I just got a Matco MD1022 scan tool. While I was looking at the live data on my '95 Blazer, I noticed that the spark advance seemed erratic. At idle, it would vary between 5*-25*. (All the numbers were negative, i.e. -5, -15, etc. I'm assuming that's degrees BTDC.) Sometimes it would drop to zero.

I'm used to timing my old, points distributor 350. With the vacuum advance unplugged, it's a rock steady 12* BTDC, and it increases with rpm, up to the max amount. (35-38, I can't remember exactly.)

I have the EDI distributor, with the horizontal spark plug towers. I believe the computer controls the timing. Is it normal for it to vary that much? Maybe there is too much wear inside the distributor? The idle is very stable and smooth.

Sorry if this has been brought up before, I tried a search and didn't come up with anything.

 

Anybody? I guess nobody else has noticed this on their truck?

 

It's normal not erratic. Timing is computer controlled and at idle, it is constantly being modified. It should be much more stable under load.

Comparing it with an old manual/vacuum system is like comparing an Atari to a PS3.

 

Ok, thanks. That's what I was hoping for. I really don't want to mess with the distributor!

I agree with the Atari/PS3 analogy. This obd stuff is a steep learning curve!