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  1. #1
    Join Date
    Sep 2017
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    16

    Sudden reduced Gas mileage

    I've owned my 90sc for about a year and a half now. Up until about 2 months ago I was getting 16-17 mpg city and 20-22 mpg highway but now it has suddenly dropped to 11 mpg city and 18 mpg highway. Anyone know what could cause this sudden loss in mpg?

  2. #2
    Join Date
    Dec 2007
    Location
    Salem OR
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    7,174
    I can tell you I had a very similar drop on my '90 (12 city) and nothing helped until I did the head gaskets (pass. side rear fire ring was pushed in) and injectors. MPG finally got so bad from those two problems it took out the catalytic converters. Hot starts were a coin toss so I expect one or more injectors were leaking.

    After repairs, I was shocked to see 25 mpg hiway.

    I'd suggest you do an exhaust gas check in the coolant, and then see if you can determine how many miles are on your injectors. Check codes at the same time, of course.

  3. #3
    Join Date
    Sep 2017
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    16
    Quote Originally Posted by KMT View Post
    I can tell you I had a very similar drop on my '90 (12 city) and nothing helped until I did the head gaskets (pass. side rear fire ring was pushed in) and injectors. MPG finally got so bad from those two problems it took out the catalytic converters. Hot starts were a coin toss so I expect one or more injectors were leaking.

    After repairs, I was shocked to see 25 mpg hiway.

    I'd suggest you do an exhaust gas check in the coolant, and then see if you can determine how many miles are on your injectors. Check codes at the same time, of course.
    I replaced injectors along with the fuel rail last year so they have less than 7k miles on them. I can try the coolant test though.

  4. #4
    Join Date
    Dec 2007
    Location
    Salem OR
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    AutoZone has a test kit you can rent to see if there are exhaust gases in the coolant.

    And while there is a list of things that can drop MPG all of a sudden, from a leaking fuel pressure regulator (pull the vacuum line on it and check for raw gas), to dragging brakes, to someone stealing gas, are there any other symptoms, or recent work?

  5. #5
    Join Date
    Sep 2017
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    16
    Quote Originally Posted by KMT View Post
    AutoZone has a test kit you can rent to see if there are exhaust gases in the coolant.

    And while there is a list of things that can drop MPG all of a sudden, from a leaking fuel pressure regulator (pull the vacuum line on it and check for raw gas), to dragging brakes, to someone stealing gas, are there any other symptoms, or recent work?
    I recently installed a new PCM, DIS, Coil pack, Cam sensor, and both 02 sensors.

  6. #6
    Join Date
    Dec 2007
    Location
    Salem OR
    Posts
    7,174
    Quote Originally Posted by 66_Fairlane_289 View Post
    I recently installed a new PCM, DIS, Coil pack, Cam sensor, and both 02 sensors.
    Was that before or after the mpg drop? If before, what were the symptoms then that you where chasing that prompted firing the parts cannon at it?

    How many miles on the heads? What do the plugs look like now?

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