AFM Related Issues

SLong · March 20, 2013

Guys, I've been doing a little reading on AFM and would like your opinions on a couple of things. My truck is a 2011 Z71 with 67K miles. Haven't had any problems. I've always noticed a slight shudder when coasting at a slow speed (15-20 mph), but when I hit the gas it stops. It's more of a vibration in the dash. If I drive in M5, locking out the AFM, it doesn't do it when I coast at the same speed, which tells me this is AFM related when the truck goes down to V4 mode at that speed. It's a little annoying, but I'm mostly concerned to know if it is doing any damage.

Next question, I noticed a soot type discoloration on the chrome of my bumper right above my tailpipe. Do you guys think this is related to the oil consumption issue that I have read about with AFM motors. I've never noticed that my truck burned oil and have never had my low oil light come on. I change my oil myself and have run the Mobil 1 Full Synthetic that is dexos approved since day one. I'm thinking I should bring the truck to the dealer and drive it with the diagnostic hooked up to the truck to see what the slow speed shudder is, and to also have thenm run a consumption test on it? Would like to address any issues before my warranty is up. Thanks in advance for any feedback.

music · March 20, 2013

Mine is a 4-spd auto, so I don't have the stutter... but yes, I'm going to guess that the truck still in 6th w/ the TC locked, and AFM activates... which is probably too low of rpm for that condition. That's a guess.

Re: the "soot"... that's normal. The tailpipe location is such that exhaust condenses on the cold bumper. It doesn't help that the engine runs rich on startup until "warm", which produces extra carbon.

Gurrzt · March 20, 2013

"Soot" in the tailpipe USUALLY indicates that your A/F ratio is "to rich", that is to say there is not enough air induction with the ratio of the fuel, usually it is a MAF calibration setting in the PCM.

SLong · March 20, 2013

Thanks, I haven't hand washed my truck since before hunting season, so the soot build up on the bumper is probably from as far back as September. I doubt the drive through wash gets the bumper soot off. I'm going to bring it in to the dealer to get it checked out.

mferry22 · March 20, 2013

Have you noticed if your oil is low. My truck was almost 3 quarts low and my oil light did not come on. I ended up getting all my pistons and rings replaced.

GMCustomerService · March 20, 2013

SLong,

Keep us posted on your progress with the dealer. Let me know if you need anything in the meantime. Thank you.

Tricia, GM Customer Service.

SLong · March 20, 2013

MFerry22, I don't think my oil has been low. I checked it a couple of weeks back, and it was just below the center mark on the dipstick, but I think that was not long after coming home from somewhere. I don't think the truck sat long enough for the oil to settle back in the pan. When I do an oil change and check the level after putting the 6 quarts in, it's right at the top mark on the dipstick. I actually changed my oil yesterday evening, and will check it 1000 miles from now to see if I lost any.

Tricia, thanks and I'll let you know what the dealer has to say.

6Sixathome · March 20, 2013

Have you noticed if your oil is low. My truck was almost 3 quarts low and my oil light did not come on. I ended up getting all my pistons and rings replaced.

Your 6.2 used oil or a previous truck that you had?