Spark plugs for the 78 Coupe Deville 7 liter engine
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  • 2 Post By jayoldschool
RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, Spark plugs for the 78 Coupe Deville 7 liter engine in Past Cadillac Vehicle Discussion; Autozone recommended platinum spark plugs, so I bought and installed them. The car was already hard to start when cold, ...
  1. #1
    ruttrow's Avatar
    ruttrow is offline Cadillac Owners Member
    Automobile(s): 78 Coupe Deville
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    Spark plugs for the 78 Coupe Deville 7 liter engine

    Autozone recommended platinum spark plugs, so I bought and installed them. The car was already hard to start when cold, but now it is worse. I have to crank the peddle about 6 - 10 times, and/or wait after pumping the pedal. And the motor is still a little rough at idle.

    A mechanic told me I have the wrong plugs. Platinum is why the car is running rough.

    Please let me know what you know if I need to pull the plugs and put copper ones in their place.

    Thanks

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  3. #2
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    I am a fan of regular AC Delco plugs in GM cars.
    1980coupe and 1980coupe like this.

  4. #3
    ruttrow's Avatar
    ruttrow is offline Cadillac Owners Member
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    It was a real pain installing the plugs near the firewall on each side. The other six were not bad. If I thought it would make a significant difference I would take out the platinum.

  5. #4
    SafariOne is offline Cadillac Owners Fanatic
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    Has she had a tune up lately?? Maybe a week coil or out of timing!! Definately use the old school plugs, in your case R45NSX.

  6. #5
    Fleetwood472 is offline Cadillac Owners Fanatic
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    Check the pickup coil inside the distributor. Not the coil in the distributor cap but the coil that is in the distributor housing. The wires I think are green and yellow. I've had them break on me a couple of times. The coil is probably weak and thats why it runs like crap with the platinums.

  7. #6
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    Check the gap on the plugs. I know people who take plugs out of the box and install them without checking the gap. Gaps can change from shipping, or the factory sets the gap for a specific application, they just happen to fit the criteria for your engine.

  8. #7
    1980coupe's Avatar
    1980coupe is offline Cadillac Owners Connoisseur
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    Quote Originally Posted by jayoldschool View Post
    I am a fan of regular AC Delco plugs in GM cars.
    X2,
    I was recommended the exact ones for my 78 and 80 coupe and I said no. Give me the Auto Lites or AC Delco ones, which the 78 got Auto Lites and I got the AC Delco for my 80.
    Gap on both is .60

  9. #8
    GeneLaw1 is offline Cadillac Owners Fanatic
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    I have used the AC Delco platinum in both my Caddies without issue.....your problem may lie else where in the ignition system.

  10. #9
    ruttrow's Avatar
    ruttrow is offline Cadillac Owners Member
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    Thanks for the replies! I heard I need to check two things next. One, the carb fuel float level, and two, the choke. I will
    take her back to my mechanic at the end of the month to look at these. I do not know how to check these things. That stuff
    is too deep for me (But you guys could do this with your eyes closed I bet!)

  11. #10
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    It sounds like you need a simple choke adjustment; 5-10 minute job. Platinum plugs won't hurt anything. I have used Bosch Platinums in all my vehicles for 15 years with no problems yet The Platimuns in my Seville are about 15 years old but only 40k miles. I can't get AC's to last 2 years and I'm too lazy to replace them all the time.

  12. #11
    dennis93coupe's Avatar
    dennis93coupe is offline Cadillac Owners Connoisseur
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    Re: Spark plugs for the 78 Coupe Deville 7 liter engine

    What about the spark plug wires? Have they been replaced lately?

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