Piston Slap: Charting the Changes, To What End?

TTAC commentator wannabewannabe writes:

Hey Sajeev,

I wrote you a while back about my 1990 Chevy pickup, but I’ve since moved on to more interesting cars.

I have a 1994 Cadillac Fleetwood Brougham (B/D-Body with an LT1 FTW!) that looks literally identical to this one, same color inside and out. It currently has just under 184k miles on it, is my daily driver, and is in pretty good shape. Very reliable car!

My question concerns fuel economy, and it’s stumped me. I commute between 75-80 miles round trip every day, which makes my weekly mileage about 400 miles, so I fill up once a week. I always fill up at the same gas station, I drive the same routes on the same days, no one else drives my car, and because my commute is mostly freeway, I’m a copious user of cruise control. Also, I should mention that because I live in the Bay Area in California, our weather doesn’t vary wildly; it’s usually relatively mild. Yet given all this, my gas mileage routine varies up to 15%. Some weeks I average about 21mpg, while other weeks I average about 24mpg. To be fair, the average is in the 22.5mpg range, and you’ll see a good number of data points there too.

I would expect some variation based on the automatic shut off on the pumps and to a certain degree, variances in temperature, particularly when filling up, but I’m surprised that my fuel economy swings as much as it does. I’ve included a rudimentary chart to show you the swings, though it doesn’t have the last month of data included. Thoughts?

Sajeev answers:

Nice rig! You’ve certainly done a great job explaining your situation..and is it ever a doosie!

The most logical choice, given your input, is the gas pumps do shut off at different “full” levels. Perhaps there’s something wrong with your Caddy’s vapor recovery system that’s making it happen. Usually that throws a trouble code and a light on the dash, so probably not applicable here.

Perhaps the weather may feel about the same, but the problem is exacerbated by a change in barometric pressure? Is that even possible?

If, by any chance, you haven’t changed your Oxygen (o2) Sensors, do it! The engine computer is only as efficient as the information it receives. And flaky sensors could cause the variance we see here, especially if they are original at this mileage. Worn out, but not enough to throw a trouble code. It’s possible!

The O2 sensors sound more likely, the more I ponder this. Off to you, Best and Brightest!

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.