1976 Eldorado -- Vibration when idling

[LenInLA]

I had the pleasure of driving a 1998 Deville over the past few days.  Rides very nicely. And unlike my 1976 Eldorado has very little vibration when idling.

I had the Eldorado tuned recently; the spark plugs and wires have been replaced.  The mechanic adjusted the carburetor to factory settings.  For some reason (mechanic doesn't know) the settings had been changed.  Yes - the car vibrates on idle.  Not horrible vibration, but definitely there; when I put stuff on the passenger seat I can definitely see it move.

So, my question is:  is that just because it's a carbureted engine and a 40 year old car? And the reason the 1998 Deville is much quieter is due to it being a newer and fuel injected car?

Thanks,
Leonard Grayver 

[Scot Minesinger]

Leonard,

Don't let anyone ever tell you that something like a rough idle that shakes stuff on passenger seat is normal.  That is a wonderful driving car in proper repair.  Anyone who says that is just how these old cars are is not correct.  Think back to 1976 and imagine the owner of a brand new Cadillac shaking and rough - no way. 

Sometimes when I'm sitting at the red light, I forget my engine is even running just like a modern car.  I had to take up working on my 1970 Cadillacs out of necessity, as there are no good mechanics in my area.  Changing the spark plugs on this car is so easy and there is so much room, you can almost do it in a white dinner jacket and not get dirt on it. 

While it is very difficult to diagnose a rough running 472/500 Cadillac over the internet, the most common reason these idle rough is a vacuum leak.  Vacuum leaks can be bad vacuum hoses (replace one at a time-requires no tools) or bad actuators (test with hand held vacuum pump - used for break bleeding sold at any auto parts store).  If the carb is original to the car it may need to be rebuilt just because the rubber pieces and gaskets are probably bad from age and the new gasoline even if miles are low.

[76eldo]

Len,

When my 76 Eldo is running you cannot feel the engine.  Idling in D at a light feels like it stalled.
Vibration as you describe is totally unacceptable and not normal.

There are so many possibilities that there is no way to diagnose it remotely.  Some people use the feel method when adjusting a carb, some use a tach or vacuum gauge.  Carbs do require rebuilding from time to time as Scott said and you may want to check a few things first.

1.  Improper air mixture idle screw adjustment.
2.  Carb has internal or external vacuum leak
3.  Carb is loose on the manifold
4.  Vacuum hose(s) old, cracked, or deteriorated
5.  Worn distributor cap or rotor
6.  Fouled spark plug
7.  Worn or unseated spark plug wire
8.  Burned or worn valve(s)  Need to do a compression test.
9.  Contaminated fuel
10. Collapsed lifter or worn cam or bent pushrod.
11.  Bent fan blade or defective harmonic balancer or other issue with a rotating component out of spec.

You need to ground out one spark plug at a time to see if there is any cylinder that doesn't make the RPM drop when you do this and then diagnose that cylinder.  If none of the cylinders being grounded out (eliminating the spark to the plug) drops the RPM then you have a global fuel or ignition issue, or mechanical issue with the valve train.

Could be a lot of things.  A fuel additive or treatment to eliminate carbon buildup may help too but the basics need to be checked out.

When these engines are running correctly they are really trouble free.  My 76 sits for months at a time with a battery tender and any time I want to take it out it starts and runs perfectly and it has 99,700 miles on it, so it's no low mileage never driven car, but was maintained by the first owner for 25 years with fanatical care.

Brian

Brian

[Dan LeBlanc]

I'm going through the same thing with my 70 DeVille Convertible I just bought.

I seem to have it traced to the intake manifold gasket potentially incorrectly installed.  At idle, I have that pffft pffft pffft miss at the tailpipe and you can feel it (and the hood shakes slightly when open).  I did calm it down by readjusting the carburetor, setting the timing, and tuning in the dwell.  It's much better now but still not quite right.  If I put my hand over the carburetor to artificially enrichen the fuel mixture, the car smooths out and the vacuum guage goes into the normal range with a steady needle (it's the same with or without vacuum accessories connected to the engine).

My engine was opened up to have an upper end refreshing.  After talking with Scot and Brian, this is the initial diagnosis we have come up with as the intake manifold gasket/valley pan combo must be sealed well to the valley pan otherwise it will idle rough.

Starting there first.  Carburetor was a Holley Remanufactured unit in 2013 (appears to be a non-a/c one as according to the shop manual, I appear to be missing a port).

So, given all this, hook a vacuum gauge on the car.  Try holding your hand over the primaries of the carburetor lightly and see if the idle changes.  Try and stall the engine with your hand cutting off the air on the primary side.  If it stalls, your vacuum leak is minor.  If it continues to run, you have a really bad one.

["Cadillac Kid" Greg Surfas 15364]

Len,
I know you don't want to hear something simply, but I might suggest that the idle speed could be a tad low.  You might try adjusting the idle speed screw (on the drivers side of the carb) 1/8 of a turn "in" at a time and see if that improves the vibration without adding excessive creep when in drive.
Greg Surfas

PS Administrator, this should be in the tech section I believe

[bcroe]

Old cars can be very smooth.  My 62 Jetfire was so smooth, passengers would
ask at a stop, if the engine was still running.  I could only tell, by the gauges. 
Bruce Roe