Rumble Strips

jdw2469 · January 27, 2015

I have a 2008 Silverado 5.3L V8 with approximately 100K miles on it. I have recently noticed that it feels like I'm driving over rumble strips when I come to a stop. After giving hundreds of $$$ to the local mechanic ONLY to tell me it is the Flow Master exhaust....NOT!!! However, it didn't start doing it until I had the duel exhaust installed. AND.... I even replaced the Flow Masters with Magnaflow to be sure! Anyway, I started paying attention to the exact timing of when it does it. What I have found is that it happens just before I come to a stop, when it switches from V4 mode to V8 mode. WHY??? Is there a sensor involved here that I can change out? I'm not interested in disengaging the AFM with an aftermarket programmer or chip, I just want to "fix" the problem that is causing this.

Thanks,

David

Doug_Scott · January 27, 2015

Have to wonder why it would even use the v4 mode at city driving speeds. It cannot possibly make enough of a difference in mileage to not have it engage until 60km/h (36mph for those south of the border).

There have been many people complaining about the exhaust sound when in v4 mode with aftermarket mufflers. Wonder if a small resonator would make it better? You would likely need two, one for each tailpipe though.

Mossyoakglock · January 27, 2015

Unfortunately there is no "fix". It's just the nature of the truck/AFM system. My '08 did the exact same thing before I turned off AFM. I did a "test" (very unscientific) to see if it was indeed the AFM causing it and in my case it was.

Put your DIC on the Instant MPG/V4-V8mode screen. Go into a neighborhood/parking lot, etc and from a stop go about 10-12 mph, let off the gas and start to slow down and stop. Watch the "mode" and make sure it doesn't drop into V4 slowing down (I believe AFM only activates over 18 mph). Then do the same but go to about 20 mph and then slow down and it should kick into V4 and feel the shudder.

The only way to prevent that is to disable AFM with either a Range or tuner. I disabled mine and it's like a whole new truck. MPG didn't suffer at all either and it might have increased a little.