70 472 dieseling...
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500, 472, 425, 368 Discussion, 70 472 dieseling... in Cadillac Engine Technical Discussion; Turning off the engine causes it to after-run couple of times. I cleaned the throttle body, everything seems to be ...
  1. #1
    cdv70 is offline Cadillac Owners Member
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    70 472 dieseling...

    Turning off the engine causes it to after-run couple of times. I cleaned the throttle body, everything seems to be working fine (quadrajet carb).

    Also, after "heavy" driving, when changing from D-to-P (or N) engine revs up a little bit. Re-running the engine - the idle is back to ok.
    What can be wrong?

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    1969cadillaceldorado's Avatar
    1969cadillaceldorado is offline Cadillac Owners Fanatic
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    Re: 70 472 dieseling...

    Quote Originally Posted by cdv70
    Turning off the engine causes it to after-run couple of times. I cleaned the throttle body, everything seems to be working fine (quadrajet carb).

    Also, after "heavy" driving, when changing from D-to-P (or N) engine revs up a little bit. Re-running the engine - the idle is back to ok.
    What can be wrong?
    my cadillac did the same thing try seafoam works very well might want to use a old set of spark plugs when u use seafoam or clean your plugs off rite after u are done but seafoam should fix your problem

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    eldorado99's Avatar
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    Re: 70 472 dieseling...

    Also make sure your idle is set low enough.

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    BluEyes's Avatar
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    Re: 70 472 dieseling...

    Idle RPM being too high can definately have a big effect as eldorado99 suggested. Also, not enough ignition advance (haha, that again) can do it. Factory settings should all be fine though.

    The idle speed should increase some when shifting from D or R to P or N. The extra drag from the transmission brings the RPM's down slightly in the forward or reverse gears. Computer controlled cars automatically adjust for this, carb'd cars just idle a bit faster out of gear...

    As an intermediate solution for when the g/f is in the car, turn the engine off with the trans still in D. The drag from the trans will usually slow the engine down fast enough to avoid dieseling. Then shift to P and set the parking brake and remove the key the rest of the way. Just don't let this replace finding the real cause of the problem!

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    cdv70 is offline Cadillac Owners Member
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    Re: 70 472 dieseling...

    Proper cleaning of the carb got rid of the dieseling, finally...
    But - then the engine has reached its normal temperature the idle is still too high (in gear or P/N). At some point it's hard to keep the car still.
    I assume idle RPM cannot be set too high because No problems when running COLD, starting etc everything works as it should.

    What to look for?

  7. #6
    Sasquatch's Avatar
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    Re: 70 472 dieseling...

    Check that vac advance can at the distributor. Also the plate and wieghts in the dist. to make sure they are not sticking.

  8. #7
    BluEyes's Avatar
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    Re: 70 472 dieseling...

    Quote Originally Posted by cdv70
    But - then the engine has reached its normal temperature the idle is still too high (in gear or P/N). At some point it's hard to keep the car still.
    I assume idle RPM cannot be set too high because No problems when running COLD, starting etc everything works as it should.

    What to look for?
    If the advance plates in the distributor aren't sticking, idle RPM is set too high. But, first you need to make sure your choke is operating correctly. See, in the choke linkage there is what is called the fast idle cam. It is there to hold the idle RPM higher when the engine is cold. As the choke coil heats up and opens the choke, it also shifts this cam to let the idle speed down untill finally the choke is finally open and the fast idle cam is past its lowest step and the idle is set by the regular idle speed set screw. The adjusting screw for the fast idle cam is over on the passengers side of the carb. The screw is kind of buried down in the choke linkage but it is the only one over there so it's not too hard to find. Now, alot of people will have a problem with the engine dying while cold and just turn up the idle speed, not the fast idle cam as they should.

    Being as some former owner likely monkeyed with the carb, we can't assume that the fas idle cam is adjusted correctly. You will need some sort of a tach if you don't already have one. I have one of the voltmeters that has a built in tach for carb tuning purposes. You can get one at most any parts store or Sears.

    When the car is cold, check that the choke flap is closed. If you remove the aircleaner lid, the choke flap is the oblong skinny one at the front of the carb in a sort of 'tower'.

    With the car cold, fully depress the gas pedal once to set the choke and then make sure the flap is closed. Then start the car and note the idle RPM when cold. Check this in P in case the fast idle cam wasn't adjusted and the engine won't idle in D later, this will just be used as a reference rpm. Now drive the car around untill it has reached normal temp (with the air cleaner lid on of course!). With the engine warm, again check the flap and it should be completley open. If you poke it with your finger it should be held open with a slight bit of spring tension. If that is all in working order, then you are good to turn down the idle speed.

    The idle speed screw is on the drivers sie of the carb right where the throttle linkage attaches. If you have the AC compressor on the car, this screw can be a real pain to reach so you may need one of those flexible extensions (the Caddy compressor generally makes all carb adjustments more of a pain than on other engines, sigh) Factory idle speed (in D) is 600rpm, but on an older engine I will tend to set it a bit above that to make sure the oil pump isn't having to struggle. 650-700rpm is fine and still plenty low. The engine should shut off cleanly with this idle speed.

    Next time you start the car cold, hook up the tach again and make sure the idle RPM is the same as it was on the previous cold start. If not, turn up the fast idle screw - NOT the idle speed screw - untill the RPM's match the previous cold start RPM's. At this point, you should be good to go.

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