Vintage & Classic Cadillacs 1902-1977 Cadillac Models

Spark knock/pinging/dieseling 68 Fleetwood

Old 09-27-2016, 01:49 PM
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Default Spark knock/pinging/dieseling 68 Fleetwood

Hi all. I have a 1968 Fleetwood 75 that i drive almost everyday. Recently it has developed some odd noises that i haven't quite figured out. starts right up when cold but has loud random spark knocking coming from only one tail pipe (i have duals that dump right before the rear wheels) it definitely does not sound mechanical and is not a steady knock that matches engine rpm. it's just random . as soon as the engine is warmed up after i drive it the sound goes away. Runs great down the road good and strong, however when i shut it down immediately at my destination it diesels and finishes off with a puff of smoke. i do have a temp gauge on it and have noticed if i let it sit and idle after i drive it and let it cool itself down to 185 (from 190) and then shut it down it wont diesel. of course the obvious thing is timing which i have checked many times and played with many times as well. right now the timing is dead on. like i said i have played with the timing and even retarded it way back to see if the spark knock would go away when cold but it remains the same. as soon as i finish writing this i am going to do a compression check on all cylinders to confirm the head gaskets are indeed good or bad as i thought that was a possibility.however there are no real signs of head gasket failure. it doesn't seem to overheat. Runs in the range of 175 - 195 which i think is normal . i would appreciate any suggestions or things to check . Sorry for the lengthy post but thought i'd put in everything i could to help identify my problem
Old 09-28-2016, 02:34 PM
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The car was not designed for unleaded and 87 octane, back then regular was 89. The valves and valve seats were changed in the early seventies when leaded fuel was going to be phased out. The valve seats can were over time because of the unleaded gas. I'm not sure what to tell you about noise but you should have a reputable shop look at it locally. The spark knock can be caused by carbon build up. You could try to decarbon it. Does engine use oil? Your temp is normal I would install 180 thermostat and flush cooling system. Make sure belts are tight.
Old 09-28-2016, 07:56 PM
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i run premium in it an sometimes 93 when i feel like it. i will take it on a hot lap and see if i can burn the carbon out.
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