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Terrible gas mileage!!

This is a discussion on Terrible gas mileage!! within the V6 forums, part of the Vehicle Specific category; Ok, so I have a 99 v6 firebird. Car has 152,000 miles but the new engine has 6,000. This car ...

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    Member flybird's Avatar
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    Terrible gas mileage!!

    Ok, so I have a 99 v6 firebird. Car has 152,000 miles but the new engine has 6,000. This car is my daily driver. I am getting 11MPG!!!!! I checked for a vaccum leak but i cant find it. i know i have an o2 sensor out but i dont think it would bring my mpg that much!

    So i am thinking that maybe an additional problem besides the o2 sensor, is my fuel filter.. I noticed exhaust fumes and also the bad mileage, and sometimes hard start. those symptoms are fuel filter.

    ANY suggestions??? ps: tire pressure is okay and calipers are okay, so no drag..

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    expensive tires az gt eater's Avatar
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    I dont know jack about the v6's, but if it has a MAF sensor, you should clean that with MAF cleaner, and also change you air filter. If that dont fix it, change the O2's, then plugs and finally wires. It could be hard to start with a plugged fuel filter, or a going out fuel pump, but that shouldn't affect your mpg. It could also be hard to start because of fouled, or worn off plugs.

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    Member flybird's Avatar
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    Plugs are new, they came along with the new engine. I checked the maf sensor, its clean. My k&n cold air intake is cleaned and oiled. Just one o2 sensor is out. I dnt believe it would cause a huge drop in mpg.

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    Senior Member 98maro's Avatar
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    i had a similar problem on my ls1. yours is a v6 but it shouldnt matter..
    my car was also lazy, i changed the fuel filter, and plugs the car was fine..

    the o2 could be the problem too, but try to check your fuel filter, also the plugs could be the problem..what plugs did they put on?

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    Member flybird's Avatar
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    Quote Originally Posted by 98maro:2723305
    i had a similar problem on my ls1. yours is a v6 but it shouldnt matter..
    my car was also lazy, i changed the fuel filter, and plugs the car was fine..

    the o2 could be the problem too, but try to check your fuel filter, also the plugs could be the problem..what plugs did they put on?
    Prestolite spark plugs. All fairly new. Maybe coil pack problem?

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    Senior Member 98maro's Avatar
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    i never heard about those plugs, that could be the problem too, this cars run with ac delco better some people put NGKs too, i leaned that if you put the wrong spark plugs on a car, can waist more gas than usual.
    if the motor is shaking alot, could be the coils, but if the car runs fine on idle, that is not the problem,
    change the o2 sensor if you haven't,
    also clean the MAF, i dont know alot about this cars.. just a guess

    any engine codes?

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    Member flybird's Avatar
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    Codes tht poped up are knockin sensor defective, heated o2 sensor defective, and air temp circut I believe.

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    I would seafoam it to. I had a new engine put in mine to and recently changed the fuel filter and get way better mpg. Around 23 city and 30-32 hwy

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    Which 02 is bad??? You have 2 front and one rear.. If it's a front one than you can bet it's the issue..

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    Member flybird's Avatar
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    Smittro, so glad you stepped in. I have just replaced my fuel filter, didnt do much. Im about to put in fuel system cleaner to help. The o2 sensor tht is bad is bank 1, sensor 1. Im 17 still learning, but Im not sure where that is located.

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    Quote Originally Posted by flybird View Post
    Smittro, so glad you stepped in. I have just replaced my fuel filter, didnt do much. Im about to put in fuel system cleaner to help. The o2 sensor tht is bad is bank 1, sensor 1. Im 17 still learning, but Im not sure where that is located.
    Bank one corresponds with number one cylinder..

    Both front 02's are located @ bottom of the manifolds just above where the "Y" pipe bolts to the mani's...

    It's a very cramped space, and unless you are absolutely sure about unbolting the "Y" I'd try to leave it be.

    Attempting to remove the bolts from the manifold can create even more problems as they are likely to be nearly rusted though as we speak..

    They will most assuredly break off if you try messing with them especially if they are the old factory ones..

    (ask me how I know)..

    If they break off it's damn' near impossible to drill them out with the manifolds still on the car..

    I drilled mine on the car and it was a total pita from start to finish there's just no darn room to work..

    I even soaked them with PB blaster for 3 days prior, then tried to heat them with a torch and they still snapped off..

    Reaching the 02's from the top is out of the question unless you can bend your forearm in the middle..lol
    Last edited by Smittro; 12-03-2011 at 03:12 PM.

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    Quote Originally Posted by flybird View Post
    Codes tht poped up are knockin sensor defective, heated o2 sensor defective, and air temp circut I believe.
    the knock sensor should get replaced ASAP if it is bad, they can cause serious damage if they are not working properly.
    Heated o2 sensor is bad if its the heated o2 "circuit" then that shouldn't affect your mpg because the heated circuit only is on when the car is cold. if its the sensor its self then it can cause a huge drop in MPG
    also air temp sensor if that is bad then the computer can not tell what the temp of the air is coming in to the engine to adjust the timing of the engine and what not if that is bad replace it.

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    Quote Originally Posted by My00Z28 View Post
    the knock sensor should get replaced ASAP if it is bad, they can cause serious damage if they are not working properly.
    Heated o2 sensor is bad if its the heated o2 "circuit" then that shouldn't affect your mpg because the heated circuit only is on when the car is cold. if its the sensor its self then it can cause a huge drop in MPG
    also air temp sensor if that is bad then the computer can not tell what the temp of the air is coming in to the engine to adjust the timing of the engine and what not if that is bad replace it.
    A bad o2 on the 3800 can and will cause a knock code as the AFR gets thrown off by the faulty o2.

    The o2 should be changed first as the black smoke it can cause due to poor mixture/combustion can foul the rear o2 and plug the cat..

    3800 will run with unchanged MPG's even if the air temp sensor is unpluged it will just have a diving idle..
    Last edited by Smittro; 12-08-2011 at 06:41 PM.

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    Member flybird's Avatar
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    Hey smittro, yeah replacing the fuel filter and cleaing the fuel system did not do much..i will be getting that o2 sensor replaced sometime next 2weeks when I get paid again. I do have BLACK smoke coming through the exhaust just on the start. I will update this forum once done. Thank you everyone for the help, I much appriciate it.

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    Quote Originally Posted by flybird View Post
    Hey smittro, yeah replacing the fuel filter and cleaing the fuel system did not do much..i will be getting that o2 sensor replaced sometime next 2weeks when I get paid again. I do have BLACK smoke coming through the exhaust just on the start. I will update this forum once done. Thank you everyone for the help, I much appriciate it.
    No prob..

    Bad knock sensors are not common with the RWD 3800-II.

    Again, I'd start with the 02 sensor first have the codes cleared and go from there.
    Last edited by Smittro; 12-08-2011 at 07:24 PM.

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    Oh dam, almost forgot, don't forget to disconnect the battery before you unplug anything.

    Once the sensor is changed reconnect the batt, and let her idle for 5-10 minutes before you take off..

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    Quote Originally Posted by Smittro View Post
    A bad o2 on the 3800 can and will cause a knock code as the AFR gets thrown off by the faulty o2.

    The o2 should be changed first as the black smoke it can cause due to poor mixture/combustion can foul the rear o2 and plug the cat..

    3800 will run with unchanged MPG's even if the air temp sensor is unpluged it will just have a diving idle..
    but if its the heater circuit then that would have no affect of the car except when it is cold therefore would not hurt MPG o2's on any engine do not operate when they are not at a correct operating temp. thats what the heater circuit is for to raise the temp faster to get them to turn on.... no matter what i would replace them but how would the heater circuit of an o2 sensor make it so the knock sensor would trip?

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    Quote Originally Posted by My00Z28 View Post
    but if its the heater circuit then that would have no affect of the car except when it is cold therefore would not hurt MPG o2's on any engine do not operate when they are not at a correct operating temp. thats what the heater circuit is for to raise the temp faster to get them to turn on.... no matter what i would replace them but how would the heater circuit of an o2 sensor make it so the knock sensor would trip?
    SImply put, the code would be for the heater curcuit if that was the issue, and a bank code for the sensor itself..

    If the sensor never "turns on" guess what happens to the AFR reading?

    3800-II knock sensor is block mounted and rarely if ever go bad durring the life of those engines.
    Last edited by Smittro; 12-08-2011 at 08:12 PM.

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    Quote Originally Posted by Smittro View Post
    SImply put, the code would be for the heater curcuit if that was the issue, and a bank code for the sensor itself..

    If the sensor never "turns on" guess what happens to the AFR reading?

    3800-II knock sensor is block mounted and rarely if ever go bad durring the life of those engines.
    if the sensor doesnt turn on then you get a "oxygen sensor no activity bank 1 sensor 1" blah blah blah if the sensor isnt working if it is just the heater circuit then you would get an "heater oxygen sensor circuit bank 1 sensor 1" and so on....

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    Quote Originally Posted by My00Z28 View Post
    if the sensor doesnt turn on then you get a "oxygen sensor no activity bank 1 sensor 1" blah blah blah if the sensor isnt working if it is just the heater circuit then you would get an "heater oxygen sensor circuit bank 1 sensor 1" and so on....
    Right!

    Both are indications of a faulty 02 sensor..

    Which currently according to the OP are abviously dirrectly effecting AFR's after the engine is warm hense the poor MPG's.

    There is nothing stated about a loss of power which would be an indication of a bad knock sensor pulling too much or not enough timing.

    Again poor afr's in turn as I already stated can/will cause a knock issue.

    If the heater curcuit is bad then the sensor is bad, if it does'nt turn on it can be assumed it also bad.

    It does'nt matter whether it's cold or hot a faulty 02 is a faulty 02..

    Target MPG's AND AFR's come after closed loop when the engine is @ operating temp, SO, if the only the heater curcuit is bad then MPG's nor AFR's would not be effected once the designated temp was achieved as it could be assumed the sensor is working aside from the heater curcuit..

    The odds of an 02 and the knock going bad @ the exact same time are slim @ best.

    If it's the knock sensor then I'll eat my hat..

    But I am confident once the 02 issue is corrected the knock sensor code once cleared will not return..
    Last edited by Smittro; 12-08-2011 at 09:45 PM.

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