Just traded out of a 2004 z71 suburban. Great truck. Now into a 2007 z71 suburban. Flex fuel 5.3. After interstate run 80 mph 30 min at first stoplight the engine vibrates. Take out of gear and the engine smoothes out no check engine light. I asked the dealer to correct the problem. They say this is normal. Due to the v4 v8 mode. They say they checked the usual culperates like vacuum leaks and found nothing outside specifications. The exterior of the exhaust pipe has a black crust. The tach wobbles between 500 and 550 no other issues present except a stale odor from the evaporator when first running the ac. I find it hard to believe the 5.3 would not idle smoothly it was perfect in the 2004. I ran techron and high test through the engine made no difference sure would appreciate help resolving the issue. Any help greatly appreciated. They dealer also said the computer was up to date. Can a black bear tune solve the problem.?

 

Don't just throw parts at it - that's a hit and miss.
If there's a black crust, it could be a little excess carbon. My first bet would be you're running a little rich. You might see a small drop in fuel economy. Maybe do a tune-up. 75k miles isn't too far off for OEM parts, so it's worth a shot. And then at least you've done a tune up that was probly due.

 

Make sure your plug wires are seated good.

Since the engine idles in V8 mode, not sure how AFM could be the cause.

 

Mine with 82k on it was doing it a little and when it was in recently they said there was some carbon build up on the intake, wanted $150 for a Fuel system cleaning. Went and got a can of STP Fuel system cleaner for $5 and she is much better. No advantage to run premium in these motors, E85 is another story. Will probably drop another can in the tank in another 30days just to make sure everything gets good and clean...

 

Techron is also a good product, I have been using it since I purchased my 07, about a bottle every 7-10 fill ups... I have been wanting to try the BG44k product but its not as readily available at my preferred auto parts supply...