340 Engine Shake

plumkrazee70 said

I saw those update instructions, but Ive never had trouble getting the IAC to respond, so I never touched the back screw. Of course I haven't driven it either. lol. do you think the MAP reading will show something off if the seal is squished? Im worried about removing that front cover and checking it, if I don't have to, mainly due to how much is packed in there.

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Yeah, it wasn't fun digging in there. Supposedly, the MAP will read correct, but the computer sees many more updates per second than can be seen on the dashboard, and intermittent leaks at the MAP can cause issues without showing up there. Frustrating. I wouldn't dig in there unless or until you're forced to.

The reason to tweak both front/rear butterflies isn't for IAC, I think it has to do with the fuel injectors being above the butterflies. If the flies are too far closed, the fuel doesn't get past them very well and causes issues with distribution which can cause idle issues. That's a guess, but it makes sense in my pea-brain.

I know if I look down the barrels, the fuel does like to bead on the butterflies sometimes, and it makes me wonder... I do know that it's one of the reasons you don't want to rely on IAC much for idle control - it's basically a vacuum leak to vary things slightly. At idle, it can cause more of a headache than anything. It's most useful in the decel regime where we don't want manifold vac to spike too high when snapping the throttle shut (this causes the fuel to vaporize and 'dry' the manifold, which then causes a leak-spike when the throttle re-opens). That's a whole discussion on it's own, but it has nothing to do with an idle shake.