knock retard only on maf

[draz53]

I'm getting bursts of knock retard but only when the maf is enabled, not in SD. This is in closed loop, not in PE mode, low rpms (1800-2400 area), tps around 30 after ramping up. It is not a lean a/f issue - both ST and LT fuel trims are close to zero and steady when this happens (before, during, after). This is running the 1-bar enhanced OS. Is there anything about SD that makes it less sensitive to knock? (I doubt it.) I'd think if there was a problem with the maf itself, or the maf calibration, that would show up as fuel trims out of whack, which is not the case. Could a really brief lean spike not show up during logging?

It's a 2000 ss m6 btw.

[POWERZONE]

Is your MAF screened or descreened?

[draz53]

Maf is screened, but it's one of those honeycomb screens from saxonpc.com. Anyway if that's the cause wouldn't it affect the fuel trims?

[2xLS1]

What does this table look line in your tune?

Burst Knock Table.JPG

[POWERZONE]

Maf is screened, but it's one of those honeycomb screens from saxonpc.com. Anyway if that's the cause wouldn't it affect the fuel trims?

I only asked because I have seen some cases where cars with stock MAFs that were descreened have false KR sometimes due to the PCM anticipating and reacting to the unlinear airflow. I could literally trigger the KR by playing with the throttle. This does not apply to all cases but just some I personally observed.

But for the best responses from the community, you will need to post a log or your tune.

[draz53]

Interesting - the honeycomb is definitely less obstruction than the stock screen, but should do a good job against turbulence, the cells are much longer than the stock one. But maybe it's just different enough to confuse the PCM.

2xLS1: my burst knock retard table is exactly the same as that one, I've never touched it.

Here's a scan. The interesting KR events are at frames 12297 and 13964.

scan23e-1.hpl

[draz53]

Well for the record it now looks like it's not the maf after all, since it's started having KR in SD mode. I guess I'll have to put back the exact SD tune/OS that had 0 KR, which is before the VE table was tuned, and see if still has 0 KR. Another variable is that the KR only showed up after I started using the 1-bar SD enhanced OS.

[tramotorsports]

Are you tuning with a wideband? It's possible that the fuel trims are not picking up the lean spike fast enough and giving a false reading.

[johnm545]

They're both at low rpm under a fair bit of load, probably real knock but the knock retard is more than needed and takes too long to decay. You could try doubling the KR decay rate and halving the attack rate. I'd remove a couple of degrees of timing in that area too.

Fueling looks fine.

[rpturbo]

They're both at low rpm under a fair bit of load, probably real knock but the knock retard is more than needed and takes too long to decay. You could try doubling the KR decay rate and halving the attack rate. I'd remove a couple of degrees of timing in that area too.

Fueling looks fine.

Can you explain to me how you can say any of that, when there isn't a tune posted (at least that I see)? Can you tell all of that just from a scan? I am really asking about the KR decay rate and attack rate. Thanks