Heavy stutter / knock at shift point but light throttle and engine load ...

[Bill Hooper]

Hi,

2009 CTSV, 2.45 upper pulley, ported snout, injectors, 90mm TB, cai, very mild LT1Z 1.0 cam. Engine starts fine, idles fine, WOT fine. Issue is that at normal acceleration from a stop, and / or at a shift point, the engine stutter and sputter for 1-2 seconds like small and sometimes larger backfires . The log shows boost will jump sometime up to 8 or more pounds, but the throttle is steady and low (like a backfire is causing the intake pressure rather than throttle / supercharger). I have not been able to figure this out.

Normally at WOT I get little to no knock, may a degree or two but not much. When it spits and sputters just off idle or at the first or second shift, there usually isn't a lot of knock associated but the first event I captured at 13.00.19 was significant.

Any ideas?

See attached log ... great example is at time marker 13.00.19, and another at marker 13.01.46.

[GHuggins]

Your at 9psi and still at stoic - might be some of the trouble - post the tune too...

[Bill Hooper]

That is my point. That boost spike shouldn't be there at all. Look at the rpm and throttle position. They are low. If I have any boost at 35% throttle at all, it is low like 1-4 lbs, again if any. That boost spike is cause because of the sputter / backfire through the intake and pressurizing the intake. It is exactly that I'm trying to resolve. I get it right off idle just as I press the gas, and again many times at the first shift point, sometimes the second shift point. It only happens right off idle, or at the shift point. That is it.

Bill

Your at 9psi and still at stoic - might be some of the trouble - post the tune too...

[Cruze]

Your tune would help. Do you have a IAT or ECT sensor fault? Fuel pressure is at Min pressure. Not saying that that is the cause of the pressure spike but maybe you have another faulty sensor issue going on as well.

[nofilter83]

Had the same issue. Is ur ve tables been touched.

[kingtal0n]

So the MAP goes from -30" Hg (-15psi) to 0psi right? Before any boost is applied you are at 0psi.

Your normal driving is 15" of vacuum, to 10" Hg right? Cruise 15 to 10. Idle 15 to 20. Its up there.

Those regions of normal driving is fine to have 15:1 a/f or 14.5:1 is fine.

Once you pass, say, 8" of Hg, headed to 0psi, you shouldnt be at 15 anymore. It should be 14 flat or so.

And as you near 3" getting closer to 0psi, it should be more like 13.8 or 13.5 or even 13 flat is good.

Then as you cross boost threshold 0psi to 3psi, shoot for 12.5:1 or so a/f
finally 5psi to 8psi is around 12:1 or 11.8:1
and 10-15psi can be 11.5 to 10.5 depends on the engine/temp of thing.

Point here is, without looking at ur maps etc... it sounds like you are tipping into boost lean at low throttle positions, and perhaps too much timing. Pop out the intake tract? Is a backfire through an intake valve, which means the intake valve was open. Sounds like a really hot combustion chamber, poorly phased injector, volatile gasoline, too hot of spark plug, all combined or one or the other and its getting lit before the plug even fires.

The fix would be to enable PE sooner, get that a/f drop down sooner before it can touch 0psi of boost.

If you already have a good low cool a/f ratio and it still happens, I would say get a colder plug, lower coolant temp, test for the issue with a cold engine. If you still get it with a cold engine, it lends a whole new diagnostic tree of possibility.
random ideas;
- Could be a coil firing early, "have seen this happen when DWELL is set too high or Cross-talk from another wire" -unlikely
- transmission torque management / rev limiters interfering. I had an engine sort of hang before a shift and it turned out to be the limiter was cutting fuel just before the shift.

[lukearmstrong1990]

fast torque exit, dynamic rpm values in SD, desoot enable switch, PE and BE values can all play a part. post the file