Vacuum leak? Marc has me confused now!
 

Hey guys,

My vacuum pump (under the pass. headlight) cycles on for a moment every 4 to 5 seconds, when the ignition is on with the car not running. I've come to understand that this indicates a vacuum leak somewhere under the plenum. A no-brainer, right?

BUT, on page 19 of the latest HOTB, Marc says in his troubleshooting tips:

"If the pump cycles on and off the vacuum connections up to the secondary port solenoid are tight. As long as the pump will turn off for one second, the secondary port throttles will operate normally. If the pump shuts off for longer than five seconds it indicates that the vacuum system connections are tighter than average."

So, according to Marc, my vacuum system connections are tight? Am I misunderstanding what I've just read? Anybody willing to help confuse me further?

Thanks!
Chris

Re: Vacuum leak? Marc has me confused now!
 

Also, forgot to mention that my car idles normally...no high idle.

Re: Vacuum leak? Marc has me confused now!
 

Mine cycles like yours. Runs for 2-3 seconds, off for 4-5 seconds. I took my car to Marc last fall for injectors, coils, radiator clean, and tuning - he said mine was perfectly normal. He drove it with his laptop hooked up to it as well - and all was good....

Re: Vacuum leak? Marc has me confused now!
 

Mine does the same. I also thought I had a leak. Glad to know otherwise.

Re: Vacuum leak? Marc has me confused now!
 

WOW, I'm kinda shocked...lots of views but not many replies. Anybody have any thoughts on this?

Re: Vacuum leak? Marc has me confused now!
 

Thanks Racinfan, good to know Marc gave yours his seal of approval!

Re: Vacuum leak? Marc has me confused now!
 

A good working secondary system is a good setup. My 95 works fine...for now. My 91 has no secondary hardware and will never have a leak from that area.

Re: Vacuum leak? Marc has me confused now!
 

Thanks for getting back, Brett! But my question for you and the rest of our brothers is this:

The conventional wisdom (or my rookie understanding of it) is that a cycling pump means an under-the-plenum vacuum leak. Is that correct? Does my cycling pump mean that I have a vacuum leak? Or do I understand what I read from Marc correctly? If so, John and Racinfan and I don't have leaks. What are your thoughts?

Everybody?

Re: Vacuum leak? Marc has me confused now!
 

Honestly, I'm not the person with that specific knowledge. I trust in Marc and Pete. I truly wish I was the expert to give you the answer.

Sincerely
Brett

Re: Vacuum leak? Marc has me confused now!
 

Thanks Brett. I'm with you on trusting Marc. I woke up today thinking I had a leak. Now I don't and I didn't have to pull the plenum...I'd call that a good day! LOL

Great night all!
Chris

Re: Vacuum leak? Marc has me confused now!
 

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

Re: Vacuum leak? Marc has me confused now!
 

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.

Re: Vacuum leak? Marc has me confused now!
 

Thanks XFire, makes sense to me.

Re: Vacuum leak? Marc has me confused now!
 

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.

Re: Vacuum leak? Marc has me confused now!
 

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

Re: Vacuum leak? Marc has me confused now!
 

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

Re: Vacuum leak? Marc has me confused now!
 

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.

Re: Vacuum leak? Marc has me confused now!
 

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.

Re: Vacuum leak? Marc has me confused now!
 

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. [-X

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

Re: Vacuum leak? Marc has me confused now!
 

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.


http://i187.photobucket.com/albums/x...pse4c2b39b.jpg

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!

Re: Vacuum leak? Marc has me confused now!
 

Pump is there to make up for problem.

Re: Vacuum leak? Marc has me confused now!
 

http://i185.photobucket.com/albums/x...LargeLarge.jpg

Marc should correct me if I'm wrong, but I don't believe he's saying a short "OFF" cycle doesn't need attention, but rather if the leak is small enough that the vacuum pump can attain sufficient vacuum to reach pump cut-off threshold, then the the SPTs actuators will not be inhibited for lack of vacuum (provided code 61 is not triggered). (Referring to the diagram above)

The SPT circuit is divided into two parts, partitioned by the SECONDARY PORT THROTTLE VALVE (switch):

1. The pump and the engine vacuum charge the reservoir side of the (VALVE) and
2. the other half of the circuit connects the (valve) to the vacuum actuators.

So, IF the vacuum pump will cycle OFF - for even a second, then sufficient vacuum exists to operate the SPT actuators, at least up to the SPT VALVE (valve). Now, however, if the circuit on the actuator side of the (valve) is severely compromised to the point of triggering code 61 when the ECM calls for the secondaries to open, we know the actuator side of the circuit is what is severely compromised.

Back to Scottfab's point about the short "OFF" cycling and pump longevity:

If a pump is short (off) cycling, I wouldn't argue there isn't an issue to be attended to. However, most of the time the engine provides the vacuum sufficient to charge and maintain the reservoir side of the (valve). The pump doesn't run at all until there is a sustained load to raise air pressure above the point the pump is triggered "ON". This significantly mitigates the concern for pump being over-worked (but I'd want to run that down at the earliest convenience). But, then again, during the 24 hour run, I would think the pump ran (ed. as needed) majority of the time, especially at/near WOT!

Of course one could perform the secondary delete mod and NEVER worry about this entire topic again...But, that is another debate.

Re: Vacuum leak? Marc has me confused now!
 

'Preciate the time to shoot a pic and post it but, you didn't need to do that. I have FSMs from 1982-2012 and have read that page a number of times.Even Marc's 15 years of experience, which I greatly respect (heck, I've had him work on my '04 and I use his cals in my '95) cannot negate the fact that when an engine like the LT5 is at WOT, there is virtually no vacuum present. If you don't believe me, connect your scan tester and go for a road test. Pick third or fourth gear then accelerate at 100% TPS and watch the MAP reading. It will be very close to atmospheric, ie: virtually no vacuum.

With 90-92, almost all time at WOT, the SPTs are commanded open and with 93-95 it's all the time. Thus, for most of the time the SPTs are open, while vacuum may be present to open them, there's little or no vacuum to keep them open, so the primary source of vacuum is not MAP but is, first, the vacuum tank, followed afterwards, by the pump.I never saw the Record Run engine with the plenum off so I can't verify the existence of port throttles on it. Might be a moot question because for the entire run (5000-km, 24-hr and 5000-mi), the engine was never run at WOT. It had a throttle stop, so there was always some level of vacuum present. At high rpm, it may have been quite a bit of vacuum because of the restriction part throttle must have been in the intake tract. It wasn't until after they set the 5000-mi. mark that the final driver, Stu Hayner, was told to remove the throttle stop and take a few laps at WOT.

There was a so-called "touch the sky" test done during the final stages of ZR-1 development. I think it was done at the now-defunct Desert Proving Ground where a ZR-1 was run on the high-banked track at WOT for the duration of a tank of fuel so the aux. vac. pump will last at least that long in continuous use.That is NOT why there is a pump.

HIB, your post was moderated for "some" of its content. PM sent.

Re: Vacuum leak? Marc has me confused now!
 

My sense for it is that you are mostly correct. However, running that close to failure is not the best solution. In fact the system is on the verge of failure (code being set) and will dip in and out of failure. This is not by any means or stretch of the imagination a good approach. Why not just fix it and be done with it. To propagate the notion that 1sec between cycles is "OK" is like saying you can run your tires at 25psi and that it's ok to ignore filling the tires until the "Low Tire Pressure" light comes on. I do not recommend either.

Getting above 15sec and in fact achieving 30sec or more cycle time on the pump is not hard. Attention do detail is the key. Really going after and fixing leaks and contamination is key (if you live in a heavy dust filled and/or dirt filled environment then all bets are off)

Yes, one can wait until there is total failure just as one can keep driving when the "RESERVE" notice is given on the dash. I prefer not to and would never suggest that members think it is OK to keep driving when the "RESERVE" light is lit. Similarly, a 1sec pump interval is a BIG TIME warning. AND I submit is not proven to NOT set a code at 1sec pump interval time. In fact I know positively that you'll get a roll off of power at about 4k and NOT see a code 61.
(marginal condition)

Agreed, most of the time if the engine is running. However, I stated
"key on engine off". Not a great concern because it is usually rare for key on engine off but it does happen. This is not a fish pump we're talking about. It is not intended to run 100% duty cycle 24/7.
If a pump is cycling every 1sec with key on engine off, just fix it.
Gezzz

Interesting, I wonder about "most of the time". My guess would be there is enough vacuum at 174mph to run the actuators with out the pump "most of the time" :)
On each curve and WOT maybe it's on. Straightaways? dunno

yes that would "fix" it. But then would it be an LT5? One wonders.
I suppose it is. I mean the block is still there.

Back in 99 I came up with a solenoid actuator replacement that pulled a cable. I was convince it would be a permanent solution. However about that time I ended up
actually fixing the secondary system and it has stayed fix all of these years, save the time the rebuilt pump failed (poor selection of replacement diaphragm material) . At that time (99) I announced I would, upon another secondary system failure, put that solenoid in and make public the part numbers etc. So far it hasn't come to that.
I do still have all the parts etc in a box. I think ripping out the secondary is out of the question for me for several reasons.
eh, but then that's just me. I still keep the spare tire and the owner's manual too :)
Respectfully submitted:

Re: Vacuum leak? Marc has me confused now!
 

Guys...the Record Run engine was throttle stopped so there was always vacuum present in the plenum. If that engine had port throttles, which I suspect it did because the it came right off the LT5 Line at Stillwater, it's likely the pump didn't run that much.

Re: Vacuum leak? Marc has me confused now!
 

Didn't see your previous post to mine.
Do you know or suspect what the thinking was to put the stop in?

i am never confused by Marc's advice
 

I Concur with Marc :cheers:
If the vacuum pump cycles 5, 10, 15 seconds or at all....the vacuum system is NOT TIGHT but normal and functional for the age of the vacuum system with components not replaced with NEW LINES AND NEW RUBBER (soft) CONNECTORS.

Disconnect the drivers side center plenum vacuum source and hold your finger over the rubber connector disconnected (Engine OFF but Key ON). IF THE SECONDARIES (Secondary Canisters) FUNCTION AND HOLD POSITION WHEN TESTING SECONDARIES WITH PLENUM IN PLACE (Key on) Verifying Secondary Actuator Operation with the Plenum in Place END OF DISCUSSION ;)

I will think further on this but for now I am busy planting corn and beans ;)

Re: i am never confused by Marc's advice
 

I'll try this. Thanks!!

Re: Vacuum leak? Marc has me confused now!
 

With key on and engine off how long does the pump run and how long does it stay off (approximately) ?

Re: Vacuum leak? Marc has me confused now!
 

It runs for only an instant...less than a second. Then, it stays off for 4 to 5 seconds. Then repeat.