Electronic spark control problems

[cyclone85]

I did a timing check on my 85 silverado (LE-9 305) the other day and found some weird problems. First of all, with the vacuum advance hooked up I am getting over 20* of advance at idle, and when I rev the engine up it does not advance any, it just stays at 20*. If I unhook my vacuum advance, plug the line, and then set the timing at the recommended spec (4*BTDC), I have to retard it a bunch as soon as I hook the vacuum advance back up in order to get it to idle smoothly. In fact, the timing is set so far back that with the vacuum advance unhooked it will barely idle. I have read the article on disabling the ESC and I wonder, is that what I should do? Will doing so kill all advance functions on my distributor and turn it into a dog?

Also, is the tip in vacuum switch really needed? Can I just run a jumper across the wires at it to prevent it from retarding my timing whenever I accelerate?

[chev74]

are you plugging your vacuum advance after you unhook it? if im correct, i believe youre supposed to do that... correct me if i am wrong though

[cyclone85]

Yes I plugged the vacuum line, read my post again.

I also forgot to add this truck has a big spark knock problem at cruising speed. It only occurs during light throttle (such as climbing a slight hill), if you push it on down it goes away. It does not spark knock any other time, only during cruise.

[greg0380]

i have an 85 that used to have the 4.3l V6 with the ESC and I had a similar problem with the ESC module and a light cruise pinging (knock). I bypassed the ESC module (there is a good write up by swervin ervin) but then you will need to get an adjustable vacuum advance canister. The stock canisters have a lot of advance and I think the spring tension wears after so many years. Under light throttle (high vacuum) the vac canister is adding alot of advance, which caused the pinging. as soon as you give it more throttle the vacuum signal drops and the distributor is not advance by the vacuum canister and the pinging stops. As soon as I eased off the throttle it started to ping again because the vacuum got stronger and the vac canister was advancing the distributor too much.

I used a Crane adjustable vac canister. It wasn't hard to dial in and it stopped the problem.

[86-C10-V6]

Sounds like you may have multiple problems.
You need to examine the weights and springs that perform the mechanical advance. The easiest thing to do is to remove the distributor and replace the weights(lubricate well) and springs and the vacuum advance.
If you want maximum flexibility, use one of the crane adjustable vacuum advances, but just replacing the old one with a new one may fix part of your problem.
You can buy weight kits with springs relatively cheap.

The ESC generally does not advance the timing, it only retards it.
This is accomplished with the tip in vacuum switch and the spark knock sensor on the engine block.

If you are getting to much advance, it is not from the ESC doing too much, it is from the ESC not working correctly and bypassing it will not fix your issues.

The ESC should not impacting your timing at idle.
Once you get your mechanical and vacuum advance fixed, you should be able to check the ESC operation by using your timing light and tapping on the exhaust manifold with a light ball pen hammer. This emulates spark knock and you should be able to observe the timing retard when you do this.
Hope this helps.