I think I may have stumbled on something...

While replacing the IAC, I found the hose that exits the bottom of the throttle body where the IAC sits is T'd off into the mechanical pump vent line and the VST vent line. This doesn't make sense to me. When I rebuilt the motor, I replaced all of the old cracked vent lines with new lines. Those old lines were completely blocked with sludge. I routed all of the new hoses in the same manner that I had found them and really didn't think twice about where things were routed as I assumed it was correct. I have been searching trying to find a diagram of the vacuum and vent hose lines, but I can't seem to find one. So...

Can anyone tell me the correct routing of both the vacuum and vent hose lines??? Is there anything that is supposed to be T'd into the line running from the fuel pressure regulator to the intake plenum? Right now it's just one short line running from regulator to intake plenum.

Specifically, the hose barb directly below the IAC is what is currently T'd off to the mechanical vent (overflow) and VST vent lines... Is that correct?
http://www.mercruiserparts.com/conte...43A12_550.jpeg

Yes that is the correct routing. There is a yellow fuel vent line from the mechanical pump diaphragm dry side that goes to the T-fitting. There is also a yellow vent line from the top of the VST diaphragm dry side that goes to that T fitting. From there it plugs into the throttle body. Under normal conditions those lines should be dry and free of liquid (fuel).

The line from the mechanical pump is to allow fuel from a ruptured mechanical pump diaphragm to flow into the intake manifold rather than your bilge.

The line from the VST is basically the same safety net as the one from the mechanical pump. Allows for an over pressure in the VST to bleed into the intake manifold (or as in the case I mentioned previously when that inner VST diaphragm degrades).

Check to see that your vacuum line from the fuel pressure regulator on the fuel rail is in good shape and connected to the intake manifold. Without that working properly your fuel pressure could be lacking. There will be nothing T'd into that line. Its simply referencing manifold pressure.

Thanks! Will do...

Saga continues!

Got everything back together with the new IAC installed and it idles beautifully. MUCH better than before. I can also chop the throttle back and it settles right in with zero surge or stutter, so thanks for getting that figured out guys!

On to the the new issue... If I bring the throttle up past idle, the fuel pressure starts to come up and down and then plummets to zero and the engine dies. Re-prime the fuel pump with the key, pressure back up to about 38 (and holds). Idles great, but plummets again throttling up past idle.

Could air in the fuel system cause this? I ran a lot of air out via the fuel pressure gauge relief button while continuing to prime the pump. It's solid fuel running through the relief valve with no bubbles...wondering if air trapped somewhere could cause this?

Thoughts?

Went over all fuel line connections and everything was tight. I measured the volatge at the high pressure pump when the pressure drops to zero and the engine dies and it still maintains a solid 12 volts. So the pump is not stopping.

Another interesting detail...when the pressure drops to zero and I throttle up even higher, the fuel pressure will recover, come back up to 35, hold for a second and then dive to zero again. Its like the pump catches up and then it drops again.

Pulling my hair out over this as I was planning to use the boat all week!

Anyone? Ideas?

pump is not losing voltage. doesn't mean it is not stopping. I don't think a regulator will pop open and show 0 psi. but might be a good time to put clear howe on return and watch return flow. if high flow and no pressure, return-regulator issue. if no psi and no return, no fuel.

In case someone stumbles across this thread in the future, the problem was the high pressure pump in the VST tank. I swapped the old pump back in and pressure was back up and holding. Not sure if the new pump was faulty or incompatible. Had the boat out the past two days and it ran excellent. Best it ever has with the new IAC sensor.

Thanks for the help!

Great feedback. Thanks!

Great to see results posted and glad you git it figured out.