Ok 88 4.0 blow back. Cleaned up motor changed oil pan and valve cover gaskets. Changed front and back vacume harness and CCV still blowing back. What could cause this? Oil psi runs 40 at start up then goes down to 20. Witch looks ok. But as soon as I get on highway I get blowback. Please help.

SOURCE: Valve cover and Oil pan bolt torque

Inside to outside for torque pattern, 7 foot lbs. or 144 inch lbs.

SOURCE: i have an 86 honda

There are two slots in the valve cover right by the cam shaft sprocket that have to be honda bonded or black rtv. look closely at the valve cover right above the gasket underneath the timing belt cover. Hope that helps.

SOURCE: Motor Oil leaks from 1999 Honda Civic EX

make sure they are tight
could be time for head gaskets
make sure the old gasket from the old filter came off the block sometimes they come off old filter and stick this causes a leak due to improper seal.

SOURCE: Oil not circulating ? 1999 Nissan Altima DOHC,

Well, from the oil pump it goes into your oil filter so the lines going from the pump into the filter could be clogged. check that and see. Also on most imports I have found that they have to be primed after the oil pump has been replaced. Buy a oil pressure gauge and hook it up. Disconnect your fuel and spark delivery and then just crank over your motor to make sure you have good pressure. It takes up to 30 seconds to build on some cars. hope that helps.

SOURCE: 2000 silverado changed oil pan gasket rear main

Ouch. A main, rod, or cam bearing failure will cause these symptoms. Did this happen before or after this work was performed?