whitec4vette Posted June 12, 2008 Report Share Posted June 12, 2008 Hello, My mother has a 1991 Caddy Deville and has been running perfect for years. In the last month she started complaining about a vibration that no one else could sense, and now it has gotten worse. with such a broad range of potential issues, I thought i would start here and see what you can come up with. After driving the car for long periods, hours or so, the car will begin to shudder when you accelerate. If you let your foot off the gas, the car will regain its smooth ride. As soon as you accelerate again the car shakes. It won't shake when your are coasting, and it does it no matter what speed you are traveling. The car still shifts smoothly, but there is that quite obvious shaking that begins. We thought it was spark plugs so we got a tune up and still nothing improved. Please help us ASAP, Mother loves and needs her car back! Walter Link to comment Share on other sites More sharing options...
KHE Posted June 12, 2008 Report Share Posted June 12, 2008 Hook up a fuel pressure gage to the fuel rail and duct tape it to the face of the windshield. Take the car on a test drive and when the shuddering occurs, read the fuel pressure. If it drops off when the shuddering occurs, the fuel pump is failing. Kevin '93 Fleetwood Brougham '05 Deville '04 Deville 2013 Silverado Z71 Link to comment Share on other sites More sharing options...
whitec4vette Posted June 12, 2008 Author Report Share Posted June 12, 2008 Hook up a fuel pressure gage to the fuel rail and duct tape it to the face of the windshield. Take the car on a test drive and when the shuddering occurs, read the fuel pressure. If it drops off when the shuddering occurs, the fuel pump is failing. Thank you! I will try that. The shuddering just seems quite strong for a fuel issue. The engine soulds as if it is running the same even while shaking. Link to comment Share on other sites More sharing options...
Cadillac Jim Posted June 12, 2008 Report Share Posted June 12, 2008 Check the motor mounts, the torque wishbones (they do have those on the 4.9 liter ?), and the CV joints. -- Click Here for CaddyInfo page on "How To" Read Your OBD Codes-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data -- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC Yes, I was Jims_97_ETC before I changed cars. Link to comment Share on other sites More sharing options...
whitec4vette Posted June 12, 2008 Author Report Share Posted June 12, 2008 Thanks you all, I will do some research. I just got back from a transmission center and they drove and attempted to do diagnostic on the car. They said that it seemed to be some sort of engine miss. It just had a spark plug change, so I guess the next step is wires and such. Thanks in advance for your help! Link to comment Share on other sites More sharing options...
navion Posted June 12, 2008 Report Share Posted June 12, 2008 As I was reading your post, I was thinking that it was missing under load. Did you change the spark plugs yourself? If so, did one of them look really dark or black on the end that goes into the cylinder? Is so, and you can remember which cylinder you removed it from, then that should be the cylinder that is miss-firing. It is fairly common for spark-plug wires to fail and cause a miss. Another place is the distributor cap. If the cap & rotor haven't been replaced in recent memory, it would be prudent to replace them along with the ignition wires. If replacing the cap & rotor and the wires doesn't cure the miss, let us know. There are people on this board that have a lot of 4.9 and 4.5 liter experience. I'm positive that we can help your mom get her Caddy back. Good Luck, Britt PS: I'm curious. Was the miss the reason that the plugs were changed, or did it develop after the plug change. If the miss developed after the plug change, then it is likely that one or more of the plug wires was damaged internally when they were removed from the old plugs. The other possibility would be that one of the new spark plugs was cracked. If a new plug was dropped before installation, it is probably cracked and that will cause a miss. Britt Link to comment Share on other sites More sharing options...
Texas Jim Posted June 13, 2008 Report Share Posted June 13, 2008 When I was in the Air Force.... ANY plug that was DROPPED was thrown away. A plug was never used in an aircraft engine, if it had been dropped. Link to comment Share on other sites More sharing options...
adallak Posted June 13, 2008 Report Share Posted June 13, 2008 Wires. Any trouble codes? Press Warmer and OFF simultaneously for a few seconds and write down any codes on driver info center. Post here. The saddest thing in life is wasted talent Link to comment Share on other sites More sharing options...
Logan Posted June 13, 2008 Report Share Posted June 13, 2008 That era of Cadillac..1991.. Long time ago.. That era of engine had a engine ground issue. The ground mounted on the engine block very near the starter. Could cause crazy stuff. Next guess would be green wires /connections in the distributor. Look for bad / cracked connections to the IC and AC trigger pickup. After that, maybe a bad ECM. Logan Diagnostic LLC www.airbagcrash.com www.logandieselusa.com www.ledfix.com www.ledfix.com/yukontaillightrepair.html www.ledfix.com/ledreplacements.html www.ledfix.com/j42385toolrental.html Link to comment Share on other sites More sharing options...
Cadillac Jim Posted June 13, 2008 Report Share Posted June 13, 2008 Remember that this car has a distributor cap. I would change the cap and rotor ASAP. The next thing to look at is the spark plug wires, then the new plugs. -- Click Here for CaddyInfo page on "How To" Read Your OBD Codes-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data -- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC Yes, I was Jims_97_ETC before I changed cars. Link to comment Share on other sites More sharing options...
catmendo Posted June 13, 2008 Report Share Posted June 13, 2008 Remember that this car has a distributor cap. I would change the cap and rotor ASAP. The next thing to look at is the spark plug wires, then the new plugs. pop the hood at night and look for dancing sparks and although i have never had the need to do it , take a spray bottle of water and spray it around the wires and look for ol' sparky and "the dance of the devil (Lane)" ... Link to comment Share on other sites More sharing options...
navion Posted June 17, 2008 Report Share Posted June 17, 2008 When I was in the Air Force.... ANY plug that was DROPPED was thrown away. A plug was never used in an aircraft engine, if it had been dropped. That is still true today. I have had aircraft mechanics drop Iridium fine wire spark plugs that list for over $90.00 each. They are JUNK as soon as they hit the floor. I allow one plug drop per mechanic. If he drops another, he gets to pay for it. And, Yes, I have dropped a couple of them myself! It is amazing that no matter how much noise is in the shop, everyone seems to hear when a plug hits the floor. It is a humiliating experience. And Yes, I have had to pay for them. Now, auto mechanics are not under the same restrictions that aircraft mechanics are. While any plug that has been dropped should go straight into the trash, some people will give the plug a visual examination, and it they don't see a crack, they will go ahead and install it. Sometimes they get away with it, sometimes the plug sill short out and cause a dead miss and sometimes the electrode will fall into the combustion chamber and destroy the piston & cylinder head! Even the most expensive spark plug is not as costly as a comeback is. Take Care, Britt Britt Link to comment Share on other sites More sharing options...
adallak Posted June 17, 2008 Report Share Posted June 17, 2008 When I was in the Air Force.... ANY plug that was DROPPED was thrown away. A plug was never used in an aircraft engine, if it had been dropped. That is still true today. I have had aircraft mechanics drop Iridium fine wire spark plugs that list for over $90.00 each. They are JUNK as soon as they hit the floor. I allow one plug drop per mechanic. If he drops another, he gets to pay for it. And, Yes, I have dropped a couple of them myself! It is amazing that no matter how much noise is in the shop, everyone seems to hear when a plug hits the floor. It is a humiliating experience. And Yes, I have had to pay for them. Now, auto mechanics are not under the same restrictions that aircraft mechanics are. While any plug that has been dropped should go straight into the trash, some people will give the plug a visual examination, and it they don't see a crack, they will go ahead and install it. Sometimes they get away with it, sometimes the plug sill short out and cause a dead miss and sometimes the electrode will fall into the combustion chamber and destroy the piston & cylinder head! Even the most expensive spark plug is not as costly as a comeback is. Take Care, Britt Sounds scary... How many times the plugs we buy were dropped during shipping and later at store... They do not have any shock protection. The saddest thing in life is wasted talent Link to comment Share on other sites More sharing options...
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