I hope you guys can give me some advice. Thanks in advance!

I have a 99 auto Civic DX w/120k miles.

I purchased the car this summer, and have been absolutely **** about checking gas mileage. I reset the trip-meter and divide the miles by gallons each time I fill the tank. In city driving, I have gotten between 29 and 33 mpg every tank.

BUT, my last tank of gas (same city driving as always) got 19 mpg! And the current tank is heading in that direction. This drop was immediate -- the tank before got 31, then I got 19.

My timing belt was changed at 80k, so it's not that. My CEL is not illuminated. I don't see any fuel spillage where I park, but I park on slush so it's hard to tell. What's weird is that the performance seems the same, and the engine doesn't sound noticeably different -- it runs a bit harsh at startup, but I figure that's because of the cold (I live in Rhode Island).

I know that cold weather affects mileage, but enough for a 10-12 mpg drop?

Any thoughts would be greatly appreciated. Thanks!

 

Is the CEL ON?

***When was the last basic tune up (plugs/wires/distributor cap/rotor)?

Does the engine reach normal operating temperature at an expected rate?

 

Check the timing anyway, as this is a major reason for bad gas mileage. Like Ron suggested, if the engine doesn't warm up fully it will guzzle gas too.

 

Thanks for your replies.

The CEL is not on (and it's not burned out).

I don't know when the last basic tuneup was. When I bought the car, I got the records for the timing belt, and looked at the others (oil changes, etc.), but didn't take them.

I'm not sure what the proper rate for reaching the normal operating temp is. It gets to the normal temp within 3-5 minutes of driving.

Would a timing belt go out of whack so suddenly? I figured it'd be a gradual change....

 

I'd maybe check the water temperature sensor that sends the signal to the ECU. Normally there are 2 sensors (or 'sending' units), 1 sends a signal to your temp gauge, and the other sends a signal to the car's computer. BUT, if the sensor that sends the signal to the computer fails, it also trips the CEL. So, since you are certain you have no CEL, that wouldn't be it.... hmmm.

 

Danomatic has a good point about the coolant temperature sensor. Sometimes a sensor can be out of specification but not far enough that the CEL comes on. In the service manual, there is a way to test it with an ohmmeter. The intake air temperature sensor should also be tested the same way.

 

Thanks for all your suggestions! I'll post back when I get the chance to address everything.