If diesel fuel ends upÂ in a gas tank by mistake, it's unlikely that the engine will even start. However, if it does start, theÂ car's performance will decline because the engine's timing will be off orÂ could cause engine knocking.
Diesel engines do not have spark plugs because the engine gets hot enough to light the fuel. But, in gas cars, the engine doesn't get hot enough so the gas relies on the spark plugs to light the gas on fire. Diesel fuel in a gas car won't light properly, which is what leads to the timing issues and, in some cases, knocking issues. Furthermore, diesel can clog fuel lines and injectors. If diesel accidentally ends up in a gas car, the best thing to do is to siphon it out completely and replace with regular gas.