Q: Diesel in gasoline tank

If your fuel tank capacity is 20 gallons and you presently have 0.5 gallons of diesel in a full tank, only 2.5% of your total fuel (the 20 gallons) is diesel. That level of contamination (2.5%) will reduce the average octane of your 20 gallons of fuel by about 1 point (from 87 to 86 octane). Normally, the effect on engine operation will be fairly minor. What you can do is add an octane enhancer to the tank from any auto parts store to offset the reduction and, as well, as you burn off fuel, just refill with premium grade fuel. Although only 2.5% of your fuel is contaminated, if you do notice pinging or knocking, reduce the load on the engine, that is reduce speed until the pinging disappears. If the check engine light comes on due to misfiring, you would have to stop and drain the tank (misfiring and pinging is damaging to an engine). The bottom line is at 2.5% contamination you have a minimal amount of diesel, there will be some degradation to the average octane rating, with consequent negative effects on engine operation but if you do NOT detect pinging or knocking, you can probably run the car safely enough, and long enough, that you just keep adding fresh gas (as you use up the gas in the tank and make room for new gas) which will have the effect of further diluting the diesel from its already low percentage of 2.5%. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.