In searching for engine, if it comes to that. Do your homework. So many will claim, low miles and or JDM engine (no verified 4.7L JDM, ever in USA). Get pictures of VIN # plate on BK2 head. Run carfax and vehicle history. Look for regular oil, filters and coolant services. Some of the "red flags" are: Any coolant issues and starter R&R history. Starter R&R, all to often, top of engine was not cleaned before intake manifold pulled off. Sand drops into intake ports. This is a compression killer.
#4 & 8 being adjacent to #6 cylinders, lower at 125 PSI, is interesting. Does suggest head gasket. Which is usually, from warp head. Which is usually due to overheating. But numbers suggest, not extremely warped if that the cause of #5 55PSI.
During compression test:
Was fuel cut off, throttle body wide open and cranking speed 250 RPM or better?
Was engine cold, warm or at operating temp?
One trick is listen at oil fill spout, while engine idling. Same as we do when using a leak down tester.
I suggest:
- Do a wet compression test.
- Borescope inspection of cylinder walls and valves. Directional borescope very hand here.
- Coolant combustion gas test?
- Inspect head gasket around outside?
Some additional consideration:
- Have you been loosing coolant and or coolant in radiator low (not to very top)?
- Any white smoke?
- Oil look like milkshake?
- Did #6 have any thread repair in heads, spark plugs capture threads?
- Was starter replaced?
Coolant in oil, looks like light browne milkshake. Bearings, cam journals & oil pump take a beating.
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Head gasket leaks
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Debris in cylinder, spark plug thread repair.
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Minor scoring on cylinder wall. Lower but passable compression.
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Major scoring cylinder wall, resulting in loss of compression.
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Decarbonize piston top, indicating coolant enter cylinder.
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