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HORRIBLE gas mileage question?

 
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Old 01-14-2014, 09:36 AM
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HORRIBLE gas mileage question?

I have a 99 lude. Straight piped and I have a CAI. I filled up 3 days ago, premium, and I've driven 50 miles... I'm almost halfway down a tank. I had a check engine light thrown up Friday and it was from the downstream o2 sensor I believe because I have an exhaust leak before it... Other than that I just got my oil changed. It idles KINDA rough, but its alright. I drive decently aggressive, anybody have any ideas?
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Old 01-14-2014, 11:26 AM
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Re: HORRIBLE gas mileage question?

6mpg is not normal for these cars.
But, when you fix that O2 sensor issue you should see dramatic mpg gains.

See, the sensor thinks you're running too lean and throws more fuel into the mixture.
They sniff out unspent fuel. If it doesn't sense any unburned fuel it thinks there's not enough.
If it's not functioning, it's not sniffing. Therefore the mpg issue.
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Old 01-14-2014, 11:46 AM
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Re: HORRIBLE gas mileage question?

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Originally Posted by bykfixer View Post
6mpg is not normal for these cars.
But, when you fix that O2 sensor issue you should see dramatic mpg gains.

See, the sensor thinks you're running too lean and throws more fuel into the mixture.
They sniff out unspent fuel. If it doesn't sense any unburned fuel it thinks there's not enough.
If it's not functioning, it's not sniffing. Therefore the mpg issue.
I'm getting it fixed this weekend, it should be good after that right?
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Old 01-14-2014, 02:32 PM
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Re: HORRIBLE gas mileage question?

I did a writeup on bad gas mileage and covered almost every angle... Check it out, it's in my signature
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Old 01-14-2014, 02:50 PM
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Re: HORRIBLE gas mileage question?

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Originally Posted by ProjectPrelude95 View Post
I did a writeup on bad gas mileage and covered almost every angle... Check it out, it's in my signature
just went through it. Thank you! But I have a quick question, isn't the o2 sensor in the cat? I don't have a cat or a resonator I'm just straight piped, would I have an o2 sensor, if not it wouldn't effect my gas mileage would it?
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Old 01-14-2014, 03:09 PM
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Re: HORRIBLE gas mileage question?

There are two of them.
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Old 01-14-2014, 04:30 PM
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Re: HORRIBLE gas mileage question?

Either way you need your o2sensors, one by the exhaust manifold, one right before where the cat should be on the 5th gen I believe 4th gens only have 1
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Old 01-14-2014, 04:33 PM
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Re: HORRIBLE gas mileage question?

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Originally Posted by ProjectPrelude95 View Post
Either way you need your o2sensors, one by the exhaust manifold, one right before where the cat should be on the 5th gen I believe 4th gens only have 1
I'm gonna go take it to get the leak fixed this Friday and see where that takes me
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Old 01-14-2014, 06:43 PM
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Re: HORRIBLE gas mileage question?

Its because you have a straight pipe, both o2 sensors are reading the same reading and the c.e.l. will pop up. Same thing happened to me, I had a straight pipe no cat and no resonator, just had one o2 plugged into the downpipe and the other in the straight pipe. Ur o2 sensors are prolly fine but you deff need a cat. to make the 2nd bank o2 that's in ur straight pipe read a different reading then wat ur downpipe is at. I recommend wat I have is a high flow magnaflow catalytic converter, its cheap a good quality part. with this solved you should get better gas mileage in the long run.
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Old 01-14-2014, 10:13 PM
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Re: HORRIBLE gas mileage question?

Yea, I have the magnaflow hi flow too, good quality, like $100 and if your o2 sensor is after the cat, than you need it to run proper fuel ratio, unless you chip and tune your ecu, which would also mean an obd1 conversion.... Way more expensive route lol
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Old 01-15-2014, 08:32 AM
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Re: HORRIBLE gas mileage question?

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Originally Posted by ProjectPrelude95 View Post
Yea, I have the magnaflow hi flow too, good quality, like $100 and if your o2 sensor is after the cat, than you need it to run proper fuel ratio, unless you chip and tune your ecu, which would also mean an obd1 conversion.... Way more expensive route lol
So my gas mileage will be crappy either way? Unless I get a cat?
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Old 01-15-2014, 09:04 AM
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Re: HORRIBLE gas mileage question?

I'm pretty sure when your o2 sensor fails or the ecu thinks it does, you will run rich. Which means more fuel. So you would suffer on gas mileage. 6mpg sounds way off though.... I went through 2 o2 sensor issues and never had 6mpg.....

I would tell you to do 2 things. Number one, check all your fuel system components for proper operation. If your fuel pressure regulator is stuck open it will cause you to run rich.

Do a full tune up. If sufficient air, or spark is not being provided you will run rich so to speak

And actually a 3rd thing. Check for any fuel leaks from the gas tank all the way to the injectors. Make sure you check your filler neck when the car is running too. I've seen cases where the fuel comes out the gas cap when the car is running due to pressure. In that case check your evap canister.
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Old 01-15-2014, 09:28 AM
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Re: HORRIBLE gas mileage question?

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Originally Posted by ProjectPrelude95 View Post
I'm pretty sure when your o2 sensor fails or the ecu thinks it does, you will run rich. Which means more fuel. So you would suffer on gas mileage. 6mpg sounds way off though.... I went through 2 o2 sensor issues and never had 6mpg.....

I would tell you to do 2 things. Number one, check all your fuel system components for proper operation. If your fuel pressure regulator is stuck open it will cause you to run rich.

Do a full tune up. If sufficient air, or spark is not being provided you will run rich so to speak

And actually a 3rd thing. Check for any fuel leaks from the gas tank all the way to the injectors. Make sure you check your filler neck when the car is running too. I've seen cases where the fuel comes out the gas cap when the car is running due to pressure. In that case check your evap canister.
I'm taking it Saturday to get it checked out and to get my exhaust leak fixed, I'll have my mechanic look it over and see what's up, in the mean time do you think an o2 simulator would do anything for me?
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Old 01-15-2014, 02:34 PM
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Re: HORRIBLE gas mileage question?

I personally would not put one in my car, unless my car was highly modified, and I knew my air/fuel ratio at all times (gauge) and a wideband at that... The reason I wouldn't do it is because the o2 readings can tell your computer that you are running lean and correct the problem if it's not mechanical.... You do not want to run lean....

Why not just put in a catalytic converter??? Get a high flow, the restriction won't be that much of a difference........ They made high flows to make it as close as possible to no cat without crossing the borderline for emissions... Either that or spend a lot more money trying to keep your cat less exhaust in place.
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Old 01-16-2014, 09:47 AM
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Re: HORRIBLE gas mileage question?

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Originally Posted by ProjectPrelude95 View Post
I personally would not put one in my car, unless my car was highly modified, and I knew my air/fuel ratio at all times (gauge) and a wideband at that... The reason I wouldn't do it is because the o2 readings can tell your computer that you are running lean and correct the problem if it's not mechanical.... You do not want to run lean....

Why not just put in a catalytic converter??? Get a high flow, the restriction won't be that much of a difference........ They made high flows to make it as close as possible to no cat without crossing the borderline for emissions... Either that or spend a lot more money trying to keep your cat less exhaust in place.
So I just got to half a tank, I'm at 61.3 miles, this is with warming it up for 10 minutes in the morning 4 days in a row and driving 61 miles... There has to be a problem right?
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Old 04-22-2014, 11:48 AM
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Re: HORRIBLE gas mileage question?

My 01 a/t lude got what I perceived to be pretty poor milage when I bought it. Part of my issue stemmed from the fact that the car has a tiny gas tank lol.
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Old 04-22-2014, 01:13 PM
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Re: HORRIBLE gas mileage question?

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Originally Posted by richardsalem View Post
So I just got to half a tank, I'm at 61.3 miles, this is with warming it up for 10 minutes in the morning 4 days in a row and driving 61 miles... There has to be a problem right?
1) Keep in mind that a fuel gauge is just a reference. Just because your gauge says 1/2 tank, it doesnt mean that it has exactly 7.95 gallons.

2) Letting your car warm up for 10 minutes every morning has a negative impact on your mileage. Try cutting it to 2-3 minutes.

3) Also, you asked for advice, and everyone is telling you put a cat on your car and fix your exhaust leak. It's not nice to ask for advice and then disregard it when it's given. As bykfixer said, your exhaust leak is tricking your O2 sensor (and ultimately your car's computer) into thinking that the car is running lean. And it's dumping fuel to compensate. Fix your $hit.
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