The effects of putting syrup in the gas tank of a car depend on how far the syrup gets into the fuel system. If the car is started and run for any length of time, the syrup can end up clogging the fuel filter, which can cause the flow of fuel to be restricted, and in turn can cause the engine to stop running. If the syrup gets to the filter, the gas tank may need to be replaced.
At the very least, the gas tank and fuel filter need to be cleaned when syrup is put into the gas tank. If the syrup gets past the fuel filter, the syrup will travel to the cylinders and valve. This will cause the valve to get stuck in an open position, and when syrup comes into contact with the cylinder, carbon deposits will be left. Both of these mechanical parts are essential for the engine functioning properly, so the engine will shut off if syrup passes through the fuel filter.
Syrup in the gas tank may also cause the spark plugs to malfunction. If this is the case, the spark plugs will need to be replaced. The best thing to do when syrup has been poured into the gas tank is to have a mechanic flush the fuel lines, gas tank and clean any affected parts. Ultimately, the longer a car is driven with syrup in the gas tank, the more damage that is done to the car.
It is important to note that if sugar was placed in the gas tank, the sugar will not get past the tank because it will sink to the bottom of the tank. However, syrup is capable of mixing with the fuel and getting into many different parts of the engine.