7mpg average! Help if anyone can.
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HT4100 4.1, 4.5, 4.9 Discussion, 7mpg average! Help if anyone can. in Cadillac Engine Technical Discussion; OK so I am averaging around 7mpg total driving city and highway. I notice whenever I use the gas pedal ...
  1. #1
    Forgoten214's Avatar
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    7mpg average! Help if anyone can.

    OK so I am averaging around 7mpg total driving city and highway. I notice whenever I use the gas pedal to accelerate I'm getting around 2mpg or 4mpg.

    I don't have a lead foot by any means. I drive pretty much like an old man so I'm told. I just dont know why Im getting such terrible gas mileage.

    I also only use premium gas from Shell or Exxon or Hess so that's not the case.

    I know the EGR valve needs to be replaced due to a E48 error code.

    I also have a E49 code.

    Water pump and Idler pulley also needs to be replaced and I have a hole in my exhaust manifold.

    Can any of these cause such horrible gas mileage? What would be the reason for the vehicle running so rough? It also shakes pretty violently during hard acceleration. It does have abit of hesitation to it.

    I'm just pretty frustrated with the vehicle at the moment but its a work in progress I guess I love Cadillacs and DeVilles and refuse to give up on this car. Its a 90 Deville with the 4.5.

    Any thoughts or help or advise would gladly be appreciated.

    Thanks.

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    Sevillian273's Avatar
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    Re: 7mpg average! Help if anyone can.

    Quote Originally Posted by Forgoten214 View Post
    I notice whenever I use the gas pedal to accelerate I'm getting around 2mpg or 4mpg.


    Its normal to see that on the Inst Mpg display. Take your foot off on the hwy and it will read 70mpg.


    Sounds like you have alot of issues going on at once. I would start with a tune up (plugs, wires, cap, rotor) Then onto the E048 EGR. Also check the resistance of your fuel injectors. All of the above systems can cause your symptoms although I would think something serious would be going on to cause such low mileage. Have you checked the actual fuel mileage via the trip odometer between fill-ups? There would be gray/black smoke coming out of the tailpipe all the time if that much gas was going through the engine.

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    Re: 7mpg average! Help if anyone can.

    Quote Originally Posted by Sevillian273 View Post
    Its normal to see that on the Inst Mpg display. Take your foot off on the hwy and it will read 70mpg.


    Sounds like you have alot of issues going on at once. I would start with a tune up (plugs, wires, cap, rotor) Then onto the E048 EGR. Also check the resistance of your fuel injectors. All of the above systems can cause your symptoms although I would think something serious would be going on to cause such low mileage. Have you checked the actual fuel mileage via the trip odometer between fill-ups? There would be gray/black smoke coming out of the tailpipe all the time if that much gas was going through the engine.
    Yes I have checked it, No I have no smoke. I already did a full tune up, Plugs, Wires, Filters, cap and rotor.

    Im not sure what the problem is with the gas mileage. Yea I did notice that on the highway If I'm going 65 and take my foot off the gas it will read 70mpg. But once I get back on it just to coast the car or cruise I will get about 9mpg on the highway.

    The car shakes violently at those speeds as well.

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    Re: 7mpg average! Help if anyone can.

    I noticed I used about 6 gallons for about 80 miles as well.

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    Re: 7mpg average! Help if anyone can.

    Replace the exhaust manifold and repair the EGR. The hole in the exhaust manifold is causing that bank's O2 sensor to read lean, triggering E49 and giving your bad mileage.

    The EGR problem is likely causing your mileage issues as well.

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    Re: 7mpg average! Help if anyone can.

    Quote Originally Posted by drewsdeville View Post
    Replace the exhaust manifold and repair the EGR. The hole in the exhaust manifold is causing that bank's O2 sensor to read lean, triggering E49 and giving your bad mileage.

    The EGR problem is likely causing your mileage issues as well.
    Oh ok thanks, I was quoted over the weekend for 270 to replace the Exhaust Manifold. Is that a fair price?

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    Re: 7mpg average! Help if anyone can.

    Quote Originally Posted by Forgoten214 View Post
    I noticed I used about 6 gallons for about 80 miles as well.
    That's 13 miles to the gallon. Is that city or highway?

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    Re: 7mpg average! Help if anyone can.

    yeah sounds like a 'fuel dump mode' situation where the pcm can't tune so it just uses as much fuel as possible to make sure there's enough in the cylinder to ignite

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    Re: 7mpg average! Help if anyone can.

    Quote Originally Posted by the recluse View Post
    That's 13 miles to the gallon. Is that city or highway?
    It's highway, yea I was thinking the same thing.

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    Re: 7mpg average! Help if anyone can.

    Ive only been getting about 12 mpg's with my 4.5 right now . i think the cat might be bad though , but should i take a look at the egr valve as well thought about it but i don't have any codes.

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    Re: 7mpg average! Help if anyone can.

    Quote Originally Posted by quins90lac View Post
    Ive only been getting about 12 mpg's with my 4.5 right now . i think the cat might be bad though , but should i take a look at the egr valve as well thought about it but i don't have any codes.
    Quin, you said before that you have 19lb/hr injectors in the 4.5. That is your problem. Its been discussed before in other threads. The 4.5 uses 16lb/hr injectors and the computer cant fully compensate for the extra flow. I get 15 mpg on mine.

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    Re: 7mpg average! Help if anyone can.

    ^^^This.

    Now that I've switched to 16lb, I averaged 19-20 in the warm weather, up to 27.5 on the highway. I'm down to 18 now that it's cold out, which is still far better than I ever got with the 19's.

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    Re: 7mpg average! Help if anyone can.

    i had the 19's in till about a month ago when i first noticed the drop in mpgs . it went up a little bit not much im still at 12 mpg that's why i was thinking it might be the cat gone bad.

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    Re: 7mpg average! Help if anyone can.

    I could definitely see a rich condition from big injectors causing a cat to gum up and fail. The 19's in my 4.5 caused carbon build up in the combustion chambers over time which led to heavy spark knock.

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    Re: 7mpg average! Help if anyone can.

    Yup. Raw fuel in the converter will burn there and superheat the converter, causing meltdown.

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