: 02 Deville rough idle & codes

04-03-09, 08:11 PM
Hello to all members,
I'm a new member and I have been following this forum for a while. I have a 02 base Deville with 140k miles and a rough idle issue. The car was purchased used about 2 years ago and it has been fine up to now. The problems started when I went on a overnight trip and on the way home I stopped and noticed the car had a rough idle, usually it was very smooth. When I got back into town I went to Auto Zone and they told me it was a bad MAF sensor and I purchased one and replaced it the next day. The car ran fine for about two weeks and then the rough idle came back. Auto Zone rechecked and found no codes. A few days later the service engine light came on and they pulled a code out for the bank two O2 sensor and I purchased and replaced. Car was fine for a day or two and rough idle returned. After learning here in the forum that I could pull up the codes myself I now have these codes


I have changed fuel filter, air filter, cleaned throttle body, cleaned EGR, checked the FPR for defective operation, disconnected battery & cleaned both posts and the battery ground to the floor pan. I also have checked vacuum lines for leaks and the fresh air tube for leaks between MAF and throttle body and found it to be okay. Both crank sensors were replaced a month after purchase. Tomorrow I have plans to change spark plugs. I had the dealership pull up and print the service records for the car and didn't see where the plugs were ever changed, so I don't know if I'll find the original plugs or not. I also used a bottle of STP injector cleaner in the tank on the last fill up, no change. The car doesn't stall just rough idle. Thanks for help in advance, sorry for the long explanation, but I wanted to be thorough.

04-03-09, 09:01 PM
Forget Autozone. See the sticky at the top of the page and pull your own codes. Then do a search for "Plenum".

04-03-09, 10:31 PM
Thanks for the info Ranger, I have found the codes and searched for plenum. I read on one of them to use brakeclean around the plenum to check for leaks. I don't have brakeclean but I do have a throttlebody cleaner. Would that be wise/ok to use instead of?


04-03-09, 10:51 PM
That'll work just as well.

04-03-09, 11:10 PM
Thanks again Ranger, I'll try it tomorrow and try to get back here and let you know what I find.


King Steve
04-04-09, 01:45 AM
I suggest coils, as it sounds like a similar problem I had before and coils were the answer.

04-04-09, 09:39 AM
Ranger, This morning I sprayed the TB cleaner around/under the plenum and I noticed a change in the RPM's. I also sprayed in the area behind the PS pump and didn't notice any change in RPM. I searched the forum for the numerous posts on the plenum and there are many. My local dealer is closed weekends now with the slow economy and I won't be able to get the new plenum until the early part of the week, depending if they have it in stock or not. I may be able to get the intake gaskets and the injector o-rings from the local parts store.

04-04-09, 09:41 AM
King Steve, thanks for the info, I'll certainly keep that in mind during this R/R.

04-04-09, 10:57 AM
Glad you got it figured out. I'm surprised that my plenum hasn't gone yet. I think it's just waiting for the warranty to run out next month.

04-04-09, 01:07 PM
I'm working on the back bank spark plugs, not too bad of a job. The plugs that I removed are Denso PT16EPR-C13. Even though the car ran well before the plenum, the plugs were definitely due for a change out. Factor Originals???

I haven't made up my mind whether or not to do the plenum replace or have the repair shop do it. May get an estimate then decide.

04-04-09, 01:56 PM
The plugs you want are ACDelco #41-987 Platinum professional. Probably made by DENSO, the OEM supplier. 13 ft/lb of torque, cold, no anti-seize. Check the boots for carbon tracks. If there was a tad of oil on the plug insulators, no sweat - the wells are sealed by O-rings and tend to weep over time.

The coil cassettes are around $185 apiece at www.rockauto.com. They only have a few original DELPHI units left with ICM (modules). Very reliable part, so probably OK.

I have had good luck with www.gmpartsdirect.com, Flow Chevrolet-Cadillac in Winston-Salem, NC; www.gmotors.com, Palm Chevrolet-Cadillac in Gainesville, FL; Luke in Parts at Lindsay Cadillac, -----------> over there.

04-07-09, 05:51 PM
I finished replacing the plugs on this past Saturday. The car seemed to be running very smooth over the weekend and Monday and the SES light actually went off also. Today I took the car in to the mechanic for the estimate on the plenum replace and twice before I arrived, the engine idle rpm's surged up to 2100 while in park. Is this a problem that is consistent with the plenum issue or is there something else happening?

04-07-09, 06:07 PM
A plenum crack will not normally cause an rpm surge as you describe. Rather, it causes a nagging low speed miss and a raggy idle.

IF you intend to have your shop do the plenum, make sure they check all the various vacuum attachments to/from the throttlebody and cam covers. That intake manifold only takes 8 ft/lb of torque on the bolts.

Sometimes an unexplained rpm surge is either an IAC (Idle Air Control) valve or a buildup of oily deposits on the engine side of the throttle butterfly. Neither is rocket science to change or properly clean. Many posts on the processes.......search.

04-07-09, 06:23 PM
Thanks submariner409 for the info. I just hung up from talking to the mechanic. He asked me if anything has been done recently to the engine. I went over with him what I've stated here and he's going to clear all codes and test drive the car to see if the codes reappear. He said the car seems to be running fine without any rough idle. He's going to keep it overnight and check it in the morning again. I'll do some more searching here on what you posted. Thanks!!

04-08-09, 08:07 AM
Glad you got it figured out. I'm surprised that my plenum hasn't gone yet. I think it's just waiting for the warranty to run out next month.Ranger I sprayed the throttle body and reving up the car pulling cleaner into plenum which probably cause the cleaner to sit in the pelnum grooves over time eating away the rubber, I didnt take it apart which i think alot of people are doing who wine up having this problem, minds crack some time after!!!:rant2:

04-08-09, 11:18 AM
You have to spray it in there when you clean the TB. Some may get in the plenum and sit in the low spot, but that stuff evaporates pretty fast and you would think that they'd use a rubber that was impervious to that type of stuff. What causes them to fail so much is beyond me.

04-08-09, 07:42 PM
Just an update for everyone. I left my car overnight with the mechanic and this morning he replaced the plenum as suggested in the forum and the original plenum did have a crack in the rubber. He charged me $154 for the labor and nearly $70 in parts. Thanks to all here in this thread and the many other threads that suggested to replace the plenum. You guys are awsome! All the other work that I did prior to the mechanic replacing the plenum would have been needed anyways and the money and time was well spent. Changing the plugs wasn't as bad as I thought it would be, I did the rear bank first thinking it would be the most difficult, which it was. Thanks again!

02-02-10, 05:32 PM
I own a 1990 Deville 4.5 and 1994 Deville 4.9, they both developed a rough idle, miss and poor gas mileage to say the least. I have the GM manuals for both cars that I got cheap from ebay, they are priceless to own. I'm not a fan of throwing parts at a car, esspecially a cadillac. Pulling the codes sure does help and both had bad TP sensors. After replacing them, they still ran rough and bogged down under load. The computer system is great, but doesn't tell you everything. I thought I'd run the test for the fuel injectors, since the pressure seemed well within range. Guess what, four of the injectors were bad on each car. After replacing the injectors .. no more miss, bogging down and restored the gas mileage! It's not a simple job, but an easy test to see if it's the problem.