With varying effects, the simple answer is yes.
Leaking injectors that are allowing fuel to flow after shutdown, will allow fuel to flow into the cylinder. This fuel will mix with the oil and dilute it's lubricrating ability. This will cause engine wear and will eventually cause serious engine damage.
It will really mess up the fuel mileage,and driveability
Biodiesel most certainly does NOT ruin your engine. Most engine manufacturer in the world approves the use of biodiesel in their engines, unless of course your engine is not made to run on diesel fuel. Gasoline engines should not be ran on Biodiesel.Actually, if you don't change out your fuel pump, injectors, and fuel lines to accommodate the much thicker biodiesel fuel, it CAN ruin your engine.
It is not a great idea, you will be likely to ruin your fuel system. Jet fuel is low lubricity compared to diesel fuel. Perhaps someone can identify a commerially available lubricity additive. The US military has a lubricity additive to make jet fuel acceptable for ground vehicle use.
Yes. If a coil is bad causing one or more cylinders to not fire, this allows raw unburned fuel to enter the catalytic converter, where it is burned. This can ruin a converter.
A catalytic converter's function is to filter most pollutants out of the exhaust so it probably wouldn't mess up the computer in the car or lower gas mileage. On an OBDII vehicle(post 1996) the computer watches the O2 sensors to monitor converter efficiency If the converter has failed(or is missing) the computer will adjust fuel ratios to try to compensate This can ruin economy,
Timing is off, misfiring spark plug, or overly-rich fuel/air mixture. This will ruin you catalytic converter.
NO, if you put it in the fuel it will ruin the car.
Black smoke from the exhaust is a sign of an overly rich fuel/air mixture. This can ruin your catalytic converter, plus it hurts your mileage, so get this repaired asap.
It can. Backfires are caused by raw fuel in the exhaust pipe and lit off by hot gases from the cylinders. If enough fuel enters the exhaust system and the muffler or catalytic converter, it can cause enough pressure once lit to split pipes and mufflers.
Black smoke is an indication of an overly rich fuel/air mixture. Your check engine light should be on with this condition. You can ruin the O2 sensor and the catalytic converter if you do not get this repaired ASAP.
Dirty injectors, dirty fuel filter, need new spark plugs and wires. Do the cheapest first. DO NOT stick a feeler gauge in the spark plugs, you will ruin them. They are pre-set.