Bad Gas Causing Bad mileage, or is it Something Else?
 

1995 E320
206K Miles

This past weekend I filled about half my tank (8 gallons) with gas from a Getty station in Delaware, since it was the only one available before getting on the interstate. I normally use only Sunoco 93/94 or Exxon 93. (The Getty was 92.)

Since then, my fuel economy appears to have dropped noticeably, perhaps as much as 30%. Could this be the result of bad or watered-down gas?

The only other thing I did to the car that weekend was replace my neutral safety switch, during which time the car was jacked up pretty high at the driver-side front, and also during which time I occasionally pushed against a fairly thick rubber hose (vacuum??) adjacent to the transmission and the neutral safety switch. The neutral switch also needs to be correctly realigned, but I don’t think its misalignment should impact fuel economy.

In addition, I have noticed that my oil pressure, rpm and fuel gauges seem “weaker,” sitting at lower than normal levels when the car idles, and my engine sounds quieter than normal.

My first thought was “bad gas causing weak engine performance” and my second thought was “electrical fault in instrument cluster tricking me into thinking my fuel consumption is greater than it is.”

I plan to let the tank run empty before refueling, to burn off as much of the existing fuel as I can. Anyone who has experienced this, please share our thoughts.

You need to fill up, drive some reasonable distance, then fill up again. Then you will know what MPG you are really getting. Gauges can have problems as you know.

Thanks for the reply.

I think it may be partly a quirky fuel guage issue. I was a hair away from empty/R and added 8 gallons -- and the gauge indicated a bit less than 3/4 full.

But mainly, I realized I am way, WAY overdue for new spark plugs and I will address that first.

Not sure if I need to do that yet(?) :confused:

My last spark plug change was approx 40,000 miles ago (Jan. 2006), which included a new engine wiring harness, new spark plug wire set, and new OVP relay.

Isn't the spark plug change interval 100,000 miles or 5 years, whichever comes first?

No, spark plugs are every 30k miles.

Thanks. Plugs have now moved (back) to priority one.

Since all ignition wires were recently replaced, I assume the boots are still ok?

Boots should be part of a new wire set and should still be OK.

Normality is restored, and mileage may even be slightly better than before.

Odd thing is, the return to normal fuel consumption did not happen immediately, but rather over the course of two or three days.