Putting diesel fuel into a gasoline engine also causes serious consequences, but the mechanism is different from the reverse case. Here is the technical analysis:

1. Causes and Mechanism

• Gasoline: Highly volatile, ignites at low temperature, ignited by spark plugs.
• Diesel: Less volatile, ignites only under high compression, denser and more viscous than gasoline.

When diesel enters a gasoline system:

• Difficult to ignite under spark plug conditions → leads to incomplete combustion.
• Heavier and more viscous nature → clogs injectors, spark plugs, and catalytic converters.

2. Technical Consequences

1. Hard starting or failure to start
   • Diesel does not vaporize properly under spark ignition → results in an overly rich air-fuel mixture.
   • Result: Long cranking, engine may not start, or stalls immediately after starting.
2. Misfiring & power loss
   • Gasoline burns but diesel remains unburned → misfire, vibration, black smoke.
   • Effect: Weak engine, poor acceleration, possible stalling on the road.
3. Rapid soot & carbon deposit build-up
   • Incomplete diesel combustion produces soot that coats spark plugs, valves, and pistons.
   • Consequence: Valve sticking, reduced combustion efficiency.
4. Clogged fuel system & gasoline injectors
   • Diesel’s high viscosity clogs fuel filters and injectors designed for lighter gasoline.
   • Result: Fuel pump and injector damage.
5. Damage to catalytic converter
   • Unburned diesel produces soot, HC, and SOx → deposits clog and damage the catalyst.
   • Repair cost: Very expensive catalytic converter replacement.

3. Severity Depending on Mixing Ratio

• Below 5% diesel: Car may still run but with smoke and poor performance. Fill up with gasoline to dilute and drive to a workshop.
• Above 10–20% diesel: Engine hardly runs, risk of stalling and fuel system damage.
• Full tank of diesel: Engine will not start, requires immediate intervention.

4. How to Handle If Detected

1. Before starting:
   • Do NOT crank the engine.
   • Drain all fuel, flush the tank and lines.
   • Replace fuel filter, refill with clean gasoline.
2. After starting:
   • Stop the car immediately.
   • Tow to a workshop, drain and clean the fuel system, replace filter, clean combustion chamber and spark plugs.
   • Check catalytic converter and fuel injectors.

Conclusion

Filling diesel into a gasoline car causes:

• Short-term: Hard starting, stalling, black smoke, vibration.
• Long-term: Clogged filters, damaged injectors, spark plugs, catalytic converter → high repair costs.