufactured) or they overheat and get soft. Once they stop sealing well, they don't transfer heat to the valve seat very well and they get hotter and softer. The valve can also crack and so can the valve seat.
If you pull the head yourself and bring it to a machine shop, you can save a fortune. Valve jobs are not terribly expensive when done this way.