For about the last six months my gas mileage has decreased quite a bit. I used to be able to go about 260 miles on a single tank. Now I'm lucky to get 220. All of the regular tune up parts (including fuel pump and distributor) were installed over the past two months with no change in mileage. My driving habits haven't changed and tires are at the proper pressure. The only other piece of the this puzzle that I could come up with would be the timing belt. It's a little overdue for replacement, but I'm wondering if that's causing my mpg issue. Is this even possible or am I missing something here?

 

did you even bother to replace the spark plugs, plug wires and air filter?

 

Simple answer, no.

Is this the first winter you've had your car?

By the way, what car do you have which only gets 220-260 miles per tank?

 

@230k miles, there are many factors that can contribute to a reduction in gas mileage.

What have you done as far as maintenance to keep it running efficiently?

Spark plugs, wires, cap, rotor, ignition coil? Air filter?

When is the last time your O2 sensor has been changed?

What is the compression of the cylinders?

How much % loss do you have during a leakdown test?

Have you cleaned all of the sensors on the intake?

Have you performed a valve adjustment?

 

This is a key factor. Gas mileage always drops in colder weather.

Caoboy's comment above is also very relevant, especially the last line

 

one thing not mentioned....OP, are you loading up the vehicle?? For example, if you have a truck and the bed is loaded down with mulch, the overall weight is more than a truck with nothing in the bed. Gas mileage will suffer...

 

My car is a 98 Integra that I bought new, so not my first winter with this car. And yes, I'm well aware that gas is shitty in winter. Plugs, wires, distributor, air filter, fuel filter, and fuel pump are all new. Valves were done last month. Oil/filter changes are done about every 4k miles along with tire rotation. I also put in Lucas fuel injector cleaner every 4-5 tank fills. O2 sensors are original. I have not cleaned any of the intake sensors or had a compression/leak down test done. The car is not loaded down either.

Update: Got my google on and it seems some new o2 sensors are on the menu. I read a few different articles that say old o2 sensors can decrease mileage about 15%. The decrease in mileage I'm seeing is about 15%. I figured no CEL for the sensors must mean they're working fine. I guess not. Would it be safe to assume that Denso is the go-to brand since OEM is ridiculously overpriced?

 

would have saved all of our assumptions if you have mentioned this in your initial post. We aren't mind readers.

Denso O2 sensors should be fine. Bosch will work too but some have complained having issues with that brand.

 

A few comments:

• Lose the Lucas fuel injector cleaner; snake oil
• Yes, a lazy O2 sensor may well be the culprit for the lower fuel economy.
• I would STRONGLY recommend against Bosch O2 sensors, especially in Honda engines; they often more problems than they solve.
• A double yes on the Denso O2 sensor.

 

Denso and it uld be that your o2 sensors are lazy but not yet faulty.

 

Hey look! We agree! lol.
Yes, everything said here I agree with.

For the O2 sensor though, get NTK brand from rockauto.com
NTK=NGK, and that is oem brand and is much better than bosch and a bit better than denso

On the note of worse gas mileage in winter discussed above, it's not because of crappy gas. It's because colder air is denser and requires more fuel for optimal burn.

 

I ended up ordering Denso sensors through Amazon. I can't argue your logic about the crappy gas, but out here in CA the fuel companies change up the mix around late fall. That usually begins subtle decline in mileage.

 

Thats not how that works,bud.

Lower FE(fuel economy ) has everything to do with the blend and nothing to do with air density. In fact,colder air is more fuel efficent becuase it is cold.

 

Quick update: On my way to work this morning, my CEL comes on. I go home during lunch to hook up my code reader and up pops the codes P0420 and P1705. I had planned on changing out the O2 sensors tomorrow, so hopefully that'll take care of the P0420. I'll have to google the other one.

 

No need to mess around with the downstream sensor, just replace the upstream sensor(s), preferably with Denso or NTK units; whatever you do, do not use Bosch units, they don't play nice with Hondas.

 

Update: Changed out both o2 sensors and mileage is back to normal.

 

You can ignore the rear O2 until it gives a heater circuit code....