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1968 Cadillac DeVille - overheating, engine stalls when warm

Started by Lexman, June 21, 2021, 07:29:53 AM

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Lexman

Hello,

I desperately need your help:

My 68 Cadillac DeVille Convertible causes problems when the engine is warm. When cold, it runs fine in idle and full load. After about 10 miles in a warm state, it has no more power, only takes on moderate gas and dies. The engine and cooling system get extremely hot.

The following parts are already new:
Water pump, alternator, starter, radiator, engine wiring harness, ignition coil, base plate, spark plugs and cables, ignition lock, battery, fuel pump, carburetor cleaned and adjusted, ignition adjusted.

I'm slowly losing my nerve as I've been fixing the car for 4 years and haven't driven 30 miles in a row.

I ask for your expert assessment because I don't know what to do next.

I'm guessing a vacuum problem. Is there a trick how I can find the cause or which can even bring the vacuum system to bypass in order not to bring the cooling system to a boil when it is warm?

Please excuse my English. I come from Bavaria and I am very grateful for any help.

Many Thanks!

wheikkila


Lexman


Lexman


DeVille68

Hello

Does it smell like unburned gas from the tailpipe?

I would tend to say the engine runs way too lean. This will give not a good combustion, which could smell like its running rich, when in fact it runs lean.



  • With the engine cold and the cooling system cold, open the radiator cap and start the engine.
    Look inside the cap to verify that the thermostat is letting water through after the engine heated up.
    Put the cap on before you shut off the engine, otherwise water will come out like a geyser.
  • Make sure the belt is tight for the fan
  • Check the timing according to shop manual, too little timing will waste energy, too much should be heard by a knocking sound.
  • If you think it is running lean after the choke is fully open, you could try to unhook the choke coil and close the flap manually a bit and see if you can bring down the temperature by letting run the engine a bit. You should also feel the engine running better as soon as you start to close the choke flap again.
  • what is your reading on the temp gauge? Slightly above 1/4 is ok, more than 1/2 is definitely not ok

Best regards,
Nicolas

P.s: if you think you did not understand me correctly, I can write in German no problem.
1968 Cadillac DeVille Convertible (silver pine green)

Lexman

Nicolas, thank you for your detailed instructions. I really appreciate that. I had the car in a specialist workshop and had the carburetor and ignition adjusted. But that didn't make it any better.
The thermostat opens and the fan strap fits perfectly. I let it run for 15 minutes and it got very hot again. The choke flap was now fully open and I closed it a little manually. But I couldn't find any change in temperature, and it didn't run much better either. My temp gauge is out of work and I will repeat this again tomorrow with an external thermometer and deal with the carburetor again.
It always smells a bit like gasoline. But it's hard because I have no comparison to before. In addition, it is my first real US car. I'll stay tuned and keep you updated.

Thanks again,
Alex

Scot Minesinger

Have you checked the timing?  What about timing chain - could it have slipped or maybe been installed wrong if replaced?
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

35-709

Make sure also that the heat riser valve is moving freely and that it opens readily as the engine warms.  If it doesn't, free it up (they can rust open or closed), wire it open and see what that does for you.
It should look something like the one pictured below.  Those things have been causing miseries for car owners for years, important to inspect them often and keep them free --- or just eliminate them.  My opinion. 
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

Lexman

The ignition system and carburetor adjustment were set in the specialist workshop. But I will fine-tune the air valve again to exclude the carburetor.

Will also check the thermal vacuum valve.

Thank you all!


Lexman

Sorry, I confused the heat riser with the thermal valve.

DeVille68

Quote from: Lexman on June 21, 2021, 01:38:30 PM
were set in the specialist workshop.

well,....those few words would start to concern me a lot!

Please report back on each tip we gave, so that we can assess the situation.

Best regards,
Nicolas
1968 Cadillac DeVille Convertible (silver pine green)

fishnjim

Not sure but I think by '68 they start with the thermostatic clutch on the fan.   If it has one and it's slipping, not turning, it'll heat up quickly.   
Make sure no other owner induced changes around the radiator like an electric fan.   They can cause headaches by restricting air flow.   
I'd inspect for that and replace if necessary or change to a conventional fan or one with more blades.   When all else fails read the instructions.   Get a '68 shop manual.   

Gas engines are heat transfer limited on the air side, so when you come across an overheating problem, look to the air flow first to solve it.   Then you can look at the water side for fouling/scaling/pluggage, if the air side is functioning correctly.   It could just be the block isn't filled properly, has an air pocket, but they usually erupt when that's the case which wasn't indicated here.   Make sure all the splash plates are installed so the air is directed properly to the radiator.   
Otherwise, you got a head gasket leak/cracked head or block and it's directly bypassing hot gas into the cooling system, and not out the exhaust.   You'll see oil or discoloration, gassing in the coolant.

DeVille68

That is true about the fan clutch, however I replaced mine a year ago because it was not working, actually never worked. I never noticed. As long as the temp is around 1/4 the clutch should not engage. If you are idling with the A/C on on a hot day, maybe the clutch engages.
However, he has problems right after the engine is warm, so that should be about 10 min, I doubt that there is a fan clutch issue here. Just make sure the belts are tight and not slipping.

Best regards,
Nicolas.
1968 Cadillac DeVille Convertible (silver pine green)

Lexman

OK thanks. The fan is original and it works. All belts are tight. The oil is clean and the cooling water is ok.
I can rule out any damage to the block or the seal. Will check the other parts tomorrow.

Thanks again,
Alex

"Cadillac Kid" Greg Surfas 15364

Alex
You say hot. How hot is hot and what and where are you measuring the temperature.  I would make sure the vacuum advance is working and hooked up to manifold vacuum, and that you are running the correct initial advance, with the timing light on the correct (first one on the passenger side) #1 cylinder.
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-

dadscad

Just curious, I haven't seen any mention of checking manifold vacuum. Have you put a vacuum gage on it? Is the A/C and heater vacuum system in good leak free condition? Put a gage on it and watch it while driving to see if vacuum remains consistent. Vacuum leaks can cause overheating and stalling.
Enjoy The Ride,
David Thomas CLC #14765
1963 Coupe deVille

Lexman

Hello,

I found the causes for the problems: The upper thermostat in the radiator cap was defective and the outer regulator of the heat riser was hanging loosely next to the manifold.
I'll get the parts and replace the front end right away.

You guys have helped me a lot!

Thank you again for your time and detailed instructions.

Have a great summer,
Alex

The Tassie Devil(le)

Alex,

Not sure what you have been told, but there is no upper thermostat in the Radiator Cap, just a pressure-release spring.

There is only one Thermostat, and that is in the top radiator hose fitting that is bolted to the front of the engine.

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

Lexman

Bruce, you are right of course. I mean the cap with release spring.

Thanks,
Alex

35-709

"... the outer regulator of the heat riser was hanging loosely next to the manifold."

No surprise, really dislike those things, you won't find one on any of my old cars.  More trouble (to me) than they are worth and often overlooked and misunderstood until the damage is done.
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