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Thread: Sudden reduced Gas mileage

  1. #1
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    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
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    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
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    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
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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
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    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
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    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?

  7. #7
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    Quote Originally Posted by KMT View Post
    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?
    This was before I noticed an mpg drop except for the o2 sensors, those were after. The symptoms that the parts cannon was for was the tach was losing signal and the car was bucking and surging really bad randomly. It ended up being the cam sensor but I replaced the PCM bevause I got a code saying internal short in the PCM. The car has almost 106k miles on it, no cats, New pugs/wires installed at 94k.

  8. #8
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    Did you have any trouble finding a matching PCM?

    No codes now? Starts cold/hot ok? Obvious raw fuel smell out the exhaust or hear the pass-side headlight/fender? Looked for vacuum leaks?

  9. #9
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    Quote Originally Posted by KMT View Post
    Did you have any trouble finding a matching PCM?

    No codes now? Starts cold/hot ok? Obvious raw fuel smell out the exhaust or hear the pass-side headlight/fender? Looked for vacuum leaks?
    I did not, I got a remanned PCM from A1-Cardone through Rock auto. The only codes I have are EGR malfunction and an intermittent code for Knock sensor not detected (both of which I've had since I bought the car). Starts fine everytime wether hot or cold, and no obvious raw fuel smell anywhere. The only weird smell is that recently the exhaust has been smelling really strong (also smells really bad and I cant pinpoint exactly what it smells like but it doesnt smell like its running rich). And I have no vacuum leaks that I know of (vacuum guage steadily reads about 18-19in of mercury at idle)

  10. #10
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    Pull the vacuum line at the FPR and check for raw fuel. Wouldn't hurt to check fuel pressure just to be sure it is in spec at all times.

    Go over as many vacuum lines/connections as you can, like it was court ordered. Check the valve cover bolts to see if any are loose. Make sure the cross-over tube from the air intake tube to the driver side valve cover is in place.

    Check the oil for any evidence of coolant, and/or fuel smell.

    Air up the tires and try to check if any calipers/wheel bearings are dragging. Check the fuel filter for evidence of leaks.

    See if the place you normally buy fuel has been selling a winter grade. Buy fuel from another station next time you fill up. Toss in a small bottle of fuel dryer at the same time.

    Replace the PCV and it's grommet. I'll assume you've replaced the air filter. Inspect the throttle body plate and bore for carbon/oil buildup, and the plate pivot shaft and bearings for excessive play...you're looking for unmetered air leaks where the shaft moves in the throttle body. Might need to find a smoke machine to check for other leak sources, such as injector o-rings. IC tube joints, cracked dipstick tube....

    Check the crank pulley to see if it is loose or wobbles, or looks like it is coming apart at the rubber bond. Might need to do a base timing check to see if the balancer has shifted.

    Still no joy, you might want to do a compression test. I'll dig out the factory 'engine/emissions diagnosis' manual later to see if I missed anything.
    Last edited by KMT; 03-08-2019 at 06:07 PM.

  11. #11
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    Quote Originally Posted by KMT View Post
    Pull the vacuum line at the FPR and check for raw fuel. Wouldn't hurt to check fuel pressure just to be sure it is in spec at all times.

    Go over as many vacuum lines/connections as you can, like it was court ordered. Check the valve cover bolts to see if any are loose. Make sure the cross-over tube from the air intake tube to the driver side valve cover is in place.

    Check the oil for any evidence of coolant, and/or fuel smell.

    Air up the tires and try to check if any calipers/wheel bearings are dragging. Check the fuel filter for evidence of leaks.

    See if the place you normally buy fuel has been selling a winter grade. Buy fuel from another station next time you fill up. Toss in a small bottle of fuel dryer at the same time.

    Replace the PCV and it's grommet. I'll assume you've replaced the air filter. Inspect the throttle body plate and bore for carbon/oil buildup, and the plate pivot shaft and bearings for excessive play...you're looking for unmetered air leaks where the shaft moves in the throttle body. Might need to find a smoke machine to check for other leak sources, such as injector o-rings. IC tube joints, cracked dipstick tube....

    Check the crank pulley to see if it is loose or wobbles, or looks like it is coming apart at the rubber bond. Might need to do a base timing check to see if the balancer has shifted.

    Still no joy, you might want to do a compression test. I'll dig out the factory 'engine/emissions diagnosis' manual later to see if I missed anything.
    Sorry for the late reply. I think the computer just needed some time to recalibrate because I am now getting 22.25mpg city! Regards to your other suggestions, the compression was good when I changed spark plugs, I have previously installed a BHJ balancer, cleaned throttle body, replaced PCV, ect.... Clearly everything is working as its supposed to now though. Thank you for your help!

  12. #12
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    Nice! Thanks for the follow up.

    Ken

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