Looking for advice. I have a 74 F250 with a 390 that smokes only when I use the engine compression going down a hill. It's a four speed, so for example, if I downshift to second going down a hill, kick RPM 2000+ after about 10-15 seconds it will start to smoke (light gray) ... the longer I go the more it will smoke.

As soon as I stop using the engine for compression, it stops.

Also, for example, if I start in first and hit 3000 RPM before I shift each gear, as I let out the clutch, it will let a puff of light gray smoke.

I'm still a newb, so not exactly sure what is going on. Does this mean the rings are shot?

Thanks!

 

Guess is either valve stem seals or worn valve guides. High vacuum on downshifting causes oil to be sucked down around the stems into the combustion chamber. I'm leaning more towards the guides rather than the stem seals.

 

Thanks Mike. How can I tell for sure if it's the guides? If I pull the valve covers, is there something I can look for?

Thanks again.

 

Not really. The stems seals will be obvious, there will be chunks and bits and pieces of hardened rubber laying around the valve area of the head and clogging the oil return passages.
However, stem/guide wear can only be diagnosed with disassembly.
If you are REALLY brave you could probably pressurize the cylinder with compressed air like you're going to replace the stem seals. Remove the locks, retainer and valve spring. Then put a clothes pin on the valve stem, relieve the air pressure and check for excess play in the valve stem.....but don't drop the valve.

EDIT: One thing you could check. If the oil returns are clogged, it will allow oil to puddle around the valve stems giving the symptoms of worn guides when it is really the stem seals. Pull the valve covers and see if the seals have brken up or there is sludge clogging the oil returns.