I need some opinions. I’ve lost quite a bit of fuel economy and performance. This was not a gradual decline rather a sudden step change with the car.

Some quick background info: I have a ’98 Honda Civic LX with a 5 speed manual transmission. The car has 163,000 miles on it. I drive it 80% on the highway and I normally average 40 mpg. I changed the oil and filter every 5,000 miles and use Mobil 5000 10w30. My driving habits have not changed and oddly, I have NOT gotten the CEL/MIL. I live in Tulsa, OK and granted this has occurred in the winter season, this is not a seasonal effect. My mileage has dropped too much and we’ve had some weeks of warm weather and my poor performance/mileage issues continue.

My Honda seems sluggish and the mileage is averaging 34 mpg, a drop of nearly 20%. This occurred sometime in early December. I searched these forums quite a bit to look for ideas and generally I’ve been trying to make changes only 1 or 2 at a time, then wait a tank of gas to see the impact.

The mileage issues came shortly after I replaced my tires. I put a set of Yokohoma Avid H4’s keeping the same tire size as before. I’ve discounted the tires as the culprit because firstly I don’t see how new treads would impact the mileage that significantly, but more noteworthy, I had a couple tanks of gas average at my normal 40 mpg with the new tires.

Before the poor performance and mileage issue came about I had recently replaced my plugs and wires, probably about 3 months prior. So here goes my log of checks:

Checked plugs, didn’t notice a real rich condition so I assumed the O2 sensor was okay. Wanted to check the other possibilities before replacing a $90 sensor. Replaced the air filter. Mileage: 34 mpg.

Replaced rotor and cap, noticed some deterioration around the contacts. When I started the engine, the idle was noticeably stronger. I felt confident I had nailed it. Guess again. Mileage: 34 mpg.

Put a can of injector cleaner in the gas tank. I used Chemtool B-12. Mileage: 34 mpg.

2.18.08
Replaced the fuel filter, pulled spark plugs and adjusted them to a 0.040” gap. The plugs looked okay but there is carbon deposit around the outside of the electrodes on the threads. Not heavy, just a light layer of it. I also noticed the rubber inside of the plug wires looking at the ends, seem to be pulling off. Went back to Advanced Auto and got the plug wires replaced. Mileage: 33 mpg. I suspect I lost some mileage replacing the fuel filter.

2.23.08
Re-gapped plugs back to the factory spec of 0.043”. I don’t know why I hadn’t noticed it before, but the ends of the spark plugs had dried up dielectric grease on them. Cleaned them up and reinstalled. I attempted to pull a voltage measurement from the O2 sensor. I was unsuccessful. (In hindsight, I had not probed deep enough into the connector.) I disconnected the O2 sensor and tried from the other side. Checked the resistance across the O2 sensor element and didn’t get a reading. I later learned that this is a big no no. I may have also tried to check the resistance on the PCM side of the connector. My battery terminals had very heavy sulfate buildup on the terminals so I cleaned up the battery and wire cleaned the terminals and connectors. I started the car and after warm up, I’m idling very low, around 300-400 rpm. Great, appears I fried my O2 sensor. Mileage goes up though?: 36 mpg.

3.1.08
Bought a new Bosch O2 sensor from Advanced Auto. Man did I have a hard time getting the old one out!! I had to destroy it in the process. Put the new one in, pulled the negative battery wire to reset the PCM for about 15 minutes. Took a test drive and after warm up, now the car is idling at 100 rpm and nearly stalling out!! WTF! Pulled car back in the garage and I test the O2 sensor both in and out of the car. Can’t read a reading above 0.4 volts so it appears I bought a bad O2 sensor. What are the chances of that? Brought it back to Advanced and they have to order one in. They tell me to come back tomorrow. For fun, I start checking other things. I pull resistance across the primary and secondary O2 heaters and get approximately 10 ohms. One of the tests is to turn the key on and pull a voltage across the connector(PCM side) heater terminals. I don’t read a voltage. I pull it across the hot terminal to ground and get battery voltage. Hmm. I pull the P32 connector from the PCM and check across the 5/6 and 10 terminals. I get battery voltage. I also double check the #15 7.5 amp fuse for the O2 sensors. Its fine.

Now I’m wondering if I have a bad PCM. Here’s the weird thing, the only time I ever got a check engine light was when I was running the car and physically disconnected the primary O2 sensor back on Feb 23rd. When I was idling around 100 rpm, I should have seen a MIL, don’t you think? If my O2 sensor can’t get above 0.4 volts, I should have gotten a MIL for lean condition. Why am I not getting a MIL for the O2 sensor heaters? Is there a chance the PCM could have fried the new O2 sensor?

I had a similar problem with this car about 5 years ago. Bad mileage and poor performance. Car idling extremely low (~100 rpm), and the check engine light was indeed on. The codes were P0171 (fuel system too lean), and the other had to do with the fuel throttle max position. Replacing the O2 sensor fixed it and mileage returned.

Anyway, two things I want to do today, get the replacement O2 sensor back in the car, and stop by Autozone to see if there are any pending trouble codes. I’ll be sure to check the new O2 sensor before I install it.

Your thoughts or suggestions?


Dave

 

3.2.08
Got the replacement O2 sensor. Tested it on the bench and starting seeing similar results as yesterday. I was grounding the multimeter to the sensor shell. Then I trygrounding the MM to thegroundconnector pin and voltage shoots on up. That's more like it, O2 sensor responding as one should. Put the O2 sensor in the car and took for a test run. Idle speed after warm up is still very low at around 100-200 rpm. Tested the O2 sensor in the car when I got back and again seems to respond normally.

I'm going to callHonda tomorrow and see if they're able to test my PCM. I'm not able to get continuity on the ground side of the O2 sensor's nor the O2 sensor heaters. I continue to NOT see a CEL.

At this point I don't know what to test.I'm game for any ideas.

Dave

 

I've been having very bad gas mileage too, 20-22 MPG, on an auto 98 sedan.

I can't figure it out either, I replaced fuel filter, air filter, checked tires, checked spark plugs, replaced O2 sensor a month ago. Nada. No CEL's either. I do notice a burning smell coming from my rear passenger wheel, so hopefully that'll be the problem

 

OP's change in mileage may be due to winter-blend gas or sticking brakes.

 

I was gonna say Winter gas. everybody's mileage takes a dump for winter gas. but it can't be said right away.

 

Spark timing. You never mentioned checking the spark timing. How old is your timing belt?

Any time you disconnect the battery, thus clearing ECU memory, it will idle poorly for a while until the ECU recalibrates itself.

 

Appreciate all the reponses. I'm not going to say its not winter gas but I will say my mileage has never gone down THAT much. I'm sure there is an impact but a 20% hit?

I did check my tires about a month ago to check for dragging brakes. All my tires spin freely. Good suggestion though.

mk378, you certainly have a point because my idle returned to normal today!!! I drove 25 minutes to work this morning, got off the highway, depressed the clutch, and idle goes to 800 rpm, just where its supposed to be! So I went ahead and topped off my tank to start fresh. We'll see if this does it and brings my performance and mileage back. So I guess it took a bit for the PCM to "learn" the replacement O2 sensor or recalibrate itself.

Regarding the timing belt, its the original belt. I did check the timing real quick a few weeks ago. Seems like it was ok but then I didn't really spend anytime looking at it carefully.

I'm going to see what this next tank of gas does, then go from there. I'll focus on the timing. Next I guess will be the timing belt. I know I'm overdue.

Dave

 

Well that was an expensive experiment. After replacing the O2 sensor, my mileage is 33 mpg. Basically didn't change. Someone mentioned winter gas and that got a thought going. I'm thinking that its actually the ethanol blends causing this issue and that the car is just fine. Honda confirms reports of poor fuel economy with the E10 gas. So I found a gas station that says it doesn't use any ethanol. We'll see if that helps.

Anyone experience E10 gas causing mileage/performance issues?

Dave

 

Ethanol isa renewable fuel source but doesn't contain as much energy from it as gasoline does.
It is only a viable fuel source because they can renew it, it has no favorable properties other than that IMO.
It produces little power for the amount combusted, which could explain your mileage dropped.

 

it might be your ECU, it has builtin tables on how properly mix the fuel and air, and they program it only to a certain mileage, i dont beleive that it was programmed to your 163k mileage, but there might be a bug in the sw.