Mechanics

ROUGH IDLE

2004 Cadillac SLS • V8 FWD Automatic • 132,000 miles

Recently changed spark plugs due to cylinder 4 misfire. For about a week it idled and accelerated fine. The last two days now the engine shakes a little during idle and hesitates for a moment when I accelerate. After that acceleration is fine. I also notice that when at a stoplight if I touch the pedal enough to keep the rpm around 700 the shaking stops but still a little stuttering or hesitation when I first accelerate. No codes for drivetrain showing at this time. Idle is now at 500-600 rpm.
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WDWOOLEY
February 20, 2011.




. Check for vacuum leak at the boot between the throttle body and the upper intake manifold plenum, take carb cleaner and spray in this area listening for change in rpm. Have seen this problem in the shop on a number of these but usaully have codes for misfire, lean condition. Give this a try and let me know.


PROTECH1980
Feb 20, 2011.
Sprayed carb cleaner as recommended. Sprayed entire intake area including entire boot. No changes in rpm at all. Disconnected some of the sensors one by one with immediate changes in idle. Where is the EGR valve located? Also what is the sensor or valve bolted over the top of the valve cover near the fire wall. I had to remove and replace it when I changed the spark plugs.
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Tiny
WDWOOLEY
Feb 20, 2011.
Lets start here how did you know you had a misfire on cylinder #4 a week ago? Do you have access to a scan tool that shows misfire data?


PROTECH1980
Feb 20, 2011.
Took it to Autozone. They showed the misfire. When I changed plugs #4 plug was bad. Electrode was almost gone. There were no other codes relating to drivetrain. Will check again to see if any new codes show up. Still need to know what that valve is bolted over the top of back valve cover. Removed it to access coil pack. Disconnected with engine running. Once with engine cold and once when engine was hot. No change in rpms or performance either time. Could that be the EGR valve?


Tiny
WDWOOLEY
Feb 21, 2011.
I am sending a pic of the EGR valve which you would not have moved for plug replacement. That is probably a secondary air system valve. I was expecting you to find a bad intake boot. When they crack it is on the bottom. I am not talking about the boot that goes from the air filter to the throttle body, it is the boot in between the throttle body and plastic intake manifold, just to make sure we are on the page.


PROTECH1980
Feb 21, 2011.
What condition are you plug wires in, are they the original?


PROTECH1980
Feb 21, 2011.
I sprayed carb cleaner in that area as well. Will do again at the bottom to be sure. Found the EGR and unplugged it then drove the car for a while from cold to hot with no changes in engine performance. The vehicle has coil packs and they are the original ones. Checked the codes and they are all history. No new codes appeared from driving with the EGR diconnected. The current DTC codes showing all say history and they are, U1016, P1107, B1328, U1255. There is also C1226 AND C1233 which I believe is for the speed sensor on one of the wheels.


Tiny
WDWOOLEY
Feb 21, 2011.
Sprayed carb cleaner again. Sprayed entire intake system, even around injectors and entire boot areas, vaccum lines and plenum. No change in idle speed at all. I may remove entire throttle body and boots to clean every thing and to check the condition of the boots and all the sensors including the MAP, IAC, MAF, and the TPS. Will also remove and clean the EGR. I doubt if it is one of the coils or boots over the spark plugs since this only happens during idle plus the fact that idle was fine for a week before the problem started.


Tiny
WDWOOLEY
Feb 21, 2011.
Let me know if anything changes, The codes dont concern me except the p1107 which is a map sensor code, but stated earlier may have unplugged it, plugged back in. And make sure to clean that MAF good, and it might not be a bad idea to run some fuel injector cleaner thru the system as well. Let me know if you need anything.


PROTECH1980
Feb 21, 2011.
Removed both coil packs again and cleaned the connectors inside the boots. Put a little dielectric grease inside each boot again. Car is running fine now. Must have been weak sparks from bad connection. Have another problem thats been intermittent for a few weeks. DTC code C1233. RF wheel speed sensor open or shorted. Hub was replaced 10 months ago. Unplugged sensor and cleaned all terminals. The wiring looks clean with no corosion. Or breaks found. Could the hub be defective or moisture getting inside? Could someone tell me where the harness is for the module that all of speed sensors connect to. Would like to check continuity to see if circuit is complete. I have a diagram that shows what pins on the harness are for each wheel sensor but I don't know where the module is. I was told it may be under the dash near the steering column but they were not sure.


Tiny
WDWOOLEY
Feb 26, 2011.

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