I've had my '98 Civic for a little over three years now. I've averaged between 31-33 MPG. About a month ago, I got 32MPG then on the next tank I only got 27MPG and then on the following tank I got 28MPG.

Same weather conditions, same driving style, same route driven everyday. I got the gas from two different gas stations. The car seems to be running just fine as it's idling fine, running smooth, the power is there and there are no CEL or other warning lights on.

On the tank that I got 27MPG I ran a bottle of Chevron Techron fuel system cleaner, which I've run plenty of times before and had nothing but good results.

Fuel filter was replaced about three years ago, air filter (AEM SRI) was cleaned back in August, tire pressure is good and plugs look rather new.

Any ideas?

 

My best guess is that it has something to do with the two separate gas stations. To my experience (I live in PA during the school year) I t end to get slightly lower gas mileage from the popular local gas station chain Sheetz than I do as, say a big time Sunoco or Mobil station. The trade off here is that the gas at Sheetz is cheaper (slightly, but appreciably) than the gas at many other large scale and better known national brands. What contributes to this drop in milage I can't attest to. It has something to do with fuel composition but it never is accompanied by damage to the engine or fuel system so I don't think (at first thought anyways) that the mpg drop is cause for considerable alarm. The next thing to do, I think, is do two tanks at each station and calculate your fuel consumption for each to see whether the mpg difference is a causal relation to the gas station.

That's my best guess anyways from personal experience.

 

I got poor gas mileage from both stations though. I could see if I got good mileage from one and bad mileage from the other, but that's not the case. Also, the place where I got 27MPG is the same station where I usually get upwards to 35MPG during the Summer.

Is it possible that the Chevron Techron fuel system cleaner dislodged something which is now clogging up something?

 

It's possible but I think unlikely. The chemical treatment breaks up the deposits by dissolving them and so with continuous exposure to the chemical (especially when the chemical is almost constantly moving) whatever large breakoff pieces there are should dissolve as well after a short time. Is the treatment you're using particularly potent? By that I mean, is it an aggressive treatment or is it one that is formulated for frequent use (like every 3-5 thousand miles)? I ask because the odds of it breaking off a big chunk that could cause other problems is higher if the chemical is very aggressive and lower if its a moderated formula designed for frequent use as preventive maintenance.
Also, if you had a chunk, it would likely work its way towards blocking fuel to one of your cylinders partly, which may not drop your mpg considerably, but would definitely come with a noticeable drop in performance. If you have noticed that your engine is a little more sluggish or hesitates/takes longer to start, you want to look into that possibility more.
I will say though that fuel cleaners don't combust the same way as gasoline and so they are (however slightly) diluting your gas. While I doubt this dilution is enough to account for a 5 mpg drop, it's possible the cleaner itself is contributing to the symptom.

Unfortunately, I don't have any extra insight on the issue. I'm car-literate enough for most DIY stuff but beyond the basics of engine mechanics I'm afraid my knowledge drops off Hopefully one of our more experienced members can give you some new insight on the mileage drop. I would say though, check your mileage again after you've run the Chevron out of your gas system to see if you recover any of the mpg. It may offer some new info regarding the problem.

Best of luck!

 

Thanks for the info Tatsuma91.

I'll be getting gas either tomorrow or Friday and it looks as though my MPG will be just as bad as it has been the past month or so.

I'm taking it to my mechanic next week to have them check the spark plugs, wires, oxygen sensors, fuel filer, etc. to see if anything is going on.

 

Well my mechanic checked the plugs and wires and said they were fine. They road tested it with a scanner to check the oxygen sensors and said they were also fine. Air filter has been cleaned recently and tire pressure is good, so I don't know. He also said they advanced the timing a little, not sure what, if anything, that will do.

 

ah, i have the same thing happen to me.