back pressure,

I went to check out a 2001 tracker it is a 2.0 or 2.5 not sure but while it was running, i un screwed the oil fill cap and noticed the back pressure not really blow buy but strong enough to push my hand away.after putting the oil fill cap back on the tracker wouldnt stay running it stalled and sputtered and shook like a leaf on a tree.is back pressure in the crank case common on there engines or did something else come apart

Thanks in advance for any and all help