Vacuum leak? Marc has me confused now!

[PhillipsLT5]

Can you @ 3k feel the secondaries opening? A burst of power?
I think you are OK, but in the future just remove this stuff, no confusion just + 10 RWHP with tune

[XfireZ51]

The point of the secondary pump is to make certain there's enough vacuum to open the secondaries. It is not the main source of vacuum, the motor is. So w motor OFF, the pump becomes the primary source. It's normal for it to cycle. As Marc has said, all motors have some level of vacuum leak but most not enough to make a difference.

[KILLSHOTS]

Originally Posted by XfireZ51

The point of the secondary pump is to make certain there's enough vacuum to open the secondaries. It is not the main source of vacuum, the motor is. So w motor OFF, the pump becomes the primary source. It's normal for it to cycle. As Marc has said, all motors have some level of vacuum leak but most not enough to make a difference.

Thanks XFire, makes sense to me.

[KILLSHOTS]

Originally Posted by PhillipsLT5

Can you @ 3k feel the secondaries opening? A burst of power?
I think you are OK, but in the future just remove this stuff, no confusion just + 10 RWHP with tune

Hey Phillip!

Oh yeah, it's VERY strong. The fury on mine starts at more like 4000, due to the stock gearing, but I certainly never thought I was giving up any horsepower to a vacuum leak, that's for sure! And I agree, at some point I'll just remove it.

[Hog]

Originally Posted by KILLSHOTS

Hey Phillip!

Oh yeah, it's VERY strong. The fury on mine starts at more like 4000, due to the stock gearing, but I certainly never thought I was giving up any horsepower to a vacuum leak, that's for sure! And I agree, at some point I'll just remove it.

The gearing of your rearend wont affect the rpm at which your engine begins to pull. Only engine mods will affect that.

[KILLSHOTS]

Originally Posted by Hog

The gearing of your rearend wont affect the rpm at which your engine begins to pull. Only engine mods will affect that.

Hey Hog,

I was referring mainly to the PERCEPTION of when the big power comes on. As Marc says, with a 4:10 upgrade, "the really useful area begins at 3000 instead of 4000 RPM."

Chris

[Hib Halverson]

Originally Posted by XfireZ51

The point of the secondary pump is to make certain there's enough vacuum to open the secondaries. It is not the main source of vacuum, the motor is. (snip)

Not.

The engine is the source of vacuum which opens the SPTs but once MAP continues to rise, as it undoubtedly will do as the the engine enters high-part throttle and reaches wide open throttle, the pressure in the intake tract is too high (or the vacuum is too low) to keep the SPTs open.

Remember, WOT means low/no vacuum but the SPT actuators need vacuum to work.

[Hib Halverson]

Reading this thread again, got me thinking...

As a rule, I agree with Marc Haibeck on most significant issues relating to LT5 performance/drivability, but on the Secondary Port Throttle vacuum system, if Marc feels that as long as the auxiliary vacuum pump will shut off for a second or so, the port throttles will work properly, I disagree.

Key parts of the port throttle system are the vacuum motors which open and close the SPTs, a small vacuum reservoir, the auxiliary vacuum pump, various controlling/sensing devices and the plumbing. The system is controlled by the ECM and can be disabled with the "power switch" or "valet switch".

Once the power key is on and the ECM decides the port throttles should open, engine manifold absolute pressure, which is below atmospheric at the time the port throttles are to be opened, is fed to the port throttle actuators and the SPTs open. Manifold vacuum also is also fed to a small "vacuum" reservoir down under the intake plenum.

As the main throttle continues to open and load on the engine increases, manifold pressure increases to nearly atmospheric. Of course, that's not enough to continue to overcome the return springs in the SPT actuators, so...what holds the SPTs open? For a short period, the lower pressure level in the vacuum reservoir.

Ok...riddle me this: what happens during a period of high part throttle or wide open throttle operation long enough for the pressure in the vacuum reservoir to begin to rise towards MAP. What keeps the SPTs open, then?

Well, that's why GM added an auxiliary vacuum pump to the system. Once pressure in the SPT system goes rises above 61-kPa (or, drops below 41-kPa vacuum), the pump kicks on to keep the pressure in the SPT system low enough to overcome the SPT actuators' spring pressure and hold the port throttles open.

Now, what happens when the system leakage is enough that aux. vac. pump cannot maintain a low enough pressure to hold the port throttles open? If the SPTS are commanded open and MAP rises above 80.3-kPa (or, drops below 21-kPa vacuum) for more than two seconds, the ECM sets DTC61, turns the MIL on and sets the rev limit to 3000 RPM.

So when can a leaking SPT system set DTC61, try climbing a long hill at 80 mph and at higher altitude–say 4000 feet or above. Or try a long acceleration in fifth gear.

The bigger the leak in the system, the sooner that code will set and the pump running once every second seems like a pretty big leak or a weak pump to me.

Bottom line, if your secondary port throttle system leaks or the vacuum pump is too weak to hold a pressure low enough to turn off, for not just one second, but 15 seconds or so, repair the system. The instructions on troubleshooting and repairing the SPTs are in the Service Manual.

SPT fun fact: Peel off the label from the vacuum reservoir under the plenum and you'll find a Ford part number.

[scottfab]

Originally Posted by Hib Halverson

Reading this thread again, got me thinking...

As a rule, I agree with Marc Haibeck on most significant issues relating to LT5 performance/drivability, but on the Secondary Port Throttle vacuum system, if Marc feels that as long as the auxiliary vacuum pump will shut off for a second or so, the port throttles will work properly, I disagree.

....snip...

Bingo. You can now give yourself the "beacon of reality" award

I have voice this opposing view in the past also. Along with what Hib mentions about ability to hold the secondaries in, to me a 1sec cycle time is both an indication of a worsening issue AND ages the crap out of the pump (key on motor off)

Hib, conventional thinking in the 90s was that 10sec was the limit as to "better fix it" but 15 is good also. I keep mine at 30sec. When it goes below that I investigate. That way I don't find myself piss-whine and moaning about secondary failures.

PS it is less fashionable nowadays to "disagree" on the forum but
I think done correctly it's just fine.

[XfireZ51]

Here's a page from the FSM. The secondary pump is considered a "crutch" to any lack of vacuum. Don't forget, there's a reservoir and a solenoid involved. However, the secondary pump is not the primary source of vacuum at WOT.

No offense Hib, I appreciate your writing but I'll go with the guy that has worked on LT-5s for over 15 years. This issue came up just a week ago regarding a 95.
Marc was there to describe the operation of the secondaries. Of course, my ZR is bastardized since it doesn't have secondaries any more.