I just bought this car two weeks ago, it had a bad blown head gasket. I fixed that and also replaced the intake and exhaust manifold gaskets. after driving 100 miles, it would be down half a tank, there are no check engine lights, it idles perfect, seems to run good overall, no exhaust smoke, doesn't overheat, car is completely stock, no cat. new plugs, wires, cap, rotor, all tires have same air pressure, new air filter, frequent oil changes. trans shifts good, no slips.

the engine has higher mileage, over 200,000 but its been driven easy its whole life and I drive it easy now.

its a stock D15b7, automatic. is it possible after the blown headgasket, the o2 sensor got all gummed up? I double checked the timing and its fine. it also seems to be a little slugish when accellerating but fine after you get it moving.

any ideas or tests I can do to narrow it down? Thanks

 

i would check the o2 sensor first and foremost.

a dirty o2 sensor can make the car more gas than its suppose to.

also check to see if you need a wheel alignment, while driving, let go of the steering wheel on a level surface, if the car turns excessively on its own, needs aligning

 

yeah, definitely check the o2 first.

Auto eg's get significantly worse mpg than the manual ones.

I drove a 92 DX auto one winter and I don't think I ever got over 27mpg with it compared to the 40 mpg I get with my 93 Si.

 

I replaced my o2 sensor today, the car seems to run better. its not so boggy when accellerating but I haven't driven it far enough yet to see if fuel consumption has went down.

 

have you driven your car long enough to evualate your cunsumption?

 

its alot better then it was but its still not great. I think its just because its a worn out 1.5L auto with 250,000 miles. It may also be related to putting my 15" steelies and Bridgestone snow tires on, I never drove it with the stock 13" wheels and tires but the added weight has to affect it somehow.

 

wouldnt degrade mpg dramatically, an automatic tranny does get several mpg's lower than manual trannies though

may be time for a tune up and a leakdown test

 

The bigger the rims you have, the more power it takes your engine to turn those wheels. Racingmason was right. Its because of the added weight from the wheels.

Then again, if you dont have rims... go back to the O2 sensor idea.

 

I have an auto 95 DX hatch and I get 42 - 43 on the highway in good weather. The mileage is going to hell in the cold though. Have yet to figure that one out. It has 246,000 miles.

 

I noticed a decrease in mileage with my hatch in winter too when I drove it last year.

I replaced the o2 sensor already and it did help but I'm starting to think that the heavy 15" steelies and snow tires are probably hurting me a little bit. there seemed to be quite a difference in weight between the 13" steelies and the 15" steelies. I never drove the car with the 13" wheels so I don't have a comparison, I might switch and see if I get any change.

The car has had all the basic tune up stuff recently so I'm starting to think the snow tires, high mileage, winter weather and being an auto are probably hurting some of the gas mileage.

 

The bigger the tires or the heavier the rims = more rotating mass. In the winter your fuel is colder and doesn't burn as efficiently, thus your car uses more fuel.

 

I put the snow tires on early this year and I just got 43 MPG an a 410 Mi. trip. Certainly they decrease mileage but not as much as you might think. For the record in the winter I still get about 41 MPG as long as it's a straight highway trip the whole tank. Even if all of my driving is highway but broken up into multiple trips I still get bad mileage in the winter. I think it has to do with warming up. I'm installing a block heater soon to put this theory to the test.

 

This is true. Think about this too, the car calls for 13's stock I think.. and with 15s the outter circumference is greater. Meaning one full rotation of the 15 inch wheel is greater than that of the 13 inch wheel. This will cause your tripmeter to read less miles than you have actually traveled.

 

You did all the basics, to the ignition system. I didnt see you say you changed the fuel filter. Might be worth trying also.

Before you do run a little bottle of injector cleaner. Then change fuel filter, Cars that sit for a long time tend to always get uckie fuel filters also.

But that being said 2000 and under automatics just kill mpgs. I had a 2000 acura el ( civic EX for you guys ) in 5 speed and another in automatics 2 years after.

and in non winter would get 32 mpgs average in 5 speed ( thats 50/50 highway city ) and with the automatic get 25-28 mpgs average.

winter with 5 speed went down to 29-30 mpgs

and automatic is down to 20-22 mpgs . I noticed that having fresh synthetic tranny fluid has helped it get a little better.