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Yes good stuff - I think I'm getting it

...So practically speaking - a case problem...

I have not yet put in a new lower butterfly plenum on my DOHC. I have it disconnected.

When I am on a flat stretch of freeway doing about 70-75 on a windless day, (slower speed in headwind) my 4wd lights will start flashing. If I let off the gas and then step on it again to maintain my original speed - the lights will go off. Only to come on again over a short period of time or if I put a slight bit more load on the engine.

My theory is I have a vaccuum leak from the bad plenum, and this is somehow affecting the system. But isn't there less vaccuum at higher RPM's? So would it be a bad check valve?

I dont understand what the resevoir is doing either <img src="/forums/images/graemlins/confused.gif" alt="" />

Yes, go buy a plenum...somebody spare me $400. <img src="/forums/images/graemlins/barf.gif" alt="" />

I'm not willing to spend money, I'd just like some free answers <img src="/forums/images/graemlins/smile.gif" alt="" />
Sounds like you might have a problem with your check valve or maybe solenoids. They are located relatively high on right fender but still might get submerged, which very well could have happened when you got stuck <img src="/forums/images/graemlins/shiner.gif" alt="" /> a year or two ago. Sucking in dirt/water wont make solenoids happy. Intake manifold vacuum depends on throttle position. More throttle - less vacuum. That's why manual says to coast when trying to engage 4WD. Coasting with heavy foot off accelerator will create most vacuum in the system. Later on vacuum reservoir will hold some extra vacuum to engage/keep 4WD engaged even then car is driven hard. Malfunctioning check valve will most certainly defy that. Plenum leak won't have that much effect unless it's really bad but then you couldn't really drive your truck either.

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