RepairPal is your advocate for car care advice and guidance

Q: Slight rough idle. RPMs fluctuate 150± sometimes. Surges at highway speeds. on 2006 Buick Terraza

Vehicle has 121,000 miles on it. Local Oil change service company said CAM timing sensors were bad. No codes on then or now. They just thought that would be the fault. Took it to Buick dealer and replaced plugs and wires and ran BG fuel treatment through sytem. Did not replace CAM sensors. Surge going down interstate is only slight, like moving gas pedal back and forth. Rough idle is very slight at stops but can vary in degree. Dealer drove vehicle with computer attached and found nothing. They said one time while idling in shop, engine stumbled slightly but not enough for computer to register it as an engine misfire. Technician thought it may be slight injector misspray on cylinder 5. Put full tank of premium gas and some Sea Foam in (1 oz per gallon). Been running this for a week now and no improvement. I suspect a bad Oxygen sensor or possibly fuel injector. I haven't replaced fuel filter in a while but it doesn't seem to be a fuel supply problem. Not sure if Dealer checked fuel pressure. If there is a vacuum leak I haven't found one unless it is very small. Sitting at idle, the RPMs fluctuate slightly maybe 100 to 200 RPMs on the tachometer.
Get a Repair Estimate
Guaranteed by certified locations nationwide. Learn more
RepairPal estimates are guaranteed at over 1,700 quality certified locations nationwide. Learn more
This could be so many things. How are your mode 6 readings? This is the scientific data for the OBD-II system, not just scan data. See if something is borderline, like fuel trim, misfire counts, EGR ( if equipped ) and yes air/vacuum leaks. How are your coils? And how old is the mass air flow sensor? Again, check your fuel long term fuel trim, It needed to be no more than 4-5% on either bank.
Thanks for the quick response. As soon as I get more data, I'll post it. Without a definite code, the Service tech can't seem to give me a definite answer. All of the sensors, coils, etc. are original and have never been replaced.

Updated 3-1-2012: Let Buick dealer keep vehicle for 3 days. They tried everything they could think of. They swapped out catalytic converter, MAP sensor, MAF sensor. Checked fuel trim and fuel pressure. No vacuum leaks found. The dealer called GM and they said sounds like a problem with the torque converter. I don't see how the torque converter could cause the rough idle. Maybe the rough idle is causing something with the torque converter but I don't think so. Drove the vehicle home. Now will have to see if Buick will give me some kind of direction. If not, I'll trade it in and get what I can for it.

Updated 6-21-2012: Terraza finally set a code a couple of weeks ago. I checked it with my code reader and it was P0011. This indicates a problem with over advanced timing. I took it to my Buick dealer and they repaired it based on the procedures for this code. I got the vehicle back and have been driving it for over a week now and the problems are gone. No more surging or rough idle. Not sure why it took three months for the vehicle to finally set a code but it does seem to have been my problem.
Didn't find what you were looking for?