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

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Originally Posted by EagleMark

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

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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.

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

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.

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Originally Posted by 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.

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Directly off ECM system voltage?

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

Weird how the GM test procedure warns not to use 12 volt?

I need to build another test bench to have one running all the time, becuase this one is down half the time while building systems...

If the IAC motor is a stepper motor (and I think it is), GM probably doesn't want techs to simply apply straight 12V to the coils and then think it's bad because nothing happens. A stepper motor has to be fed a specific type of signal in order to make it move.

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Originally Posted by FSJ Guy

If the IAC motor is a stepper motor (and I think it is), GM probably doesn't want techs to simply apply straight 12V to the coils and then think it's bad because nothing happens. A stepper motor has to be fed a specific type of signal in order to make it move.

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pulse it at an 80Hz frequency with 12 volts and it will move as it is supposed to. i imagine the average tech (at least back then) wouldn't know what to think of a little IAC motor that moves an almost invisible fraction of an inch when power is applied, then when they remove power and reapply, the same tiny movement.

I don't know what I did the first time I had one out of TBI unit on my test bench but when I turned system on... or jumped A and B on ALDL it came flying out! There was no tiny movement! :yikes:

that would be the IAC reset function of the ECM. it pulses the IAC at either 80 or 160 Hz (or at least it does on later systems) to fully extend/retract/whatever.

obviously, it's not a single pulse that does that. :laugh: