78 DeVille has a rough idle
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500, 472, 425, 368 Discussion, 78 DeVille has a rough idle in Cadillac Engine Technical Discussion; A friends sedan developed a rough idle almost overnight. So rough, the car stalls at idle if set below 900 ...
  1. #1
    carnut is offline Cadillac Owners Master
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    78 DeVille has a rough idle

    A friends sedan developed a rough idle almost overnight. So rough, the car stalls at idle if set below 900 RPM. Spraying the carb and intake with brakekleen determined air entering on the passenger side intake to cyl head just beside the carb. If sprayed, the idle smoothed out and increased. We removed the intake and replaced an original intake valley pan gasket. We made sure all bolts and holes were clean and all surfaces spotlessly clean. Once installed we torqued to spec all bolts in proper order. Bottom line is the vacuum leak did NOT change??? Did not see a crack and no obvious gasket damage. We also changed the carb base gasket during the tear down. What's left? Need opinions!

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  3. #2
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: 78 DeVille has a rough idle

    Do make sure the original timing chain has been replaced. Original cam sprocket
    plastic teeth fall off with age, regardless of mileage. Bruce Roe

  4. #3
    drmenard is offline Cadillac Owners Enthusiast
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    Re: 78 DeVille has a rough idle

    It could be your EGR valve is bad or stuck. Have you taken it off and cleaned and inspected it?

  5. #4
    carnut is offline Cadillac Owners Master
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    Re: 78 DeVille has a rough idle

    Timing has been verified to be at spec and steady. The EGR was removed and cleaned. Thanks!

  6. #5
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: 78 DeVille has a rough idle

    Does the engine still speed up when you spray around the intake gasket? It sounds like those mating surfaces aren't flat for some reason. It looks like you may have to address that area if that's where the leak is. How low is the vacuum reading?

  7. #6
    carnut is offline Cadillac Owners Master
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    Re: 78 DeVille has a rough idle

    That's what I thought! Intake cracked and or warped. We did not put a straight edge on the manifold, DUH!

  8. #7
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: 78 DeVille has a rough idle

    Check your vacuum lines going to the manifold vacuum ports. There are several vacuum lines running over the top of the manifold and over times they burn through, dry out, and crack. The most common one to burn through is the one at the front of the carb that runs under the A/C compressor. The other that runs from the TCV to the EGR can also fail. Careful with the TCV connections. At this age they are brittle and fragile.

  9. #8
    carnut is offline Cadillac Owners Master
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    Re: 78 DeVille has a rough idle

    All vacuum hoses were replaced. Thanks.

  10. #9
    carnut is offline Cadillac Owners Master
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    Re: 78 DeVille has a rough idle

    Any others with more insight?

  11. #10
    the APEMAN is offline Cadillac Owners Fanatic
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    Re: 78 DeVille has a rough idle

    I've been fooled by qlodrajet shafts leaking before. The carb cleaner hits the warm engine and evaporates. It's then sucked into the carbonator base. Makes it look like an intake leak. Could be an incorrect base gasket too. Some of the AC compressor mounting bolts go into the intake and can leak too. Not sure about the 425 though. Been too many years.

    #1 cause of rough idle on these engines is intake valve stem carbon. i used to run a couple quarts of transmission fluid through the engine every 6 months when I drove my Cadillac full time.

    Wait until a neighbor u don't like has a picnic and the breeze is just right. If yer good the fire department may do a driveby.

    Sticking mechanical timing is another big problem with these. Be sure the distributor weights and all the associated stuff is free.

  12. #11
    carnut is offline Cadillac Owners Master
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    Re: 78 DeVille has a rough idle

    Thanks! I will double check the carb base gasket. I did my best to not spray anywhere else than the manifold to head area. And yes the Q Jet throttle shaft has worn the carb base a little but it only affects the carb coming back to curb idle. If I let return slowly, it idles about 100 RPM too high. Blip the throttle and it will return normally.

  13. #12
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: 78 DeVille has a rough idle

    The Q-Jets do have a problem with the throttle shafts and the extent of the wear can be determined by picking up on the shaft. A change in engine speed determines how much wear. A engine that is driven in traffic all the time will show more wear than an engine run at a more constant state. There used to be someone out there who would re-bush the throttle plates and replace the shafts. Most of the wear is caused by all the resistance applied to it by all the add-ons and the springs needed to return the shaft to the idle position. Some people use higher tensioned springs, when things get sticky. That is not advisable.

  14. #13
    carnut is offline Cadillac Owners Master
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    Re: 78 DeVille has a rough idle

    I managed an independent repair shop that would send the carb base to a machine shop. They would drill the shaft holes oversized and press in brass bushings to correct this problem on quadrajets. From the factory there is no bushing on this shaft.

  15. #14
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: 78 DeVille has a rough idle

    I have seen some very worn throttle shafts and the vehicle still ran perfectly fine. I don't think that will be the cause of the stalling and rough idling.

  16. #15
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: 78 DeVille has a rough idle

    Quote Originally Posted by carnut View Post
    I managed an independent repair shop that would send the carb base to a machine shop. They would drill the shaft holes oversized and press in brass bushings to correct this problem on quadrajets. From the factory there is no bushing on this shaft.

    Your right. re-bush was the wrong terminology.

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