Very poor MPG 1992 Deville ?? HELP!!??!! - Page 2
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HT4100 4.1, 4.5, 4.9 Discussion, Very poor MPG 1992 Deville ?? HELP!!??!! in Cadillac Engine Technical Discussion; To 100: If you're getting 26 actual mpg (not based on what the fuel center says), I'd say you're doing ...
  1. #16
    awadecki is offline Cadillac Owners Enthusiast
    Automobile(s): 76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    To 100:

    If you're getting 26 actual mpg (not based on what the fuel center says), I'd say you're doing really well. My cross counts really do stay above 20 about 95% of the time on the freeway. Around town I usually see about 13-18, and about 8 at idle.

    Well, regardless, for $20 and 15 minutes, I change my oxygen sensor every 30,000 miles in all of my cars (each of which is over 15 years old).

    Another tip for improving mileage for those who have replaced your EGR: make sure that you use some Permatex gasket sealer on the EGR gasket to make a nice fit between the intake manifold and the EGR. If you don't do this, and have a leak, this could also lead to poor mileage.

  2. #17
    100
    100 is offline Cadillac Owners Enthusiast
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    awadecki,

    I went back to my note and calculated the actual mpg... It was more like 24 mpg. I figured out that most of the time the actual was little less than the computer value, but sometimes, it was better than the computer one. Also the differences between the actual and computer were somewhat random, although most of the time they were not too much (< 2-3 mpg).

    Quote Originally Posted by awadecki View Post
    To 100:

    If you're getting 26 actual mpg (not based on what the fuel center says), I'd say you're doing really well.

    Well, regardless, for $20 and 15 minutes, I change my oxygen sensor every 30,000 miles in all of my cars (each of which is over 15 years old).
    Then, again, I did not see any change in cross count when I changed the O2 sensor. I changed only one to see if I get higher cross count, but I didn't. And I did not gain any increase in mpg. So, I will see when I will have to change it...

    Quote Originally Posted by awadecki View Post
    Another tip for improving mileage for those who have replaced your EGR: make sure that you use some Permatex gasket sealer on the EGR gasket to make a nice fit between the intake manifold and the EGR. If you don't do this, and have a leak, this could also lead to poor mileage.
    That's a good one. I didn't use a sealer (new gasket though). I will check. I like Permatex products, but do you know which particular one is for that EGR sealing?

  3. #18
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    I use the copper colored one (super high temperature). If you replaced this, and exhaust is leaking past the seal, it can affect the oxygen sensor reading and cause the car to run rich...Although, at 24 mpg with this engine, I'd say you're doing really well. I get about 24 on the freeway and 19-20 in mixed driving. Did you say your actual fuel mileage measured well against what the fuel center reported?

    I almost always get about 8-10% less than what the data center says, although I have beaten its reading a few times.

  4. #19
    CaddyKen is offline Cadillac Owners Member
    Automobile(s): 1992 Deville
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    Quote Originally Posted by chevelle View Post
    Before you do anything perform a simple test. Fill the tank again and reset the electronic fuel data center. Zero out the fuel used and average MPG when the tank is filled. Drive the car until the tank is low again. Fill up and record the amount of fuel added, the mileage and the data from the fuel data center...i.e....the fuel used and the average MPG. Calculate the actual MPG from the gallons added and mileage. That should match the electronic average MPG from the fuel data center. Similarily, the actual fuel used should match the electronic fuel used from the fuel data center.

    If the actual and electronic values match then it will indicate that the injectors are not leaking and the fuel pressure regulator is OK, etc.... If they do NOT match, then you are looking for a place unmetered fuel can enter the engine.

    Recommendations for replacing O2 sensors, wholesale tuneups, etc. are just shotgunning hoping to find the problem. Rarely successful and very expensive. I would wait until you know what the problem is...or if you have a problem.

    If you just drove short trips in the city and stop and go driving and like to hit the accelerator hard it is not too hard to believe 11 or 12 MPG actual. It is a relatively heavy car with a fair amount of power. It costs fuel to accelerate hard and stop and go. That is just the way it is with any car. If you aren't used to larger or heavier cars than the MPG might surprise you. Compare that to the cost of the car and how much it will cost you for gas and for the car to see what the total costs are. People panic and sell a lower MPG car for peanuts only to make payments on a much more expensive car just because it gets a few more MPG....they never save as much on gas as the car costs to get better MPG.

    With a 92 one thing you would certainly want to do is to check or have someone check the EGR tubes under the throttel body for cleanliness. Hold the throttle blade wide open with the engine off and the air cleaner off. Look down the bores. The two tubes sticking up from the bottom of the intake manifold should be clean and NOT clogged with carbon. If they are clogged rod them out with stiff wire and/or a round brush and carb cleaner. Clogged EGR tubes can affect driveability and fuel economy and are often overlooked.

    I assume the engine starts and runs smoothly and there are no severe driveability issues. If not, then a "tuneup" is not likely to do much for you.

    If an O2 sensor is failed it will likely set a diagnostic code telling you it needs to be replaced. Replacing them just as a matter of fact is pure BS....unless you sell O2 sensors.

    One thing to check is for trouble codes. Turn the key on, engine off. Simulataneously press and hold the OFF and WARMER buttons on the climate control panel until the onboard diagnostics activates. It takes about 5 seconds. When the onboard diagnostic system activates release the buttons and write down the trouble codes that display on the FDC. Post back. That will tell if there are any operating deficiences noted with the sytem.

    Checking or measuring or estimating "fuel economy" from the fuel gauge is very misleading. Similarily, unless you reset the electronic Average MPG when you filled the car the data displayed will include all the idling and driving done before you got the car. The only way to know your MPG is to fill the tank, measure the miles and gallons and calculate it ..... and if things are fine it should correlate well with the electronic data.
    Chevelle,

    You posted this some time ago so it wouldn't surprise me if nobody sees my comment.

    My actual and indicated economy have always been fairly close. However, recently they are significantly different and the idle has become rough. You suggest a leaky injector or regulator problem. Any clues to troubleshooting those to identify which? I have the factory service manual and can't seem to find help.

    Thanks
    Ken

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