You've been hit by an Octane Cheater. IMO Sunoco is the absolute worst for octane cheating but it can also happen at any gas station. As for why, look at the price difference between 87 octane and 91 or 93 octane. Fill that premium service tank with 10,000 gallons of 87 octane and your looking at a nice fat profit. Sad thing is that it may not be the individual station, it may be a fuel distributor doing the theft. Because NO, as in NONE, State has a policy of checking for Octane Fraud. Best thing you can do at this point is file a complaint with the local AND State Police. Other than that burn that gas up ASAP and find another place to get your fuel.
BTW, if you use your mileage indicator frequently this can act as an Octane Indicator. Because 87 octane results in about a 3 mpg loss in fuel economy.
You’re right that “Octane cheating” is pretty common at cheap gas stations. State inspections usually catch other cheating like dispense rates, contamination, etc, they don’t have any good way of properly testing octane (which basically requires burning it in a special test engine in a lab). Also, many states only test annually, so in some cases it’s as easy as making sure the months around the inspection are up to snuff and cheating the rest of the year. A few states also test based on consumer complaints, but I think they are in the minority.
Edit: I’m referring to true octane ratings which are half of the (R+M)/2 rating used in the US and are most relevant to high pressure in-cylinder load conditions especially for boosted engines. There exist portable test units to verify “octane index” at normal atmospheric conditions that are used by inspectors, but are dependent on infrared measurements and a derived mathematical model to predict octane.
Could also just be a batch of bad/old or contaminated gas. Add an octane booster to the tank and see if it goes away.
However, that said, if octane booster or a fresh tank of gas (possibly from a real top tier gas station) what the OP is describing sounds a bit more severe than that. I’d start by checking spark plugs for lean/rich condition, and logging data or gauges to check cylinder knock corrections, AFR, wastegate duty cycle, fuel rail pressure, LT/ST fuel trims, etc.
The engine might be pulling timing or power because of knock or AFR being slightly off, Or it’s possible it could be something like a partially plugged catalytic converter or fuel filter/pump issue or maybe an O2 sensor starting to go.
Sounds like something is “off” and recent, but just not enough or for long enough to trigger any codes. Might also check cylinder compression, and for coolant in oil or vice versa to rule out a popped head gasket, which is not out of the question running increased boost levels.
@flbchbm What mods and what tunes have you been running and what year car? It’s an off possibility, but any pop/crackle tunes might have accelerated a cat failure even if it’s not a direct cause. If your tunes are older, some may also have things like Cat CEL disable, which would also hide something like a cat failure or exhaust issues (restrictions or muffler damage) from triggering a code.
