If your having oil burn-off its most likely because the motor is worn out and the piston rings are allowing oil to get past into the cylinder. If you have overheated the motor it could be a cracked block or head. Some Chevy 350s, like the one in my impala, the factory head was too thin and would crack about 120k miles. Basically if your oil is getting in the cylinder there is major internal engine work that needs to be done.
That is extremely bad! You could have a seriously engine fire start and rupture your whole vehicle. You have some bad gaskets in that engine! You should replace them as soon as you can.
12 mpg
no... you don't want to mix diesel and gas engine parts if its not an exact match you can mess your engine up....
i would put a 700r4 that's a good one for the gas per mile and towing or 350 turbo for the other
engine timing
you need to tune the carburettor
on 1979 gmc 350 cu in engine the mech'l fuel pump is on the passenger-side towards to front end of the engine. it mounts just above the oil pan. follow the gas line coming off the carburetor. this gas line will take you right to the mech'l fuel pump.
DTC- Diagnostic Trouble Code.
The most common engine used was the 5.7L 350 V-8. But it may have the 6 cylinder in it. Count the spark plugs.
gas
That is a GAS engine.
Your engine has worn piston rings. Get it fixed asap because the oil is no good and will seize the motor. it can also come from flooding carb especially on new engine