Question: I own a 2010 Chevy 1500 Silverado with a flex-fuel 5.3-liter Vortec V-8. The motor has an Active Fuel Management system that deactivates from V-8 to V-4 under certain conditions. This is my second vehicle with this system. The first was a 2008 GMC Yukon. The system in this vehicle was seamless; if not for the Driver Information System, I wouldn't have known what cylinder mode the vehicle was in. The Silverado operated without issue until about 19,800 miles. At this time the vehicle developed a serious vibration, and fuel mileage dropped from a 20.5 average to 16 mpg. The Driver Information System showed vibration occurs only in V-4 mode. Also in V-4 mode there's a noticeable speed reduction and drop in rpm, then within seconds the engine reverts back to V-8 mode. In V-8 mode, there are no issues. This event occurs continuously; even with the cruise control on at 59 mph the engine bounces back and forth from V-8 to V-4 every 30 seconds or so. Over 70 mph, the engine remains in V-8 mode and runs fine. At low speed, the engine constantly shifts between modes. I've brought the vehicle twice to the purchasing dealer and it was returned stating "Cannot duplicate customer complaint" on the repair order.

Two trips to a second dealer resulted in the same thing. Finally, a call to GM resulted in a service manger riding with me who confirmed my complaint, but was unable to diagnose the problem. He claims the engine is showing no fault codes, and GM does not have any service bulletins on this problem. He went so far as to suggest the problem is with the Volant cold-air box, which is installed on the vehicle. This was installed during the first oil change at the Chevy dealer at 5000 miles. Keep in mind this is the same brand of cold-air box that was on my Yukon and it operated trouble-free for two years.

Answer: Active Fuel Management may sometimes be referred to as seamless, but that's often not the case. Remember, the Powertrain Control Module is shutting down cylinders in a calibrated sequence to improve fuel economy. How noticeable it is varies from drivetrain to drivetrain, but often you can detect the cylinder deactivation if you're looking for it -- although it may be as slight as hitting a small bump in the road.

It seems the service department you're working with missed one very relevant TSB: #06-06-05-001D. GM explains the notable, but normal, change in exhaust tone and slight vibration experienced in four-cylinder modes. As far as an aftermarket air box exacerbating this characteristic, stranger things have happened. The only way to determine for sure is to reinstall the factory setup. After that, see if they can put you in the same truck and do comparative road tests. If your engine's cylinder deactivation seems significantly harsher, they'll have to take diagnosis further to determine the cause. A hardcore misfire would've set a DTC and turned on the check engine light, but it's possible one of the cylinders that isn't being shut down is a little weak. This could cause roughness when only firing the remaining active cylinders. Also, be sure the technicians working on the truck have performed a "snapshot" and submitted the data to GM tech support. A snapshot involves driving the truck with a GM scan tool attached. While in snapshot mode, the technician will hit the trigger button when the condition is felt. The scan tool will then record all PCM data for a set period of time before and after the trigger point. This is an important procedure that allows engineers to analyze the problem effectively, off-scene.


If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at trucktrend@sourceinterlink.com. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.

Can't wait for help with a problem you're having with your Truck or SUV? Ask the expert we trust here at Truck Trend Garage -- visit Alex Steele at www.RealWorldAutomotive.com.