timing drop out at regular interval for no reason?

[rattle]

I cant take it anymore, i have seen this happen on many engines, mostly trucks with ls1, basically the timing at a seemingly regular interval drops 20 or so degrees for no apparent reason, see the graph in the attached gif, anyone know what causes this? this particular scan is from a 2000 4.8L sierra

[Russ K]

Can't help you, I never had the spark timing spike down like that before. Try your stock tune to see if it is a fault of the tune or not.

Russ Kemp

[rattle]

stock tune does it as well, first time i saw this was on a 6.0 litre with my old serial cable, i just thought it was glitching, now seeing it with usb cable, going to verify tommorow with tech2 just to make sure it is not hardware issue.

[Russ K]

Does the timing dip at wot as well? Any DTC's

Russ Kep

[jimmyblue]

Log the IAT and ECT sensors; only time I've seen unstable
idle timing like that, was a truck with a flaky ECT sender
connector. Every time it opened up intermittently the ECT
would jump. Every idle miss could kick it "just enough".

You might want to log the various advance / retard
"subcomponents" (IAT advance, ECT advance, AFR advance,
...) and see if any of them carry the perturbation. The
rhythmic nature of it is peculiar for sure.

[tahoe]

Oh yes, I know this one.....

It is the AC compressor cycling on and off.
There is a function somwhere in the PCM that reduce advance when the load is changing, i.e. compressor ON and OFF.

Press the AC button and at the same time direct the airflow through the facia vents, possibly also to the floor. There is an auto AC ON function to prevent you from misting up the windows. Do an RFM to find exactly what I am talking about.

When you have managed to switch the AC Off (incl auto ON function), the timing dips will no longer be there.

Br//

[Russ K]

Under the engine torque tab, find the AC torque spark table and 0 out that table. Don't know why GM wants the timing to retard every time the AC turns on and off.

Russ Kemp

[rattle]

That did it, turned a/c off and no more timing dips,

now to modify the a/c control head to not turn a/c on with defrost (gm did this to remove moisture but it usually just results in a hvac housing full of moisture to frost your window on the next start up)