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your air filter is cologged, since it cannaot get an adequate air supply through the air filter it creats a suction thus sucing oil through the crankcase breather which is located in the intake, after the air filter
Backfiring is usually an indication of a lean condition. When it does it during cranking, it's probably timing related.
Check the IAC (Idle Air Control) valve on the back of the Throttle body http://www.youtube.com/watch?v=gHjl6BWp6j4
I'm not a mechanic / technician but I believe that is installed in the upper section of the engine air filter box
Check the inside of the distributor cap for a crack or spark traces. If that's not it, check the timing. Far less common but still a possibility, if it's popping back through the air intake it could be a burnt or stuck intake valve.
Intake Manifold
The intake air temperature sensor on a 1991 Honda Civic is found on the intake. The intake air temperature reads to climate of the air that is being sucked in through to the engine.
It's intake air temp sensor so somewhere on the intake tube to throttle body-many times is an extra wire in the maf so may be part of it.
Okay, there's a little hose on the side, going into your intake. This is called the blow by tube. Chances are you need to replace your PCV (Positive Crankcase Ventilation) valve, and this may clear up your problems. This happens when pressure gets high in an engine, or when you have a blown seal or valve. The oil is being pushed up through the tube, and thus enters your air intake chamber.
To prevent backfiring make sure the belt is tight and in good working order. Also clean the air filters regularly.
over fueling egr problem restriceted intake air system or CAC tubes or clamps loose turbo problem.
To allow the air to enter the body. The air can enter through either the nose or through the mouth.