: 1979 DeVille 425 with a rough idle

07-28-11, 05:40 PM
Someone please help!! I am fighting a rough idle issue in my new to me DeVille. I replaced the plugs, wires, cap, rotor and now just rebuilt the carb. The car runs perfect except the engine shakes when on normal idle. Seems fine at fast idle. I do have choke issues, but that is separate issues, just replaced the thermostactic coil (electric), and my pull off is not working. Just wondering if there is something that normally goes bad on these engines to cause a rough idle. Like maybe a vacuum leak, egr valve etc. I did change the pcv valve and I did remove the plugs for the idle mixture, although I have not attempted to adjust the mixtures. The car is all original with 93K miles. Thanks, Kim

07-28-11, 06:07 PM
The vacuum lines would be the first place I would look; I would just replace them all and make sure they are still routed to their factory correct positions.

07-29-11, 06:57 AM
Thanks, That is what I am doing today. Anyone have a vacuum diagram for a 79 DeVille, the sticker under the hood is pretty basic. Kim

07-29-11, 07:58 AM
Try to get your hands on a vacuum gauge too to verify that it has good vacuum. Since the adjuster screws are still plugged I am a lilltle less worried about that adjustment. I would get the choke working properly and maybe run some Techron in the fuel tank if it's been sitting for a while. How much is this car driven and how long does it sit around? My 425's do run very well and smooth and they do sit around quite a bit so that's not really an issue unless it's been years. This relates to the age of your fuel too.

07-30-11, 09:48 AM
I changed a few old hoses, seems to be better. I unplugged the idle mixture screws when I had the carb apart (what a pain) but left the adjustment alone. I may have an old fuel pump/vacuum gage in my box, if so I'll check. The gas is old as I just bought the car and it was sitting not used much for a while. It had a full tank when I got it, its down to less than a quarter tank now. I added a can of Seafoam, so maybe when I get a full tank of fresh gas, things will improve. Cadillac Al, what part of Maine are you from? I just moved to Florida last year from Auburn. Kim

08-02-11, 08:44 AM
In addition to all the other suggestions---might be wise to check the "fuel filter" too.It`s a bitch to change though----the a/c compressor is in the way.
I tried changing it on my '78 Deville 425 and no luck so far.

12-10-11, 01:52 PM
I have brought 2 Cadillacs back to life, a '79 Sedan de Ville that sat for 5 years and a '79 Fleetwood Brougham that sat for 18 years, mostly inside. If I were you I would get a vacuum gauge and hook it to manifold vacuum and adjust your fuel air mixture screws so that you have your highest vacuum. I have found sometimes you may have to back the screws out as many as 6 turns. But before you do that turn the screws in until they seat lightly and count the number of turns and write that down so that you have a re ference point. If the RPM 's go up over 1000 lower them to 800. Also get a can of carb and choke cleaner and spray along the intake where it meets the cyl. head on both sides, spray around the EGR valve and spray around the base of the carb. If you have a vac leak the RPM's will go up. I hope this was helpful. You may have already fixed the problem by now as I am coming into this forum today.

12-10-11, 06:17 PM
Check timing, confirm idle speed.

12-11-11, 10:20 AM
You say U have rebuilt the carb. Do you have a Cadillac Service Manual?
The manual has a definite proceedure to follow when setting the choke rods and vaccuum dashpots. I have found that following this proceedure to the letter helps your gas mileage and performance.