The signs that an engine has been overfilled with oil include oil leaking from the engine, the smell of burning oil and irregular engine noises. Symptoms resulting from an oil overfill progressively worsen over time if the problem is not properly corrected.
When an engine is overfilled with oil, signs that the overfill occurred progressively worsen as the vehicle is driven further. The excess oil leads to increased pressure in the engine block, which can cause the rear main bearing seal to rupture, leading to a small oil leak. In cars with manual transmissions, this rupture allows oil to contact the flywheel and the clutch disc face, which can lead to front seal failure. As the leak continues to worsen, oil is spread throughout the engine bay as far out as the wheel brakes.
If enough oil is added so that there is contact with the rotating crank, then a froth of aerated oil can be formed. When this froth coats the engine bearings, there is an increase in friction because air is not a lubricant. Signs that the bearings have been coated with aerated oil include the smell of burning oil and an irregular sound emanating from the engine compartment.