New engine necessary if cylinder output low (#5 and #8)

[ckazF100]

Just wondering if jumping the gun on going from "could just be a bad spark plug, to low output on 2 cylinders, to needing a new engine"?

That is the discussion I had with my mechanic. Says must be worn out from unleaded gas use (still the original engine in 1968 Ford F100 390).

I know not a lot of detail, if this is common and jumping right to needing a new engine... Suggestions for another path prior to another engine (now were talking labor).

Thanks in advance!!

CK

[Ranchero50]

For a WAG I'd be more apt to think the old rubber valve seals turned brittle, failed and clogged the back drain ports which allowed oil to build up and leak down the valve guides to the point it's causing issues.

Define how you determined low output. RPM drop comparison while shorting the plugs?

[Busboy]

Find a mechanic that doesn't jump to conclusions. Run a compression test. Don't guess.

[colnago]

I Had an engine with low compression on two cylinders (~70 on one cylinder, ~35 on another). It ran fine for 5+ years until I dropped in a rebuilt engine. You don't need to rush to any big conclusion. Especially if you're looking at rebuilding, what are you going to hurt if you keep running it?

Joseph

[Redcrew71]

For a WAG I'd be more apt to think the old rubber valve seals turned brittle, failed and clogged the back drain ports which allowed oil to build up and leak down the valve guides to the point it's causing issues.

Define how you determined low output. RPM drop comparison while shorting the plugs?

would no valve seals also cause smoke? Found my 71 f250 w/360 to be a mosquito fogger when I got it. found out a few valve rods were bent and the one side was plugged for oil drain up top. Cleaned all that up now no smoke. But overall not a ton of power but again all original 47 yr old engine.
never bothered with cylinder pressure check...its a run about to the dump and lumber store.

[tnlprt]

Do a leak down test

[colnago]

Do a leak down test

On re-reading the original post, I concur. With what you posted, you might "just" need to rebuild the heads. Yes, this is can be an endeavor, but not as daunting as a complete engine rebuild. The leak-down test should (hopefully) show whether it's rings or valves.

Joseph

[Ranchero50]

For a WAG I'd be more apt to think the old rubber valve seals turned brittle, failed and clogged the back drain ports which allowed oil to build up and leak down the valve guides to the point it's causing issues.

Define how you determined low output. RPM drop comparison while shorting the plugs?

would no valve seals also cause smoke? Found my 71 f250 w/360 to be a mosquito fogger when I got it. found out a few valve rods were bent and the one side was plugged for oil drain up top. Cleaned all that up now no smoke. But overall not a ton of power but again all original 47 yr old engine.
never bothered with cylinder pressure check...its a run about to the dump and lumber store.

When the oil drain is clogged, especially the rear one oil will build up in the cover until it floods the valves and gets sucked into the intake path.