Symptoms: '94 850 (not a turbo). Fuel pressure regulator failed while wife was driving, engine died, didn't restart. Replaced fuel pressure regulator, starter cranked engine for several minutes until engine caught and ran very rough with blue smoke for several minutes. Turned engine off, talked with Volvo service tech buddy. Changed oil (level was OK), pulled spark plugs (dirty but undamaged). Engine restarted, ran better but still somewhat rough, no blue smoke, but loud metallic tapping noise. Turned engine off, had Volvo tech listen. Doesn't like the metallic tapping noise but is not sure whether it is bent valve or hydrolicked cylinder.

I removed spark plugs for visual inspection of piston heads while turning harmonic balancer with wrench: bit of carbon but no visible damage. I don't have a bore scope. Following a hunch, I put shish kebab skewers into spark plug holes to show piston position and turned harmonic balancer with wrench. Rotating the engine with the wrench is pretty easy until cylinder 3 nears but has not yet reached top dead center, when it feels like something hits something else. The wrench gets very hard to move at this point. I think I could rotate past it, but I don't want to break my socket extension trying. The problem occurs both clockwise and counter clockwise.

Please help with diagnosis: I suspect a bent valve or other top end problem, not bent connecting rod or other bottom end problem. You agree?

Seems like the next step is to pull the head, inspect head and piston top for visible top end damage. (I think that's easier than pulling the oil pan to look for bottom end damage.)

Thanks for the help.

-_ Bruce

Using shish kebab skewers to indicate piston position is a great example of necessity being the mother of invention.
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-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854