Initial Setup of TBI, TPI and MPFI systems. Min Air Adjust, TPS set and Fuel Pressure

[Mastiff]

I tried this procedure and when the A/B pins were shorted the IAC made noise and never stopped. Just kept sort of buzzing. Well, I didn't wait forever, but 30 seconds or so. Should it take a long time to bottom out, or is something wrong with my IAC?

Also, you specifically said to have almost no pressure on the return line. Why is that important? I've never checked it, but I am using a section of my original 1/4" steel return line, which may be on the thin side.

[EagleMark]

Yes when it's buzzing it is completly closed and ready to pull the connector.

Any pressure on return line directly increases pressure on feed line. Remember when you pinched your return line?

[Mastiff]

Any pressure on return line directly increases pressure on feed line. Remember when you pinched your return line?

Yeah, I understand that, but if there were, say, 5 PSI due to backpressure from the return line, it doesn't seem like it would make any difference. If I'm keeping 11 PSI at idle, when the least fuel is being used at the injectors, I think I'd be okay everywhere else. In other words, if I don't go over pressure at idle I shouldn't have trouble off-idle when more fuel is being used.

But if there's more to it I'd like to know. It wouldn't be too difficult to used 3/8" line all the way back if I really needed to.

[EagleMark]

I don't think that is your issue. If it were you would be over 11-13 PSI. Read other thread.

[RobertISaar]

IAC is definitely 12 volt, if you have a scope, you can confirm this, however GM schematics imply it as well as Ludis's document all imply that the IAC is driven directly off of system voltage.

[EagleMark]

IAC is definitely 12 volt, if you have a scope, you can confirm this, however GM schematics imply it as well as Ludis's document all imply that the IAC is driven directly off of system voltage.

Directly off ECM system voltage?

[RobertISaar]

yeah, the same voltage the ECM gets from the ignition circuit is what gets fed to the IAC.