Abnormally low gas mileage.

[young_]

So I just got back from a friends house and he lives an hour away. The drive is an hour for 50.9 miles. I filled up before I left, and then went there and back. It's all interstate driving. So for a total of 101.8 miles, I used almost half a tank of gas. Five gallons to be exact. So 101.8 miles divided by five gallons of gas = 20.36 mpg.

That's extremely low for a 1996 Honda civic ex, stick shift, with only an aftermarket intake and a header.

I have a CEL on for my downstream o2 sensor, and I replaced it with a Bosch o2 sensor but can't get the code to go away. I've tried all the ways to clear it. Hood ecu fuse, negative battery terminal, etc.

No real performance issues, exhaust doesn't smell like its running rich.

I know I have a small exhaust leak at the header from shitty installation (friend of mine cross threaded the bolt onto the stud so it doesn't seal all the way at one point.

The only other issue with the car is the second gear synchro is bad and sometimes second pops out, but I don't see how that could be related. But I figure it's worth mentioning.

Tire pressure is at spec all the way around. Car has a relatively fresh tune up. Plugs, wires, cap, rotor, fuel filter, and... Yeah that's it.

I also know the air filter needs to be replaced soon, but could it, alone, cause such low gas mileage?

Any ideas?


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[RonJ]

I know I have a small exhaust leak at the header from s***ty installation (friend of mine cross threaded the bolt onto the stud so it doesn't seal all the way at one point.

This^ is almost surely the problem. Fix the leak.

 

[young_]

Didn't think it could cause that much of a drop in gas mileage. I'll get to it, thanks Ron!


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[RonJ]

Didn't think it could cause that much of a drop in gas mileage. I'll get to it, thanks Ron!

The primary O2 sensor sends it's information to the ECU, which uses the information to adjust the fuel trim. When the O2 sensor reads high O2 levels (=lean), such as would falsely happen when there is a header leak, the ECU tells the fuel injectors to pump massive amounts of fuel into the cylinders. Therefore, even though the O2 sensor is constantly reading a lean condition due to the exhaust leak, your engine is actually running extremely rich because the fuel injectors are maxing out on fuel output.

Fix this problem ASAP.

 

[xxBLOOD88SHOTxx]

Was it you or someone else we were trying to figure out if you had a clogged cat?

Ron is probably right, but damn that like a drop of 12-15 mpg.

 

[young_]

The primary O2 sensor sends it's information to the ECU, which uses the information to adjust the fuel trim. When the O2 sensor reads high O2 levels (=lean), such as would falsely happen when there is a header leak, the ECU tells the fuel injectors to pump massive amounts of fuel into the cylinders. Therefore, even though the O2 sensor is constantly reading a lean condition due to the exhaust leak, your engine is actually running extremely rich because the fuel injectors are maxing out on fuel output.

Fix this problem ASAP.

Ah, makes sense. Thanks for your input Ron! I'll probably source a stock replacement for a quick fix.


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[young_]

Was it you or someone else we were trying to figure out if you had a clogged cat?

Ron is probably right, but damn that like a drop of 12-15 mpg.

That was me. And yeah, crazy low gas mileage! Cost me forty bucks to go down there and back.


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[EJackBauer8]

That was me. And yeah, crazy low gas mileage! Cost me forty bucks to go down there and back.


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40$? How do you figure? Unless you live in an area where gas is 8$ a gallon which is nowhere in the United States. I'm not trying bash, just want to see your reasoning.

 

[RonJ]

40$? How do you figure? Unless you live in an area where gas is 8$ a gallon which is nowhere in the United States. I'm not trying bash, just want to see your reasoning.

But he uses rocket fuel and cruises at 180 mph.

 

[young_]

40$? How do you figure? Unless you live in an area where gas is 8$ a gallon which is nowhere in the United States. I'm not trying bash, just want to see your reasoning.

Eh, it was more like 36 or something. It cost me 20 bucks to fill up from right below half where I was before I left, and like 16 to fill back up. So I rounded up. Lol


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[lethal6]

Lol. The fill up before you left doesn't count. The fill up AFTER you got back is what counts towards the trip. The fill before is gas that was already used for something prior so technically it cost you $16 to go there and back. :thumbsup

 

[JohnS.]

I know I have very small leak where my cat and header meet and I still get 30+mpg. I can hear it when I accelerate.

If your CEL is still on for the o2 sensor, it's still telling your ECU false information, no? Only until the CEL is cleared will it go back to normal?

 

[RonJ]

I know I have very small leak where my cat and header meet and I still get 30+mpg. I can hear it when I accelerate.

How close is the leak to your primary O2 sensor?

If your CEL is still on for the o2 sensor, it's still telling your ECU false information, no? Only until the CEL is cleared will it go back to normal?

The OP says his code is for the secondary O2 sensor, which does not contribute to the fuel trim. It would be nice, however, for the OP to post the code to verify.

 

[antie22]

RON!!! your're back!!!!

 

[young_]

The OP says his code is for the secondary O2 sensor, which does not contribute to the fuel trim. It would be nice, however, for the OP to post the code to verify.

P0141 circuit heater malfunction downstream o2 sensor.


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[JohnS.]

How close is the leak to your primary O2 sensor?



The OP says his code is for the secondary O2 sensor, which does not contribute to the fuel trim. It would be nice, however, for the OP to post the code to verify.

Inches? I forget off the top of my head since its been a while since I've actually looked at the end of the header... But it's close.

Aah I read that too quick.


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[RonJ]

P0141 circuit heater malfunction downstream o2 sensor.

Measure the resistance of the heater element of the o2 sensor. Should be 10-40 Ohms. If not, replace the O2 sensor.

 

[RonJ]

Inches? I forget off the top of my head since its been a while since I've actually looked at the end of the header... But it's close.

You probably don't have an exhaust leak there. Otherwise, the engine would be running rich and you'd be experiencing poor gas mileage.

 

[young_]

You probably don't have an exhaust leak there. Otherwise, the engine would be running rich and you'd be experiencing poor gas mileage.

Like me!


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[HeX]

Have the CEL code cleared and it should make a difference in your MPG. My local AutoZone stores offer that service for free. FYI, Ive done some recent research on O2 sensor brands and Bosch ranks quite low. Our Civics use NTk plugs (made by NGK). It may be too late to return your Bosch O2, so remember to use NTK when the Bosch fails.

If you want some more tips about improving your overall MPG, click on my signature.