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HELP: 1998 Si with Horrible Gas Mileage

Old 05-Oct-2010, 01:53 AM
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HELP: 1998 Si with Horrible Gas Mileage

Hello All!

I recently bought a 1998 Si with 333000KMs and since day one, it's been giving me horrible gas mileage. 380-400 before the light comes on. And this is a manual car shifting at 2-3k RPM. The engine light is also on so borrowed a buddy's code reader and pulls 2 codes: P0131 and P0171.

It has the following mods if it helps:

- DC 4-2-1 Headers
- Magnaflow Catback

I had a 1992 EX prior to this and I would get easy 500-550 with a heavy foot before the light came on. And it was an automatic.

Any help would be greatly appreciated! Thank you very much!

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Old 05-Oct-2010, 09:21 AM
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P0131 = Oxygen O2 Sensor Low Voltage (When it's bad, it can give you bad gas mileage)
P0171 = System Too Lean

A simple google search on the OBD reader would help you.
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Old 05-Oct-2010, 09:26 AM
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Welcome to the site.

To start off, if you googled those codes you'd find that P0131 is primary O2 sensor low voltage. P0171 is system too lean.

A bad O2 sensor can definitely contribute to bad fuel mileage. I'd start by making sure the O2 is plugged in properly and the wires are in good condition. If all that checks out, clear the code and see if it comes back. If the code comes back or there is obvious damage to O2 sensor/wires, then replace it.
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Old 06-Oct-2010, 03:20 AM
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Thank you. I know I'll enjoy my time here at TCC and learn lots.

So after I cleared the codes, the CEL came back after I got gas today. Got the reader again and pulled the following codes:

- P0170
- P0171

I'm thinking the P0131 code will come back sooner or later but the main problem is the P0171 code. It could be a plethora of things when I googled it. Ranging from injectors, vaccum leaks, primary o2 sensor, fuel regulator valve, fuel filter, etc etc..

I also forgot to mention that when the car is cold, it tends to idle rough. And at about 2-3k RPM, it will feel like I lifted off the accelerator when I didn't. All these symptoms go away once the engine is fully warmed up.

03:15

Forgot to post the reply but here's an update. It wasn't apparent before I also hear, what sounds to me, an exhaust leak somewhere between the headers and the resonator.
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Old 06-Oct-2010, 03:22 AM
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EDIT: I meant to say, "It wasn't apparent before but now I hear, what sounds to me, like an exhaust leek somewhere between the headers and the resonator."

Didn't know that I can't edit my posts. =p

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Old 06-Oct-2010, 09:14 AM
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P0170 is fuel trim malfunction.

So something is definately not right here...lol.

You say you hear an exhaust leak, see if you can pin-point hwere it is. If it's before or near the primary O2 sensor (before the cat) and it's a big enough leak, it can throw-off the O2 readings and the ecu will over-compensate causing other problems.
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Old 11-Oct-2010, 08:15 AM
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Now I am consistently getting P0170 and P0171 codes. Before, the P0131 code would just come up after.

I have taken out the Magnaflow exhaust and replaced it with a stock exhaust thanks to one of the TCC members here. There's no leaks anywhere. Didn't get a chance to look around the o2 sensor bung or the sensor itself though.

Tried to see the voltage being put out on my friends Scanguage II but didn't know how to set it up properly.

Let me try to list all of the symptoms I have noticed so far and hopefully you guys can advise me.

1. During cold starts, like in the morning:
- the idle is horrible and will go to near stalling RPMs but won't stall and when it's about to stall, it surges up to 2k.
- if I don't allow it to fully warm up, which takes 10 minutes, the engine won't pass 2.5k RPM regardless of how much throttle I'm applying and it feels like i'm letting go of the throttle when I'm not
- these symptoms will go away when the engine is fully warmed up

2. Whenever I floor it, the gas meter will go down anywhere around 1/8-1/5 of the full meter, sometimes immediately, sometimes within 5 seconds but it will go back to where it when down from

3. The exhaust smells worse than my friend's Porsche that has a straight pipe

4. After about 10 minutes of driving, if I leave the car idling for about 3-5 minutes, the temperature gauge will go up a bit. Normally, the needle is not halfway but just touching the symbol. After idling for 3-5 minutes, the gauge will go up a little bit, right about at the halfway mark and stays there. Once I start driving, the needle will go back down to normal and I will smell something the cabin and the hvac controls are off. Smells like my exhaust but not exactly. Don't know how to describe it.

5. On rainy days, If I leave the car idling for 10 minutes after 10+ minutes of driving, I see what looks to be like steam or smoke coming off the hood. There is also patches of clearcoat gone on my hood and the paint is chipping off. Suggestion of too much heat for the paint/clearcoat?

6. Gas mileage is horrible but I'm getting a P0171 code which doesn't make sense to me.

Any help would be greatly appreciated >< Thank you very much!

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Old 11-Oct-2010, 08:17 AM
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Also, to add to #6, I've already tried 1 tank of 87 and 2 tanks of 94 with fuel injector cleaner with no difference in mileage.
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Old 12-Oct-2010, 10:44 AM
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^Putting 94 in this car will not help one bit. Increasing the octane in an engine designed to run on 87 will only cause it to run worse because it won't be able to completely burn the fuel properly. All the higher octane does is resist pre-ignition/detination.

Also check to make sure your head gasket isn't on it's way out... Any blue/white smoke out the tail pipe? How does the oil and coolant look?

Imo, at this point I would look at replacing the O2 sensor and clean out the IACV and go from there. But check for the head gasket first.
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Old 12-Oct-2010, 10:52 AM
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Originally Posted by paddyhong View Post

6. Gas mileage is horrible but I'm getting a P0171 code which doesn't make sense to me.
P0171 = system too lean. This means the ecu will be trying to compensate by giving the engine more fuel. More fuel means more fuel burned. Makes sense to me.
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Old 15-Oct-2010, 07:13 AM
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Originally Posted by MPR View Post
^Putting 94 in this car will not help one bit. Increasing the octane in an engine designed to run on 87 will only cause it to run worse because it won't be able to completely burn the fuel properly. All the higher octane does is resist pre-ignition/detination.

Also check to make sure your head gasket isn't on it's way out... Any blue/white smoke out the tail pipe? How does the oil and coolant look?

Imo, at this point I would look at replacing the O2 sensor and clean out the IACV and go from there. But check for the head gasket first.
Sounds good. Will be doing that this weekend. Will post results after.

Thank you very much for your time and insight.

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Old 30-Oct-2010, 08:05 PM
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I cleaned out the IACV and changed the O2 sensor and it was fine for a week until the engine light popped back up again. Now it's giving me P0133 which is O2 sensor slow response or something like that.

Also, right after I did both, the temperature meter on the dash starts to go up to near the red ONLY when it's idling for more than 5 minutes. Also noticed that the fan is not kicking in even when it's near the red. All this goes away after I start driving it. This is becoming a serious headache. =.=''

Thanks in advance for any insight.

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Old 30-Oct-2010, 08:23 PM
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^Sounds like you've got several issues here. You should first find out why the rad fan isn't coming on. Check the fuses and if they are good check the relay.

In the mean time, try not to let it idle while the car is sitting still for too long and don't let it overheat or you'll have much bigger problems...lol
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Old 05-Nov-2010, 01:19 PM
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Fixed the overheating problem. Apparently it was low on coolant, topped it up and problem went away. The P0133 code still prevails but it is on and off. The first time it showed up was a week after the primary O2 replacement. Turned off by itself while idling infront of my house after a day or so. Turned back on today in some rush hour traffic.

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Old 11-Nov-2010, 08:56 AM
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Another update, the P0133 code came back on and went off again. I'm going to leave it until I'm getting a solid CEL for like a month or something. Gas mileage is up but still not great for a civic at 470km/tank.

Thanks MPR!

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