Normal driving seems smoke free, but when I am in gear and going downhill I get the mosquito smoker effect...not sure what is going on or how to fix? 92 F250/manual tranny - 138k miles - only put a couple hundred on it, running rotella t5 10-30, fl-1a, all other tune up related stuff has been replaced - plugs, rotor, cap, wires, o2 sensor, I am a new owner and just trying to get this baselined...this is my newest issue, thanks guys!

 

Valve seals and/or valve guides are bad. Try replacing the valve seals first and see if that takes care of it.

 

I agree.

You can also try a new PCV valve, or try unhooking/ plugging the PCV hose to try it.

When you are coasting downhill in gear, you're engine is creating HUGE vacuum, and with your throttle plate(s) closed, the engine has to pull vacuum from somewhere else. Usually past the valve seals/ guides, or piston rings, or even a PCV valve.

Just my 0.02