I have worked as a drivability technician in a dealer ship and the private shops for close to 30 years. One thing that I have faced in my career which can be a difficult variable to quantify in engine operation is bad gas. No one that I have ever seen examines their gas before or during filling their vehicle except people putting gas in cans. Yet it causes untold problems that manifest themselves in pretty bizarre ways. A random hard start, stumble, hiccup, stall or sputtering are all symptoms of contaminated fuel as well as poor mileage and often a foul smell to boot. It can cause misfires and even check engine lights.
There can be 2 kinds of bad gas. Gas that is old and has lost most of its volatile components through evaporation and has thickened to varnish or contaminated gas which has had water, dirt or some kind of other thing put in it that shouldn’t be there. Old bad gas is rare from a filling station but contaminated fuel happens all of the time. Just because gas companies have a ton of regulations and standards thrust upon them doesn’t mean that all fuel is not contaminated. Most of that comes from holding tanks that have ground water leaking into them or storm water during storms.
Gas already has additives to absorb water and it can be soluble in gas and is not a problem as long as it doesn’t dilute the gas to the point where volatility suffers. It’s when water contamination gets to a point where it can’t be absorbed anymore that it collects in globs in the fuel systems and can run through a little bit at a time causing a multitude of symptoms that can take a lot of time to diagnose.
I recently heard the owner of another establishment say that there is never bad gas and that he has never seen it in 25 years of being a technician. I can tell you from actual experience that I have seen it in at least 15 % of the cases and it’s even more prevalent during winter. As a drivability tech, I take fuel samples whenever I can to make sure that I’m not chasing a ghost when trying to diagnose a difficult engine performance problem. I have chased the ghost and when it turns out to be bad gas, I try to find it first and save myself and the customer time and money on what could otherwise be an expensive and frustrating situation. I wonder what that other guy tells his customers when he can’t find that fuel related issue.
So the bottom line is buy gas from busy station because it turns over more often and try to be aware of any performance changes after getting fuel at a certain place and if you feel a significant difference, change where you get gas and see if it’s better somewhere else. Before you take your car in for a random performance glitch occurrence, try some water remover in your tank and see if it makes a difference. Then bring it in to be looked at.