Erratic timing

[scott2093]

all of the computer's tables based on pressure are in kPa so, we don't need to convert to inches of vacuum, we just look at the scanner and it does it for us.

So when the truck is idling I'm guessing there are not supposed to be slight /jumpy 1 to 3 kPa variations. Same with timing advance...is it a matter of the chicken or the egg? Figuring out which is making the others play along in time?

 

[PlayingWithTBI]

So when the truck is idling I'm guessing there are not supposed to be slight /jumpy 1 to 3 kPa variations. Same with timing advance...is it a matter of the chicken or the egg? Figuring out which is making the others play along in time?

That all depends on what the ECM is seeing. If your RPM and/or MAP jump, it'll set SA accordingly to that cell in the Data Tables. So, a stable distributor. coil, and/or MAP feedback are/is essential. On a "built" engine with a stronger cam, you may have to flatten out your tables so, it doesn't surge so much. Here's an example of SA (timing) table. Note the lower RPM and kPa around idle (400 to 800) are flattened out to ~20° timing.

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