1969 Coupe DeVille 472 engine runs rough until it warms up

[Mihai Tarţa]

I have this problem with my 69 Coupe deville: The engine starts easily, but runs rough until it warms up to normal operating temperature. It acts as if it were not runninng in all 8 cylinders and when I press the gas there's a lot of vibration going on and it struggles to gain speed.
However, once it's warm, it runs fine.

The only change that I made this year is the oil type (I used 15W50 instead of 10W40) because I noticed that  with the 10W40 I had to add 1 Quart every 600 miles so I went with a thicker one.

Any ideas ?

[Dan LeBlanc]

Sticking choke would be my first guess.

[R Schroeder]

Sounds like the choke isn't working properly.
Closes to start it , but then doesn't open correctly , while warming up.
Roy

["Cadillac Kid" Greg Surfas 15364]

Had the same thing happen on one of my cars.  I chased the problem around the vacuum circuit, the ignition circuit, and the carburetor circuit.  As a "last resort" I pulled the plugs.All four on one side looked perfect. I started pulling plugs on the other side and the first two again looked perfect, but I went on and I found the #6 plug was discolored (fouled).  Replaced that plug and the problem went away.  Apparently (on mine anyways) once the engine is warmed up it can "drag" a cylinder or two along with little problem, but until the motor warms up it is quite noticeable.
I suggest you try the plugs.  If your motor has a few miles on it or if the valve seals have not been replaced, chances are that the oil consumption is the "clue".
Greg Surfas

[Scot Minesinger]

Agree with all advice so far - very likely, check choke as that is real easy and check plugs.  My 1970 Cadillac needed a valve job because it consumed oil and every time you stepped on the gas from the red light a puff of black smoke emitted from exhaust.  It never got to the point of rough running. 

The heads are the weak point on these engines.  However, save removing the exhaust manifolds it is not too difficult to remove the heads and have them rebuilt at a quality local engine re-builder (then you can also specify the valves for unleaded gas too).  Also Maximum Torque sells rebuilt heads already to bolt onto your car.  The heads do have the engine number in the casting, so if originality is critical get yours rebuilt.

[76eldo]

Check the choke pull-off diaphragm.  It's a little diaphragm mounted on the carb with two lines coming off of it.

When the car is cold the chokeplate completely closes when you pump the pedal.  The pull-off opens the plate slightly as soon as the engine starts and has vacuum to allow some air in the carb.

I have the same issue with my 70 and I bought an NOS pull-off at a car show but have not installed it yet.  I'll let you know if that solves my problem, but these were commonly replaced when these cars were a few year old used cars.

Brian

[cadillacmike68]

Brian, are you referring to the vacuum break diaphragm, aka dashpot?

On a cold engine it's easy to check, It should set (close completely)with a full press of the accelerator pedal. After that, you should be able to push down gently on the lower edge of the choke plate and it should open up for you. You can also visually check while running after a cold start for the choke plate to be slightly open. The car should also be on fast idle (highest setting if a completely cold engine)

Yes those can be a problem. A bad cylinder could also be the culprit. My # 7 was bad, turned out to be that a loosened up pushrod fell right out from under the rocker arm!

[Mihai Tarţa]

Thanks everybody for the replies.
I don't think it's the choke, because I've already checked it and it seems to be working OK. When I pump the pedal it closes and when the engine is warm, it's open.
I'll check the plugs. I had a problem with the plugs last year. After I switched to electronic ignition, I installed Denso iridium (IW16) plugs and they were destroyed after about three months. I thought that they are not the best match and went back to regular plugs (Bosch WR9DC) which worked ok until november of last year. Come to think of it, that's another 3 months.
The plugs are most probably the problem. I'll check them next week when I get the car out.

Thanks again !

[Mihai Tarţa]

Also, there are two vacuum units on the carburator (Quadrajet), both on the passenger side, one towards the front of the engine (vacuum brake I think) and another towards the firewall. The one towards the firewall has a vacuum line on it on one side and a little rod on the other side. I noticed that there is a metal ball stuck in the vacuum line, like bearing ball.
I bought the car like this and I have no clue as to why the previous owner would have wanted to obstruct that vacuum line.

What does that unit do ? Should I remove the ball or not?

["Cadillac Kid" Greg Surfas 15364]

Mihai,
IF IT WERE ME, and the problem just started, I would find the problem before I started making changes. Many of the QJ's did not in fact have the rear vacuum actuator.  Find the problem first.
Greg

[R Schroeder]

Just because the choke closes , ands then its open when hot, doesn't mean it is set up right.
If you do have the rear choke pull off on the car, and I say if you do, that unit opens the choke a set distance once it starts. That's all it does. The choke must then open slow enough to stay in tune with the motor warming up.
If you have ball bearings or old jeeps stuck in the vacuum line ,you have your first problem.
If it doesn't open or opens to quick, you will get a rough running engine, until its warm.
You have to check this from a cold start , with air cleaner off, and watch it while the engine warms up. You also may have to give it a quick shot on the gas once in a while .Pull back on throttle while its warming up, just a bit.

Picture of carb would help a lot. Sounds like a newer carb, if it has a rear choke pull off.

Mike , a dash pot is on the drivers side. It just slows the return action of the throttle when coming  back to idle.

[R Schroeder]

This was from Clay Tynan a.k.a. blue 68deville.
We were talking about the rear vacuum pull off on carbs, and he corrected me on this carb for his 67.
I wonder if yours is the same way.
Roy

His posting;
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The diaphragm on the rear of the 67 carb is for the idle-up function. It increases the idle speed when the a/c compressor is engaged and the car is in park/neutral.
The vacuum break/choke pull-off is low on the front pass side, should tee in to the vac port on that side of the carb. It is a full mainfold vac source. It will slightly open the choke when the eng starts, keeps it from bogging down due to a rich condition.
The vacuum advance should come off the vac port on the front driver's side, just above the idle mixture screw. It should correspond to a port in the throttle area, right above where the butterfly edge rests when at idle position. That makes it ported vacuum, it won't provide a vac signal untill the throttle is slightly opened.

[TJ Hopland]

Isn't this one of the years that has the exhaust cross over is open under the carb?   Seems like I remember reading about this symptom not that long ago and it turned out that the wrong gasket was installed and it was slowly burning through and allowing exhaust to leak into the intake. 

[cadillacmike68]

This was from Clay Tynan a.k.a. blue 68deville.
We were talking about the rear vacuum pull off on carbs, and he corrected me on this carb for his 68.
I wonder if yours is the same way.
Roy

His posting;
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The diaphragm on the rear of the 67 carb is for the idle-up function. It increases the idle speed when the a/c compressor is engaged and the car is in park/neutral.
The vacuum break/choke pull-off is low on the front pass side, should tee in to the vac port on that side of the carb. It is a full mainfold vac source. It will slightly open the choke when the eng starts, keeps it from bogging down due to a rich condition.
The vacuum advance should come off the vac port on the front driver's side, just above the idle mixture screw. It should correspond to a port in the throttle area, right above where the butterfly edge rests when at idle position. That makes it ported vacuum, it won't provide a vac signal untill the throttle is slightly opened.

Roy, thanks for the clarification.

You really can't go by a 1967 diagram. The engine was completely different. 

My 1968, which is essentially the same as the 1969, had two vacuum break diaphragms on the passenger side, one by the primaries and one by the secondaries. I had the rear one (secondaries) taken off and the line plugged, but I can put it back if i want. The Idle speed up solenoid is on the drivers side, and it does not really come into play on a cold engine because the fast idle cam will cause a much higher RPM than the idle speed up would cause. There no way a 68-74 will have the idle speed up at the rear on the passenger side.  It needs the electrical connection and that's over on the driver's side. The breaks on the passenger side were vacuum operated only.

Roy, is correct, you have to start with a cold engine, one where the exhaust manifold is cold to the touch. Have someone start it while you are looking at the choke plate. It should close then set by depressing the accelerator all the way and slowly releasing it. As soon as the engine starts, the plate should open about 1/8 inch. If it doesn't then something is stuck with the linkage or bushings, etc.

Only after this is is confirmed as not the problem should you go hunting for other issues, but  you may want to close off any loose vacuum lines as well.

And yes these years did have the exhaust crossover. I'm still thinking I should have had those ports welded off and the riser on the passenger side brazed permanently in the open position. If I ever have the intake manifold removed again, that will get done.

Here is a link on vacuum choke pull-offs, AKA vacuum break diaphragms:

http://autos.yahoo.com/maintain/repairqa/engine/ques072_0.html

Maybe I'll put my secondary one back on now!

[R Schroeder]

Mike,
Once again you didn't read all of my postings. Reply 10 is talking about the vacuum choke pull off on the rear of the carb. I am fully aware of what this does.
One of the posters posted that he didn't believe there was a vacuum pull off on the rear of the carb, for this year.  I wrote about it, IF  he had that type of carb. Some Quads  did NOT have one.
I don't know when the carbs changed over to the rear pull off.

That's why I posted one about the rear vacuum pull , ON CERTAIN YEARS , being used as a fast idle for when the air conditioning is on. Post #11
This is a TRUE set up. When you turn your air on vacuum is supplied to that rear vacuum device.
It moves a mechanical linkage to step the idle up.

Drivers side ELECTRICAL idle solenoid is on my car. It pops out when the air conditioning is on.
I never said it had anything to do with a cold start up.

On that same side , on some of the carbs , there was a unit called a dash pot.
This slowed the return of the throttle linkage , on SOME models.

So to recap;
Some Quads don't have rear vacuum brake.
Some Quads do have a rear vacuum brake.
Some Quads use the rear vacuum device as a idle control for air conditioning.

Some Quads use an electrical device to step up idle , located on the drivers side .
Some Quads use a dash pot on the drivers side to slow throttle linkage down.

And we are talking about carbs here, not so much the year of the car. There have been plenty of posters at this site that have orphan carbs on there cars. So, it was more important to find out what he was using on the car.


Guess its all moot anyway, as he has not returned to answer any questions.
Roy

[Mihai Tarţa]

Hi !
I'll go to finish up my tach project on Sunday, and I'll take some pictures of the carb and post them here.
Thanks, and sorry for the delay.

Just because the choke closes , ands then its open when hot, doesn't mean it is set up right.
If you do have the rear choke pull off on the car, and I say if you do, that unit opens the choke a set distance once it starts. That's all it does. The choke must then open slow enough to stay in tune with the motor warming up.

I'll try to take a video of the choke plate from the moment I start the engine until it warms up, with a thermometer measuring the temperature as the choke opens up. Then we'll know what's what, right?


This is how the passenger's side of the carburetor looks like. It's a crop from the service manual and my carburetor looks exactly like that. The vacuum line that is obstructed by the metal ball goes on the unit that's on the left side of the photo.

[R Schroeder]

OK. That rear vacuum device is the idle unit , for the idle when air is on. There is a vacuum line coming out of the fire wall with a white stripe on it, according to what Clay posted for me.
Vacuum comes on when air conditioning is turned on.

On newer carbs like mine , in my 78, it is a choke pull off mounted there.
Roy

[Mihai Tarţa]

Ok. So we know that it has nothing to do with the symptoms.

I'll try to take that video on Sunday, as I think that it's the best way to determine whether the choke works properly or not. If it does, at least we can rule that out as the source of the problem.

[cadillacmike68]

Mihai, please check the operation of the primary vacuum break. That's the one at the front of the carb. You can clear the obstruction on the rear one, if the break is not damaged. The line may have been plugged with the ball because a damaged break will cause a vacuum leak, and these went bad quite regularly. Because of this, removing the rear break was common and as long as the line is sealed / plugged, it does not hurt operation of the engine, because the secondaries almost never come into play on a completely cold engine.  It's better to have the line plugged than have an open line with a leaking break.

But your primary break must be working or you'll get rough idle when cold.

And now for something completely different:

OK. That rear vacuum device is the idle unit , for the idle when air is on. There is a vacuum line coming out of the fire wall with a white stripe on it, according to what Clay posted for me.
Vacuum comes on when air conditioning is turned on.

On newer carbs like mine , in my 78, it is a choke pull off mounted there.
Roy

Roy, that is the secondary venturi choke pull-off. It is not the idle speed up for when the A/C is on. There is no electrical connection, which is needed for the A/C idle speed up. From 1968-onwards, that was on the driver's side of the carb, just like yours.

And, i did read all the old posts. We can't go by 67 or earlier on these cars for just about anything in the engine compartment, they are just too different. The poster who stated that many of the QJs did not have the rear break has a 66 and a 72. the statement that 67 has the idle speed up on the passenger side is correct, but it does not apply to 68 and later cars.

Remember, in 1967 and earlier the AC compressor was waaaay over on the passenger side of the engine on a long extended bracket.  It was logical to have the A/C idle speed up on the passenger side of the carb. That's why 67 and earlier basically does not apply in the engine compartment.

1968-70 were almost identical in the carb, but are very different in many respects from everything that came before or after. I have had all three years, for over 24 years total, and all with the original carbs. They all had dual vacuum breaks. They might have not been on some later years, but the manuals for all three years all have both breaks on the carbs. (68 and 69 used the same model QJ)

A lot of changes came in 1971 as well mostly WRT emissions, which invariable changed the carbs, making these 3 years unique. Some years might not have had dual vacuum breaks, but we are discussing a 69 here which originally did, and it has a vacuum line that must be dealt with with or without the break.

[R Schroeder]

Mike, if he has a 69, why is he posting a picture of a 67 carb with the idle step up on it.
All I said is , that is , the 67 carb. HE said that was what he had on his car. If he does then we have a problem .
That is why I asked him to post a picture of the carb on the car.
To many people have carbs on there cars that didn't come with the car originally. One of the other guys with a 77 had a Chevy carb on his. Went around for days with him on that one.
A simple picture of the carb, taken from both sides would help. Pictures are better . Or a carb number.
Right ?
Roy