Re: Vacuum leak? Marc has me confused now!
 

The secondary vacuum pump runs to first charge the mini reservoir which you hear with ign on. This is just a reserve to prevent a delayed response of the secondary diaphragms.

The vacuum pump will also turn on when you step on it and open the throttle blades enough for the manifold vacuum to drop or whenever the ecm sees appropriate engine load and minimum rpm etc to open secondary ports. The vacuum pump runs at about 13" of vacuum as long as it needs to sustain the secondary high power mode. Once you back off of it, the manifold vacuum rises anyways. The pump turns off once the mini reservoir is again 'charged'.

I observed this when I hooked up a vacuum gauge to my secondary diagnostic map sensor and watched it as I drove around.

Re: Vacuum leak? Marc has me confused now!
 

It is kind of dumbed-down and runs even though there is enough vacuum to hold the diaphragms open indefinitely. I guess they assumed a leaky system and provided enough pump oomph to make it work with a minor vacuum seal issue. They also could have used another 1-way check valve, but just decided to keep the pump on until vacuum was no longer needed. it just keeps running when you are at full throttle. I saw the vac gauge needed pulsing the entire duration of full power with the pump so I know this is the case.

Re: Vacuum leak? Marc has me confused now!
 

So you're saying the secondary pump always runs at full throttle, even when you don't have any vacuum leaks? Interesting.

Re: Vacuum leak? Marc has me confused now!
 

Mike,

The point is the pump is a supplement not the primary. Why bother w the rest if the pump was able to support the secondaries on its own?
HERE's a question for Hib. On the 24 hour run, was there a secondary pump in use?

Re: Vacuum leak? Marc has me confused now!
 

I thought about that...Maybe mine leaks?, but I doubt it because it was all good a couple of years ago when I tested each line chasing vacuum leaks. My test runs were only for a few seconds because I was just tooling around town in 2nd and 3rd gear.

Also note that the secondaries stay open until you let off the pedal all the way so during those light cruise conditions, the manifold vacuum keeps 'em open via the 1-way check valve that is in circuit.

Re: Vacuum leak? Marc has me confused now!
 

Could that pump have handled running nearly non-stop for 24 hours?

Re: Vacuum leak? Marc has me confused now!
 

Well if it's the primary source for the secondaries, it would need to be, no?

Re: Vacuum leak? Marc has me confused now!
 

Maybe yours leaks a little, which is what I think mine is doing, but it doesn't seem like it must be a big deal, especially since we aren't idling high. I definitely value Hib's input and frankly, what he's saying make a lot of sense, even to my non-engineer mind. But if I parse his expert advice with Marc's expert advice, it seems like I have an issue but not one that I should be pulling the plenum tomorrow to correct.

Re: Vacuum leak? Marc has me confused now!
 

Marc said EVERY LT-5 leaks to one extent or another. That's why there's a pump.

Re: Vacuum leak? Marc has me confused now!
 

Aha, grasshoppa!

Pump not there to reveal problem, pump there to help fix problem!