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1970 Cadillac 472 near stalling on reduced throttle

Started by Scot Minesinger, May 21, 2011, 05:07:30 PM

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Scot Minesinger

Hi and thanks in advance for helping me with this problem,

The car runs well, except after it is well warmed up and say I'm going 30 or so and I let up on the gas pedal, the car wants to stall.  It runs fine under heavy throttle or at speeds above 40 mph.  It runs fine in park at idle.  It even seems to d o better at idle in drive.  It is that slowing down at low speed that seems to be a problem.  There is no back firing and so don't think it is ignition. 

Here is more information:

The carburetor was replaced with a bench tested one and this did not change anything, although it sure runs nicer now and gets better mileage.
The fuel filter and fuel line from fuel pump to carb was replaced.
The starter was replaced (this should have no effect)
The ignition coil, distributor cap, and rotor have been replaced.
The distributor vacuum advance has been tested and the rubber diaphragm is good.
The carb vacuum is piped directly to the dist rubtor advance (it does not go through the ported vacuum switch and the trans controlled solenoid valve.
The snorkel damper does not draw air across exhaust mainfold (it used to, so thanks for the help there).
The thermostat is a 180'F, and the engine does not run hot.
The car starts beautifully when this happens, no issues there (don't think it is vapor Lock).

I have not replaced the fuel pump and could this be the sign of a weak fuel pump, reducing rpms and just does pump enough gas to keep it running?

The dash pot is supposed to keep the the throttle from returning to fast and causing stalling-maybe that is it-if so where could I get one?

Speeding up the idle to a very high setting (that keeps the car at 30mph on a level road) seems to minimize the problem.

Any ideas will be appreciated.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

R Schroeder

I believe I sure would be starting with the dash pot, if it is running so well in all other areas. Just my 2 cents.
Roy

TJ Hopland

I see no mention of any work or testing of the points?  Or is this been converted to an HEI?   If its points and no one has replaced them they could be bad.   Also you need to connect a dwell meter and make sure its set right and then rev the engine and make sure the reading stays stable.  If its all over the place that indicates worn bushings in the distributor shaft which really screws up the spark and would be rpm based because the shaft tends to wobble differently at different rpms.

Does it do the same thing if you disconnect and plug the vacuum line to the distributor?    Im wondering if its a broken wire going to the points (or pickup coil if HEI) so when the advance operates it looses the connection which causes a change in vacuum so the advance moves and the connection is made again. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Scot Minesinger

Additional information, should have told you that the engine was rebuilt about 6 years and 20,000 miles ago and the ignition was converted over to electronic, so no more points and condensers.  The engine runs extremely well providing good power, even idling and respectable economy otherwise.  The car is difficult to drive with this problem after about 45 minutes of operation, because there tends to be a lot of 30 mph let up on the accelerator type driving.  From reading the shop manual the dash pot seems the likely culprit and it is original-any idea where to buy one, beside used?  Normally you can get some parts like from Chevrolet-but not this time.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

CEC #20099

I don`t think a lack of a dashpot is the issue. Decel, closed throttle , from 30/40 MPH results in high intake manifold vacuum (25 inches) and very low fuel flow from the carb. At idle, you may see 16-18 inches vac. Any vacuum leak in the intake manifild, gasketing, or attached vac accessories, may sufficiently lean the mixture to try & stall the engine. If your carb has a hot idle air bypass, this is a prime item to check.

Personally, I hate dashpots, esp on manual trans vehicles, & routinely pull & toss them. Vehicles run better. Start checking to see if your carb has the bypass, as it was commenly used in the early 70`s.
c chleboun  #20099

Scot Minesinger

CLC # 20099

Thanks I was thinking maybe I have a lean mixture on deceleration.  The problem I suffer is described as a dash pot failure in the shop manual.  The only thing with the carb bypass idea is that I switched carburetors and the same problem persisted as if nothing was done at all.  Seems to be exacerbated by a warm engine.  When I installed a 180'F thermostat (replacing the 195'F) and fixed the snorkel damper so air was not drawn across exhaust manifold after warm up resulting in a cooler running engine the problem improved and became less trouble, but still remains.  What could be a vacuum leak that develops only when the engine is thoroughly warmed up?  The car runs so well, idles smoothly and all, it does not seem like a vacuum leak although it could be.  I just replaced the dash pot with one that may be OK (can't find a new one), adjusted it and all, just need some dry weather to test it.  If the idle is turned up way to high (to run at 30mph on a level road with foot off accelerator pedal this does not happen.  Can't live with that fast of an idle.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

CEC #20099

More ideas: Power brake check valve or booster leaking?

I would spray carb cleaner ( Gumout) on the entire intake manifold gaskets & carb base gaskets, while engine is hot, idling, & has a tach hooked up. Look for an area that causes the idle speed to change. This may mess up a nicely painted engine, but I have not had good luck with WD-40 , or propane testing.

Another test: When this is just starting to happen , on Decel, shift to Neutral, & see if the engine stalls. This would tend to confirm a lean mixture issue. Be careful, you will quickly lose power braking !!

c chleboun #20099

"Cadillac Kid" Greg Surfas 15364

Scott,
Making the assumption that the dwell and timing were set correctly when the points were changed, and that you can keep the car running with additional throttle, everything you have said it sure sounds like the fault is in the idle system.  When the throttle blades close completely, the carburetor's idle system is what keeps the motor running. The purpose of the dash pot as mentioned earlier, is to keep the throttle from closing abruptly.  I did not see mention here of you setting the carb up per factory service manual by opening the idle adjustment screws a specific number of turns, adjusting the idle speed screw on the carburetor, and the repeated process of adjusting the scews and the idle speed until the correct idle is maintained.
The process is described in the manual, and if I remember correctly from a previous post, you said you had the manual.
Let us know
Greg
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-

Scot Minesinger

Guys,

I think, but don't want to say for sure, that we are closing in on this problem.  The engine was rebuilt about 20k miles ago and six years ago.  After reading CEC # 20099 about vacuum leak, I considered a flaw in the rebuild.  The intake manifold bolts several were checked and found to be half a turn lose.  Perhaps during warm engine, low throttel, and high vacuum this caused a lean mixture and near stall conditions-it makes sense.  After tightening them it seemed to have cured the problem, but can't be sure yet.  The problem developed to a painful degree after the engine ran for an hour or so, but showed after just 20 minutes.  I drove the car for half an hour and it ran better the whole time.  Will take it on a longer test drive tomorrow, weather permitting.

The carb was a rebuilt, bench tested and it really was a huge improvement.  However the carb replacement had no effect on the problem.  I have ruled out any problem with carb because the original was a twice rebuilt by 90% knowledgeable mechanics and the rebuild was independent of that, yet the problem was unchanged.

Thanks, I will post the next time I take it on a long drive and we can hopefully conclude this.

Scot
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty