Thread: Oil blow by on 390 FE

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I have a 66 F100 with a 390 engine. I bought the truck with the engine rebuilt. I have driven it for 2yrs. without any issues and yesterday when I started it up the oil pressure was low. I reved it up a little and the pressure came up, but then the oil was blowing out of the dipstick tube. Any ideas out there?

Hey there,
Sounds like possibly a cracked ring allowing compression into the crankcase. Do a good compression test to see which cylinder is the culprit. 390 Ford are known for oiling issues from what I am learning through my 63 tbird project. Good luck! Don jr

your getting pressure in the crank case look at beathers could be bad rings hole in a piston or one thats burn down the side of the piston do a vacuum check and check all the holes for compression or fast check look at the plugs see if any are oily could be some other things to but sounds like the engine is hurt

Broken rings usually make a slight ticking sound, cracked pistons knock at idle, a scored block or piston gives a scorched smell out the tailpipe and breathers.
Was it a cold morning? That'll make the oil pressure come up slowly. Maybe when you revved it cold a skirt cracked. Cast or forged pistons?

Thanks for the input. I will start checking all the tips given. When I reved the engine it was only a slight rev not a hard rev. I just drove the truck from California to Missouri with no issues. That is why I am confused about the blow by.

I recall a lot of old FEs doing that back in the day..I always figured it was inadequate crankcase ventilation

Ford ran the PCV into a manifold hole that fed 2 cylinders---they always ran a little lean and were where trouble would start---usually burnt valves---on a long steady hi speed run its possible you even burnt a piston or two
I think that it was the rear holes on the right side--later they made a longer tube that hooked into two more holes on the front left

had a nice 63 t bird years ago same thing hapened it was the fuel pump seal filling the crankcase full of gas new fuel pump and oil and filter change and no more problem just a thought ....ted

Originally Posted by ted dehaan

had a nice 63 t bird years ago same thing hapened it was the fuel pump seal filling the crankcase full of gas new fuel pump and oil and filter change and no more problem just a thought ....ted

yes a over fill crank case with fuel and thin out oil will do that to