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dieseling on shut off

Started by J, August 29, 2008, 11:57:01 PM

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J

 ok i have a 75 eldorado and it is dieseling on shut off i am going to adjust the idle it does appear to be high and i want to time it. i heard advancing it is best ,does anyone know how far or what to set it at or any tips  thanks for the help? Jessie
Cadillac got me!!!

Otto Skorzeny

First of all you should determine why it's dieseling in the first place.

I assume that the car ran normally and is now dieseling. The most common cause is gasoline that has too low octane rating. Do you normally run 93 octane? Maybe you accidentally put 87 in the tank or got some bad gas.

In any event, check the basics- cap, rotor, plugs, wires, etc. A weak spark can cause dieseling if it's not igniting all the fuel. A fuel mixture that is too lean can also cause this problem.

As for the advancing the timing, that will make it worse. Knocking can be caused by  OVERADVANCED timing. You want to RETARD the timing to  stop the problem. I had a high performance "hot rod" engine in a Ford F100. In order to run 87 octane fuel  without dieseling at shutoff, I retarded the timing somewhat and the problem went away. Of course I didn't get the performance I had before.

How far do you retard it? It's trial and error. retard it a little, shut the engine off and see what happens.

Before you go just moving the distributor around, check the timing the way it is and see if it's on target or what. If it is, then the problem is gas or the ignition components.

The most important thing is to figure out why it's dieseling and this will tell you the best course of action. Perhaps the timing chain is stretched or the gear jumped a tooth or something related to age like that.

Carbon build up on the pistons or heads can also be a cause.

fward

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#2
If the idle is too high, that alone can cause dieseling, set the idle according to specs and see what that does.  Read the shop manual for the correct procedure, it is not a simple matter of just turning screws on the carburetor, there are preliminary steps that should be taken before actually adjusting the idle speed, you will need a tachometer.  Dieseling is not uncommon and is often caused by the idle set too high by someone not adjusting it correctly.  If you don't have a shop manual then maybe this is best left to someone familiar with the proper procedures.     

I will assume your timing question was separate from your dieseling question.  As Forrest said timing can affect dieseling, especially since advancing the timing will increase your idle speed.  I believe you will have an HEI distributor, chances are the timing is OK unless someone has been fooling with it ---- again, follow the shop manual procedure for checking the timing, don't just put a light on it and start changing the timing without doing the preliminary steps first.  Check the timing BEFORE adjusting the carburetor, any timing adjustments necessary, one way or the other, will affect your idle speed.

Your low compression 500 should run fine on Regular gas if it is tuned properly, my '71 472 tuned to factory specs did fine with Regular.
:)

You might find this an interesting read ----- 
http://www.modifiedcadillac.org/forum/index.php/topic,5.0.html

     
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible - Sold
1950 Packard 2dr. Club Sedan
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

"Cadillac Kid" Greg Surfas 15364

Jessie,
As both previous posts suggested, a too fast idle speed, caused by adjusting the primary throttle blade position screw on the carburetor is the primary culprit.  For emissions (I guess) Cadillac chose to severely restrict the carbueretor's idle circuit passages and supplement them with a tad of throttle blade opening.  On the earlier 70's models this was done by an idle stop solenoid which, when the key was on (supposedly the engine would be running), the solenoid would bear against the carbueretor's throttle arm and increase the idle speed.  The adjustment of this solenoid was the method of idle speed adjustment.  When the key was turned off, the solenoid retracted and the throttle closed completely, At leas that is how it worked in theory.  Stuck solenoids frequently malfunctioned causing all sorts of problems including dieseling. Changes made to the Quadrajets included revised idle circuits so UNLESS SOMEONE HAS REPLACED YOUR CARB WITH AN EARLIER MODEL, a proper adjustment can get the idle speed down and the dieseling can be corrected.  Get the hint? Check your carb number, located on the side of the carb body.  The number should be 704523X. The fourth digit is the year.  Check this first.  It is possible and probable that if you do have the correct carburetor that it's idle circuits have become clogged with who knows what over the years and when cleaned will return the accuracy of flow necessary to idle correctly.
Good luck.
Greg
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

TJ Hopland

Whats your current 'tune up' state?   Might be worth doing if its getting close to being due.  Age is almost as hard on some of the stuff as mileage.   Does it seem to run as well as it always has?   Things like the mechanical and vacuum advance in the dizzy could be suspect causing higher than usual engine temps along with the cooling system.  A easy tip until you get it figured out is to shut it off while still in gear, the load of the transmission will usually keep it from doing it so you dont look like Uncle Buck in the parking lot.  Just remember to put it in park before you get out.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

J

thank you for all the hints  the two things that are different this year is higher  idle in park and  dieseling at shut off ignition parts are new and checked timing today and is within specs i use mid-grade 89 octane gas i will fill with higher octane i am thinking about taking the carb off and send it in to get rebuilt by someone who knows these carbs well 
Cadillac got me!!!

Rusty Shepherd CLC 6397

Until you can get the problem diagnosed and corrected, try turning the ignition off with the car still in gear and the AC compressor on; an engine under "load" is less likely to diesel.