So I was datalogging a 2011 GMC truck today with a 6.2 and VVT. The truck owner had filled the tank with premium (90 octane). At WOT, KR was pulling 10 degrees or more of timing, leaving me with 3-5 degrees of ignition timing. This is on the stock tune.

The only thing I can think of is bad gas. Any other ideas? I'm not going to try to tune this truck until I figure this out. The truck owner is going to run this tank empty, then refill from a different station.

 

What was the AFR? My truck was way lean from the factory at WOT and it would pull timing too, not that much though.

 

The truck has 5000 miles on it(roughly). No wideband, fuel trims were -3-5%, so they are actually pretty close. Alot closer than most I see. The narrowbands were reading 910-920mv at WOT.

 

Isnt 910-920 a little on the lean side?

 

900's would be a lil rich actually, It's got to be bad gas. The only thing I have seen similar is a GTO I just did. It was an auto, with long tubes, catless, stock engine. I had to reduce timing from the stock high octane table, about 3* in some spots. It ended up with about 22-23 at wot to eliminate the KR. It still ran great, but timing was a little lower than what I usually get out of those cars. Also the tables may be setup to pull ALOT of timing with a little knock. So a reduction of 2-3* may eliminate all the KR. As you said though, I'd make sure he's got some good pusholine and go from there.

 

Bump for any other ideas. Anybody run into this before?

 

you cant really pinpoint afr based on narrowband readings.

Mine is low 12s on the WB at 870 mV

Also, eventhough 90 is premium, isn't that a tad low on the octane rating for a high comp 6.2? The KR sounds excessive but some of it may be real.

 

I bet its the gas. With the weather lately I could only imagine how long 90 octave has sat around.

 

90 octane is the highest available here. I'm using it in my truck, 12 psi boost, no KR. This truck is the only one I've seen have KR problems like this, so hopefully it's just stale gas. We will see.

 

put some octane boost in that tank.