Oxygen Sensor?

[Nightfall]

What I have noticed
~~~~~~~~~~~~~
In Denver, my mpg have dropped lately from 50-55mpg down to 35mpg. I'm in Wisconsin right now and my gas mileage has increased to 40mpg.

My Conclusion
~~~~~~~~~
When an oxygen sensor fails the engine starts running really rich. Driving in Wisconsin right now, at sea level, there is more oxygen to combust with all of the gas it is dumping in (compared to at Denver's altitude) because the failing oxygen sensor is causing the engine to run rich, hence the reason my mpg increased from 35 to 40.

Question
~~~~~~
Am I correct in assuming that I need to replace the oxygen sensor? Is there any other culprit for such a large decrease in gas mileage?

195k on engine
recently replaced sparkplugs and wires
fairly recent oil change
all tires at 35psi

[Weston-work]

Yeah, that makes sense. Could be a number of things though... when was the distributor cap & rotor last changed?

Also, how were you getting 50-55mpg? What car was this in?

[Nightfall]

I was getting it in my CRX HF. What do you mean by how? I got in my car, drove, and it used one gallon every 50 to 55 miles. :confused: Haven't replaced the cap and rotor.

[Weston-work]

I just didn't realize those cars got that kind of mileage.

[Nightfall]

EPA is 45/52

CRX HF > Hybrid ;)

[Enthalpy]

The oxygen sensor can cause this, but so can a lot of other things, like an exhaust leak causing your O2 sensor to read ambient air and richen things up because it thinks things are lean.

Bring the car up to a minimum level of maintenance. Follow Honda's instructions and replace things that should be replaced like ignition components. Replace the O2 sensor if it has more than ~75,000 miles on it, as they can start to get lazy at this mileage.

[Nightfall]

I am going to replace the O2 sensor and the cap and rotor this weekend. If I still get "crappy" gas mileage after that, i'll have to leave it in Tobi's hands.

[Nightfall]

Just finished replacing the O2 sensor, cap, and rotor. I'll give it the rest of this tank of gas to adjust then record the mpg the tank after. I saw some smoke coming up from between the header and valve cover. I think it's leaking a little bit of oil from somewhere and burning off. The valve cover is clean all the way around so it's not that gasket...

[Nightfall]

One other thing that I thought of. When my engine is cold, it makes quite a bit of noise. I am a complete car nub, but I think it's the valves making a bunch of racket. I would describe it as sounding like a diesel engine IMO. I know that Honda's engines all make quite a bit of valve noise, so i'm unsure if they need to be adjusted or not. If they are off by a bit, could that drastically effect my gas mileage?

[STIBungy]

Depending on the mileage of your car and the last time you did a valve adjustment, I would say that you are overdue for one.

[Weston]

Yeah, I've had my valves be out of adjustment and cost me a couple MPG before. It's definately worth adjusting.

[computingfuture]

Has it got any oil in it?

[Nightfall]

Has it got any oil in it?
:rofl: ....yeah.

[doctorstupid]

The oxygen sensor can cause this, but so can a lot of other things, like an exhaust leak causing your O2 sensor to read ambient air and richen things up because it thinks things are lean.

Bring the car up to a minimum level of maintenance. Follow Honda's instructions and replace things that should be replaced like ignition components. Replace the O2 sensor if it has more than ~75,000 miles on it, as they can start to get lazy at this mileage.
:werd:

O2 sensors have an irritating tendency to go bad long before they throw a CEL. When healthy, their output is erratic to begin with, so it takes the ECU a while to determine that it's actually malfunctioning.

And if you have excessive valve lash, needing an adjustment, you're losing lift and duration from the extra clearance, which could effect mileage.

[Enthalpy]

When I discovered that O2 sensor issue of getting lazy LONG before they pop a check engine code, I started replacing all my O2 sensors at the 60,000 mile interval. Overkill? I don't think so. I have yet to have any of my cars drop below the average of city and highway mileage, too. The proof is in the pudding!

I also don't drive like an 18 year old, either. Consistent, steady driving and avoiding jackrabbit starts and stops is huge!

Adjusting the valve lash is DEFINITELY what I would include in my "bring the car up to Honda's minimum maintenance" statement. It's listed in the maintenance schedule, and quite a few people I know with modified Hondas inspect it every 15,000 miles!

[doctorstupid]

A really good indicator of a bad sensor is going really rich and stinking to high hell on the highway. O2 sensors are always a good maintenance item, I would defiantely agree with the 60,000 mile figure. Hell, Mitsu calls for their replacement at 60k as well.

Sensor + valve adjustment and any other maintenance that needs to be done will more than likely get you your mileage back :)

[M@]

I had to pull and clean my o2 sensors every oil change since my turbo was burning so much oil. It was clogging the hell out of them. You can deffinatley tell when an o2 sensor goes bad, gas mileage goes to SHIT.

[Nightfall]

Is there going to be a mpg difference between a Bosch O2 sensor and a Honda OEM O2 sensor, or is the difference going to be lifespan? I went cheap and bought a Bosch :(

[Enthalpy]

Some Hondas use widebands. Others use heated vs. unheated narrowbands. As long as you get the right spec sensor, I don't think there will be an issue.

[Weston]

Is there going to be a mpg difference between a Bosch O2 sensor and a Honda OEM O2 sensor, or is the difference going to be lifespan? I went cheap and bought a Bosch :(

I wouldn't say Bosch is cheap in terms of quality. I've never had any problem with their stuff, unless I smashed it or something.

[doctorstupid]

I wouldn't say Bosch is cheap in terms of quality. I've never had any problem with their stuff, unless I smashed it or something.
Hell, there's a good chance your stock sensor was Bosch anyway. I've never had a problem with them, and they do come OEM in many cars.

[Weston]

Hell, there's a good chance your stock sensor was Bosch anyway. I've never had a problem with them, and they do come OEM in many cars.

Honduh is in bed with NGK.

[Nightfall]

Last tank of gas - 53mpg

Thanks for all the help.

[M@]

Last tank of gas - 53mpg

Thanks for all the help.
Damn. That would be great if even ONE of my cars did that... :cry:

[john]

Your MAF/MAP/etc and ECU compensate for the amount of O2 being ingested. I haven't seen any difference in mileage when we travel from Castle Rock (6200 ft ASL) to Wisconsin every summer. The honda minivan gets a pretty constant 21 mpg at freeway speeds (75-85) both at altitude and closer to sea level.

It could be an O2 sensor, but it may also be a flakey MAF/MAP/etc, intake leak, who knows what else.

[Enthalpy]

Your AFR won't vary with altitude. Honda is one of the OEMs that do this the best. It's not reasonable to expect mileage changes with altitude.