78' 351m problem
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Joined: Feb 2001
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78' 351m problem
I posted last week about my 78 Bronco I bought with a ton of running problems but almost had them ironed out. Turns out what I waited to do last I should have done first - a compression check. Cyl #5 is dead, 0 compression. Wet test doesnt change a thing. The plug doesnt have any oil on it but it is wet with fuel. I pulled the valve cover to look at the springs and turned the motor over to confirm the valves were opening and closing and the y were. When I got the truck, the header gasket on this head was completly blown. I have no idea how long the previous owner drove it like this. Is it possible a blown gasket could have caused a burnt valve? Since the plug isnt oil soaked can I assume the bootom end is still intact? Thanks for any help.
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Joined: Aug 2000
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From: PacNW
78' 351m problem
0 compression is usually a pretty good indicator of a burned valve(s) and since you already checked to see if the valves were moving then it's almost a sure thing. If it were a holed piston you'd be getting excessive crankcase pressure, i.e. oil out the dipstick, etc. How the valve(s) got burned is anybody's guess. Sounds like the heads need to come off.
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