broken timing belt
Check the valve timing and the ignition timing. If both are good then check the compression in all the cylinders.
bad rings most likely burnt valves i would also suspect a blown head gasket between those 2 cylinders
No compression in three cylinders of a 454 engine can be attributed to several potential issues. Common causes include a blown head gasket, damaged piston rings, or burnt valves, all of which can lead to loss of sealing and pressure. Additionally, a cracked engine block or cylinder head could also result in a loss of compression in those cylinders. Diagnosing the specific issue would typically require a compression test and further inspection of the engine components.
It may need spark plugs/wires. And while you're at it, run a compression test. If the compression is low in one or more cylinders it will make those cylinders less likely to operate properly.
Not running on all cylinders? Check spark plugs and wires Do a compression test to see if one cylinder is dead
I'm not sure of the exact values; they should be in your Haynes repair manual. However, if you compression test all of the cylinders and find one that is lower you have a compression problem with that cylinder. It is also possible to have 2 that are lower, this would most likely indicate a warp in the head between those two cylinders. Other causes for compression loss are damaged or gunked up valves that do not close all the way or a crack in the head.
Not running on all cylinders? Bad plugs, wires, or valve? Try a compression test to see if problem is internal to the engine
sounds like bad head or gasket do compression test on cylinders
Check condition of the plugs, could also be coil pack(s) going out, if ignition looks good check compression all cylinders
It is very possible that you might have a blown head gasket. However, other causes of low/no compression could be a cracked head, a cracked piston, or burnt valves. ANOTHER ANSWER It depends on the way the compression is down. If it's low on two adjacent cylinders, it's probably a head gasket. If it's just one or more non-adjacent cylinders it could also be burned exhaust valves. If it's low on all cylinders it's probably a slipped timing chain/belt.
You think probable to compression.
Extremely low compression in two side-by-side cylinders can be caused by several factors, including worn piston rings, damaged cylinder walls, or faulty valves that fail to seal properly. Additionally, a blown head gasket can allow coolant or oil to enter the combustion chamber, leading to compression loss. Other potential issues include improper timing or a malfunctioning ignition system. Regular maintenance and diagnostics can help identify and resolve these issues.