I have a 2001 Seville SLS, with about 82,000 miles on it. A few hundred miles ago, the "Service Engine Soon" light came on with no noticeable change in performance. Then as time went on, I noticed a roughness at idle. Gradually, this became worse. I have noticed that when cold, the engine seems fine. When hot, the idle is extremely rough. When stopped at a traffic light or stop sign, it almost feels as though the transmission is down shifting. I was told that it might be the Mass Air Flow Sensor, so I removed it and tried cleaning it with a dedicated cleaner purchased at the local Auto Parts Store. No change! Also, I have made sure the gas cap is on securely. The following codes have been pulled from the instrument panel:
ABS: C1234 History, C1235 History, C1248 History.
DDM: B3832 Current, U1301 History, U1714 History.
IPC: U1016 History.
IRC: U1016 History.
MSM: B2375 History, U1000 History.
PCM: P0101 History, P0171 History, P0174 History, P0300 History.
TTM: U1016 History.
Does anyone have any suggestions on what I might try before taking it to a shop?
Poppy101

 

Try checking to see if it runs better in park or neutral compared to sitting at a stand still in gear, if so then you have a bad spark plug wire somewhere. Just had that problem on my 98 sls. plug wires show worse under a load, but can run good at idle in neutral or park. Also could be a bad oxygen sensor, normally when the 2 codes 171 and 174 come up together the oxygen sensor is bad. I had my codes read at auto zone and then i replaced the sensor and it helped but then went back to running rough again, so i remembered that one of the codes also read a misfire on cylinder #1 so I replaced that wire and runs like a champ now. Be honest with you I think that 0101 code is a misfire. Have the codes read at autozone and it will tell you exactly what cylinder it is.

 

Thanks Majbarron, I'll be going out a little later and will pay more attention to what happens when in drive or neutral. I have tried that before awhile back and think that there is no difference whether in drive or neutral, but not sure. I'll let you know what happens. I will have someone check out the codes with a more sophisticated device, but thought someone here might be able to help me with the codes off the instrument panel. I'm not sure if the instrument panel does as good a job as a shop - probably not.

 

P0101= Mass Airflow sensor system performance
P0171= Fuel trim system lean-bank 1
P0174= Fuel trim system lean-bank 2
P0300= Misfire detected(generic misfire, no specific cylinder referenced)

I would take care of the P0101 code(mass air flow) first, as a problem with that can set the system lean codes.
The P0300 doesn't really determine which cylinder it detected a misfire in, only that it has seen a slowdown in crank speed at some time, usually caused by a misfire, but could also be set by a malfunctioning crank sensor/wiring.
More diagnostics on the MAF would be prudent, to determine if it is bad, or perhaps just needs a good cleaning. A loose, or damaged air intake hose connection could create a problem with the MAF readings, also.
hth


btw..the "U" codes are communication codes, for example, the U1747 in the DDM(driver door module) indicates a DDM communication error, while the B3832 in the DDM indicates a power window lock-out fault/malfunction.
Also...the on board diagnostics on Cadillacs is usually more complete than most scan-tools, imo. Some scan tools may have communication/software issues that can, and do a lot of times, give erroneous info. Most scammers will not even access the other modules, such as the DDM, or the BCM, or ICP.
It's what you DO with the OBD info that makes the difference!

 

It could be the intake manifold. The front intake plenum may need to be replaced.
Or you could get lucky and it could be only an 02 sensor.
But the fact its going lean on 2 banks Id be more inclined that it may be the intake manifold.

Just had the same problem with my 01

 

I have the same car and have seen many of the codes you have. I keep a log of the codes that have occurred along with the mileage and date.

While they are not related to the rough idle, the DDM codes can be caused by disconnecting the battery and not resetting the windows. After every time the battery has been disconnected you need to reset the power window sensors by opening and closing each power window. This will also set the one touch operation on the windows. Then perform an all code clear. This keeps the DDM codes from showing up.

As for diagnosing the problem with the rough idle. I would first record all of the current codes, then perform the DDM step above including the clear all codes step. Next drive the car twice up to highway speed and turning the car off between each time. Then read all of the codes again. These are the codes that are currently in effect. Many of the codes in your list will go away never to be seen again. At least for a long time.

The long shot is eliminating the confusion in the PCM will fix the rough idle, but the immediate benefit is that you will be able to focus on the current and erratic indicators in the PCM by eliminating those that have occurred long ago.

The next thing I would check is the PVC valve and both the clean and dirty air tubes.

 

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