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1970 Cadillac Coupe Deville Detonation

Started by Houston747, November 28, 2019, 12:56:31 PM

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Houston747

Hello Everybody,

You might have seen me post a few questions before. I have a 1970 Cadillac Coupe Deville with 19,000 original miles. I have owned the car for almost a year now. I live overseas, and for the first few times I drove the car everything was running smoothly. Up til last May when the weather got hotter here the car had a detonation/spark knock sound to it. I was told that the car might need a HEI distributor as the original distributor might have been not working properly. I bought a new HEI distributor with some new plugs and wires, and I also bought a 4 row champion aluminum radiator, as the aftermarket one I had on it wasn’t an actual 4 row radiator, and I also still have the original radiator that was originally on the car. After installing these things I took the car out to test it in which was at least 100 degree weather, unfortunately the same denotation sound remained and it didn’t go away. I then put a clutch fan along with a 5 blade fan in order to keep the car cool in which I doubt it is overheating. Even after that, the sound was still there. I then talked to a mechanic here who told me that it might be a cold air issue, as what might be going into the air filter/intake itself is all hot air coming from the engine. So what I did was I found myself a hose which wasn’t really meant for cars and I wrapped it in aluminum tape as I did not want to spend a lot of money on something that would have a chance of not really changing anything. So I tried the car and the sound was extremely less than before and you could only hear a bit coming from the car. A couple days ago, I re-adjusted the distributor timing by retarding it. I don’t know which degree I set it to, as the timing gun I have doesn’t have a screen on it. I kept adjusting it and drove the car, the sound was even less than before, and then I tried putting that hose back on the air intake and the sound completely went away. So my question to you guys is, could this be the right solution? As when you look at mid to late 70 Cadillacs you’ll realize GM equipped them with HEI distributors and air intake hoses as they probably realized some of the mechanical problems that existed before. And now since I reduced the timing and added the air intake hose there is no sound at all. And just to let you guys know, the car drives wonderfully, it easily gets to and cruises at highway speeds, and the detonation sound only used to come when the car used to get hot especially during inner city driving, but on the highway with all the cold air there would be no sound when using the hose, but without it there would be some sound coming from the engine. So could this really be a cold air issue? I know some of you guys’ cars don’t need that, but with these cars each one is different than the other even if they are the same model year and car. Thanks a lot.

A.Alrub
A.AlRub

cadillacmike68

#1
Timing was too far advanced. 1970 was a high compression engine at 10:1 This was the last year, in 1971they dropped to something around 8.2:1 or 8.5:1 and went lower.

Without high octane fuel, say 95 or 97 octane, you will need to keep the timing back a few degrees. But this makes the engine run hotter.

You probably didn't need an HEI. I run the original points setup and my 1968 engine is 10.5:1

The larger radiator will offset the hotter running engine.
Regards,
"Cadillac" Mike

Houston747

So should I fill up the car with 93 or 98 fuel? When I had the 98 with the advanced timing it had the detonation sound, I currently have 93 in the car and with the hose there is no sound. So would it be better if I go back to 98? And should I keep the timing (retard) as it is now? And when you talked about the radiator, when you said offset what does that mean? Should should I go back to the 3 row radiator?
A.AlRub

The Tassie Devil(le)

The trouble with 98 is that that fuel is designed for late-model engines with fuel injection and high compression.

I tried to run it in my '72 Eldorado, with a modified engine, ('70 pistons, hotter cam, blocked-off heat crossover etc) and the car ran like crap.   Went down to 95, and it performed better, but now I run 91, with an octane booster, and it runs beautifully.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

rajeevx7

Leave the rad and use easy to find 93.

Yes, your motor could be running hot but it could also be running lean. For all accounts, colder intake temps are great all around, but I’m hesitant to say that is your root problem. Maybe the sound is coming out of the intake and you have just moved the ejection port farther away and into turbulent air causing a muffling effect. I and many others run open intakes with little under hood heat issues, like you cited. You are also right about variables between cars like the plastic spacer between carb and intake, or coolant jacket blockages or heated/non heated carbs or burped systems etc,etc.

Pull all the plugs, “read them” and let us know.

Dan LeBlanc

The biggest question is how is your octane rating established in your area?  Is it RON, MON, or RON+MON/2?  How they advertise the octane rating will have an effect of what goes into your fuel tank.
Dan LeBlanc
1977 Lincoln Continental Town Car

bcroe

It is very important that your timing chain set is not getting sloppy.  At
this age the plastic teeth may start coming apart regardless of low miles. 
I had the same issue till I changed mine.  Later complete failure may
severely damage the engine. 

Retarding the ignition may cure the noise, but it usually results in reduced
power and fuel economy.  good luck, Bruce Roe

"Cadillac Kid" Greg Surfas 15364

#7
A 50 year old car with 19,000 miles would indicate it saw very little highway usage and probably has a GREAT deal of carbon build up, thus the preignition you have observed. If you are overseas and you have 98 octane gasoline use it. 
As Bruce says, it is a good bet that the timing chain has gotten sloppy from disintegration and it should be checked and if found to be ANY way deteriorated, replaced.
As an aside, the later 74-76 motors with HEI and air intake hoses were quite different in their compression and timing characteristics, so adding those two items (HEI and Air inlet hose) are not necessarily an improvement to a stock 1970-472 motor.
Don't know exactly where you are (or even have the slightest clue), but I might suggest taking the car on a high speed long (100-200 mile) drive to try and clear out some of the "cobwebs".
Greg Surfas
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-

chrisntam

Quote from: Houston747 on November 28, 2019, 12:56:31 PM

... I live overseas...  --> Where?

... I was told...I then talked to a mechanic here...  -->  Highly suggest you get diagnostic  information here (not there), there is A LOT of experience with these cars here.


1) I'd suggest using the highest octane fuel you can get and set the timing per the factory service manual.  I have a '70 with a rebuilt engine with true 10 to 1 compression and I use 93 octane fuel with no issues.

2) Be sure to retain all your old parts!!!

3) There could be something in the new HEI distributor that is advancing the timing too much once you're underway (springs on the mechanical advance or an adjustment on the vacuum advance).

4) How does one check the timing chain/cam sprocket without opening up the front of the engine?

As a side note, Marty from Cadillac High Performance told me that the '70 compression was actually 9.3 to 1 (though advertised at 10 to 1), which is still high for today's normal fuels.
1970 Deville Convertible 
Dallas, Texas

bcroe

#9
Some (including me) have used water injection to deal with these
issues.  Doing this for a while will certainly clean out any carbon. 

Once I changed the chain, I did not need the water injection any more. 
Bruce Roe

cadillacmike68

Please get the service manual. they are available on fleabay for a decent price.

I would go back to the points and condenser if possible, if not, just make sure the HEI is working and you can set the base timing to the correct 5 deg BTDC, in Drive, at 600RPM, Vacuum Advance Disconnected, etc.... Then try to get stiffer springs for the mechanical advance if needed to slow the advance curve.

The radiator should not impact timing at all. and the cooler the better on the 472-500 as long as it gets up to operating temp of about 200 deg F

Cold air intake? not really needed if you are moving along. but make SURE that that spring and / or vacuum loaded trap door on the snorkel is functioning so that you  don't draw all your air from off the exhaust manifold. Or take it off if its not working. That was put on starting in 1970 to warm up the air going into the carbie on initial startup. If its hot most of the year where you are, then it's not really needed at all.

By the way, where are you?

Cheers and regards,



Regards,
"Cadillac" Mike

Jeff Rosansky CLC #28373

One thing is that there appears to have been a few changes made that may not all work well together.
Couple things-- On the air cleaner there is a tube that comes up from the driver's side exhaust. There is a little flapper valve in there that opens to allow hot air in when cold. Make sure that is Closed so that only "cold" air is fed to the carb.
Secondly, disconnect and plug all vac lines and try it. You could have a leak that is leaning you out too much. Also, I had a crack in the brake booster check valve that made a huge leak. This was the only time my temp light ever came on.
There is that temp sensing vac valve under the distributor that advances the timing when the water gets too hot. Be sure that isn't leaking and/or falsely advancing your timing.
Also, and no insult I tended, are you sure it is actual pinging and not just another sound?
Good luck,
Jeff
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

"Cadillac Kid" Greg Surfas 15364

I went back and read the original post a bit more carefully.  I agree wityh Mike on going back to the points distributor because the advance curve is correct for that motor.  The HEI advance curves are all across the board and mostly have too little mechanical advance. Be sure you have the correct vacuum advance and run it directly off intake manifold vacuum.

You can check the timing chain by either pulling the distributor or the fuel pump and you can get either a half ass'd view or use a scope to check the chain.

Bruce, I'm going to guess you tried that higher octane gas when the outdoor temperature was low.  The higher rated fuels CAN have a lower vapor pressure, depending upon engine heat to fully vaporize, thus with you heat riser blocked off,
your fuel was puddling in the intake. Just a guess.
Greg Surfas

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-

cadman56

Why can'at you just move the distributor cap off to the side, hand rotate the engine till the rotor turns then hand rotate the engine backwards till the rotor budges the other direction?  then see how much change in the harmonic balancer?  If I find much change at all I replace the timing gear set.
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville (sold)
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible (Sold)
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820

bcroe

Quote from: cadman56
Why can'at you just move the distributor cap off to the side, hand rotate the engine till the rotor turns then hand rotate the engine backwards till the rotor budges the other direction?  then see how much change in the harmonic balancer?  If I find much change at all I replace the timing gear set. 

That is what i do.  My rule of thumb is 6 degrees slop means the chain
set must be changed IMMEDIATELY.  4 degrees easily will mess up your
timing, think about getting it changed.  2 degrees might be tolerable, if
it was not so old.  Bruce Roe