Cadillac 500 Gas Mileage and 70s Cad Tips
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500, 472, 425, 368 Discussion, Cadillac 500 Gas Mileage and 70s Cad Tips in Cadillac Engine Technical Discussion; Hi all, I just made a 550-mile round trip in my 76 Eldorado with 51k miles. Car was a blast ...
  1. #1
    awadecki is offline Cadillac Owners Enthusiast
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    Cadillac 500 Gas Mileage and 70s Cad Tips

    Hi all,

    I just made a 550-mile round trip in my 76 Eldorado with 51k miles. Car was a blast to drive. Just thought I'd share that I averaged 15.8 mpg with the cruise set at 68 mph and the A/C on about 1/3 of the time.

    For an 8.2L V8 with a 3-speed auto, I couldn't believe the mileage! I get about 10 mpg city and almost 16 highway.

    I've rebuilt the QJet carb, put in new Delco plugs, wires, cap & rotor, fuelt filter, air filter, fresh oil change, new fuel pump (old one worked fine, but didn't trust it for long trips), new ignition module in distributor (again, replaced preventatively--always keep a spare in my glovebox!), gutted the converter and resonator, set the timing at 8 degrees base instead of 6, and put on a big muffler that sounds great--not too loud or quiet, just right.

    A couple of tips from my experience owning two 1976 Eldorados:

    1. If you're having problems with your blower turning on while not in "Off" or "Lo", check for a loose connection at the "Auto Relay" on the driver's side of the evaporator case. Clean off the contacts and reattach. If this doesn't work, try a new "Auto Relay". They seem to go bad.

    2. Keep the EGR valve and make sure the diaphram moves freely with firm pressure. This will allow you to run at about 8 degrees timing advance with no spark knock on 89 pump gas. This will increase power by about 5% and mileage by about 5% too.

    3. Replace the primary choke pull of (vacuum break) on the carb. It's $15 and most of them are shot. This will lead to poor cold driveability as the choke won't open properly. Takes about 30 mins to replace.

    4. Rebuild the QJet and make sure to seal the well plugs in the bottom of the float bowl. These start leaking after 30 years, creating long hot-start cranking times and that puff of black smoke on start up. Not to mention, you're wasting the gas that drains through the plugs into the intake. This will also help cold starts. Without resealing these plugs, the float bowl will drain, and you'll have to crank until the bowl is refilled. If my car sits for 3 days, all I need is ONE pump on the pedal to set the choke, and the car starts INSTANTLY. Instant hot starts, too! Rare for an old carbureted car.

    5. If you've got a "sagger" in the rear, try filling the compressor on the passenger side to 90 psi. If the shocks air up, they may hold air. If the car sits for a long time, the compressor isn't strong enough to pump them up on its own.

    6. If your car has cruise control and the speedo is jiggly/making noise, you probably need a cruise control transducer. This is also the case if, when you set the cruise, the car surges, then coasts. They cost about $120 and can be swapped in about 30 mins. I've owned 3 GM cars where I've had to replace this. If you don't replace it, the transducer will likely freeze, snapping the speedo cable from the trans.

    7. For long trips, replace the ignition module in the distributor with a new genuine GM part ($35). This part is the weakest link of 70s/80s GM cars in my opinion. If it dies, you're stranded. The car will not start. Just remove the cap and rotor and take out two screws. I always keep a spare in my glovebox and a phillips head screw driver in case one goes out--this has happened to me twice in the 6 years since I've owned about 8 70s/80s GM vehicles.

    8. Next time you change the oil, look for the hole in the bottom of the evaporator case. If you still use your A/C, clean this hole out gently with a screwdriver. It becomes plugged with dirt/debris over time and won't let the water drain out of the case. You'll then end up with water on your floor when you turn a corner on a humid day.
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  3. #2
    steelybill is offline Cadillac Owners Enthusiast
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    That's a very nice '76 !! I had a '76 Coupe DeVille some years ago, which I liked. It did about 13.5 MPG around here, city/country driving mixed.

    Had a heater problem once, that turned out to be the metal temp switch which was screwed in the back of the right cylinder head.

  4. #3
    Brother_B's Avatar
    Brother_B is offline Cadillac Owners Fanatic
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    Thanks for the tips! A lot of them sound like some stuff I need to address.

  5. #4
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    Brother B, since your cars are older, you won't have an ignition module--that's only for GM HEI systems. But, the other things I said are applicable.

  6. #5
    bicentennialcadillac is offline Cadillac Owners Enthusiast
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    Fantastic car, solid advice. Thank you.

  7. #6
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    New update:

    I've made some minor tweaks to my car and improved the mileage even further. I'm now getting 18mpg on the highway! This is no joke.

    Currently have the EGR valve passage plugged off, running about 8-9 degrees base timing advance (vs. stock 6), and burning premium gas (vs. midgrade and the EGR functional before). No spark knock, and my mileage has increased another 15%! I'll pay 8% more for premium gas to get more power and 15% better mileage.

    Plugging the EGR vacuum hose really unlocked the part throttle acceleration of this car. I now barely rest my foot on the pedal to maintain 65-70mph.

  8. #7
    Mrjb9475 is offline Cadillac Owners Member
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    very good quick tips thank you? i have a 1975 eldorado
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  9. #8
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    Nice car. It brings back memories of my '73 Eldo (same color). Only had 40K on it when I sold her in '78. Boy I wish I had her now.

  10. #9
    Angry Matt's Avatar
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    I'm in total agreement with plugging the egr. Although I'm not referring to my 472, I recently bought a car with an egr valve, and it made a great improvement in my accelleration and idle after I got rid of it. For the record the one in there was brand new. I believe, if you don't need emissions testing, you don't need all that junk, unless you're firm on keeping the car the way it was from the factory.

  11. #10
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    The plugged EGR won't really help with full throttle acceleration since there's no vacuum, but it will make the car feel peppier at full throttle. Newer cars rely on the EGR to run properly as they have much higher than the 8:1 compression ratio used in the post-1970 Cad V8s.

    I'm actually now running about 10 degrees advance--still with no spark knock. It starts coming in at around 10.5 degrees. Just drove my car another 1000 miles (I've been using it as a daily driver this summer), and am still getting my 17-18 mpg on the highway.

    I only paid $2800 for the car, so I'm driving it and enjoying it! I'm not one for trailer queens. So far, I absolutely love it.

  12. #11
    Angry Matt's Avatar
    Angry Matt is offline Cadillac Owners Enthusiast
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    I'm not one for trailer queens.
    Agreed. I bought my new car with well over 100k on it and I don't mind (although there's only 30k on the motor). Now I don't feel bad running up the odometer. The most enjoyment I get from a car is being behind the wheel.

  13. #12
    CadVetteStang's Avatar
    CadVetteStang is offline Cadillac Owners Fanatic
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    Re: Cadillac 500 Gas Mileage and 70s Cad Tips

    Go to page 6 of this MTS catalog and look at the MTS #3 grind cam. You should be able to get over 20 MPG in your car if you have this cam installed. The record so far is 22MPG in a 76 Eldo, but with your attention to detail, I wonder if you can beat the record.
    http://www.500cid.com/On-Line%20Cata...%20Catalog.pdf

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