Re: Vacuum leak? Marc has me confused now!
 

The engine had more power than GM thought was necessary to set the record.

To ensure durability, best fuel economy at the target speed and ensure no consistent throttle opening, the engine had a throttle stop.

The durability issue was the biggest reason. In 1990, no one had ever run any production car in an attempt to set the 24-hr speed record. The previous attempts (Ford, Mercedes, Audi) were done with modified or purpose-built vehicles. No one knew if the LT5 would last. In fact, GM was so worried the attempt would fail, the Record Run was done in secrecy with few media present and only limited documentation by internal GM and sponsors.

Re: Vacuum leak? Marc has me confused now!
 

It's probably the check valve (most common) but you'd want to
find the little black hose and trace it up toward the engine.
You'll find a rubber connector about half way up. Open it.
Put you finger over the hose that runs tot he pump.
The pump should run and then stop. Keep it plugged.
The pump should not run again (wait over 60sec).
If all ok then the leak is under the plenum (check valve etc).
If the pump runs again then the leak in in the diaphragm inside
the pump. This happens when a pump has cycled too many times
and has worn out the diaphragm.

Re: Vacuum leak? Marc has me confused now!
 

I'd say, you have a leak but not really a big one or the pump might have a somewhat restricted filter screen.

I just went out to the shop and checked the pump run time on my 95. After the initial run period, my aux. vac. pump runs for about a second every 15-20 seconds. I know my SPT system is in good condition. The SPT rule of thumb is, once the system is evacuated, one second every 15 seconds.

Re: Vacuum leak? Marc has me confused now!
 

Apples n Oranges??

When the LT5 performance is down, the question of the SPT system immediately comes under scrutiny, as it should - especially if a "61" occurs. However, for anyone following these "secondary" threads over the years, there is a tendency to proclaim AH-HA! prematurely! - pointing to the pump cycling as indicative of the problem being the SPT system when it isn't, regardless of whether it too needs to be serviced.

I think for the OP's benefit, the distinction to be made is between what is characteristic of a proper "tight" vacuum circuit vs. what the system will tolerate before causing an issue i.e., the SPTs not opening or throwing a "61" code.

Keep in mind the SPT is a closed circuit. So, if there were no leaks, even at WOT the pump wouldn't run but perhaps just long enough to replenish the initial reservoir vacuum expended to initially pull the actuators. So, barring any leaks, the pump would not run continuously - even at WOT.

Now, if there are some leaks, which typically there are (the many check valves, rubber connections, cracks, etc), the pump is there to maintain the vacuum...and shuts OFF when the necessary vacuum is reached - and will come back ON when before vacuum level is depleted to the point of jeopardizing the SPT system.

Marc's quick n dirty way to sort out the SPT comes from years of hands-on experience: specifically if the vacuum pump shuts off, even for a second, then one can assume there is sufficient vacuum, at least up to the secondary port throttle valve. He also discusses how to observe the actuators to verify their operation; verifying the circuit from the valve to and including the actuators (and vacuum can be monitored during that test as well.

The fine point missed (as I see it) in this/these discussion is NOT that the secondary circuit isn't perhaps in need of service - it could very well be. But! In the quest for locating a performance issue, one may be able to put the SPT system aside for latter attention while the real culprit is ferreted out. Make sense?

Re: Vacuum leak? Marc has me confused now!
 

I don't think this is a performance issue. At least reading the OP there is no indication of loss of power. The thread is about being confused. Getting apparently different or conflicting info.

Given that we're trying to clear it all up and not likely to get a consensus I'm sure the waters are now completely muddy. Oh well.

I see this whole "vacuum" issue as this.
You either have no issue with a leaking system
or you do. If you do you either decide to fix it
or let it go until you get a code 61.
Hey, but that's just me.

Re: Vacuum leak? Marc has me confused now! Th
 

Marc would say that it's operating properly. I was with him when looking at a stock ZR-1 that exhibited the same pattern. As long as the pump shuts off, the system is holding sufficient vacuum.

Re: Vacuum leak? Marc has me confused now!
 

Exactly, Scott. My Z pulls like a freight train and idles smoothly at 800. Just trying to figure out how bad my leak is. Thanks for "boiling it down".

Re: Vacuum leak? Marc has me confused now!
 

From Mr Haibecks site: "Has a broadening effect on the useful power band for the LT5. The throttle response is as good at 3000 rpm as it is at 4000 rpm with a 3.45 gear ratio"

So basically with stock gearing a downshift to 3rd gear may have been necessary to perform a passing manouver, with 4.10's, the same manouver can be performed in 4th gear.

Swapping to a lower gear increases torque multiplication to the wheels, enabling greater vehicle acceleration while engine output remains constant.

I understand what you mean.

xxxxxx
There would have been substantial engine vacuum with the LT5 throttle stopped at 70%. In addition to durability, other reason for the stop could be:
1)fuel economy
2) drivers right leg comfort, much easier to hold a pedal on a stop than holding against throttle spring tension

I do think that durability was certainly a concern, which is further evidenced by the decision to run a higher 3.07 ring/pinion. This would aid driveshaft, transmission, front accessory and engine durability, and again would aid fuel economy.

That record run was a great accomplishment.

Re: Vacuum leak? Marc has me confused now!
 

Another reason they used a 3.07 rear gear was to keep the RPMs under 6500 as the radiator was designed to bypass at higher RPMs due to higher water pressure. They had to keep it cool!!

Re: Vacuum leak? Marc has me confused now!
 

And, too: does not the coolant pump begin cavitation just north of 5500 rpm?

Also, I read a comment (in Heart Of The Beast(?)) regarding the desire of limiting the rpm to place the LT5 in its peak torque region.

Re: Vacuum leak? Marc has me confused now!
 

there also is a small vacuum switch inside the secondary pump, it could be out of adjustment and not shutting the pump off when vacuum is reached.

Re: Vacuum leak? Marc has me confused now!
 

i've had my '94 for less than a month and have very little time on the road with it..point being i dont know if my engine is at full power or not.. my pump ran for several minutes without shutting off when i tried it with key on but engine not running, i turned it off because it seemed longer than any other pump in the poll..real quick it says i have a bad leak or leaks.. would appreciate yer thoughts..

Re: Vacuum leak? Marc has me confused now!
 

Weldbead, I see its your first post Welcome... do a search here on it lots of posts on what and how to check it out. Sticky at top of this section.

http://www.zr1.net/forum/showthread.php?t=16778

Re: Vacuum leak? Marc has me confused now!
 

Your engine produces the most vacuum at idol, and very little at full throttle, that's when the secondary pump goes to work. With engine off, key on, take the hose off the secondary pump and hold your finger over the outlet. Pump should stop. If it doesn't pump is bad or needs adjusted. You can check the rest of the system for vacuums leaks with a hand held vacuum tester, about $30 at sears. The system is not that completed.

Re: Vacuum leak? Marc has me confused now!
 

That is the exact thing that happened to me when I bought mine.
You'll REALLY feel the difference when the leak is fixed.

Here is one of many vacuum leak troubleshooting sequences.
Click on the link below
Vacuum Leak Troubleshooting

Checking for LT5 Vacuum Leaks
 

As others have suggested close off the vacuum line closest to the pump with your thumb (after line is disconnected) and if the pump does not stop...bad pump or bad vacuum line connection to pump (hold thumb over pump connection to determine if bad pump). You can continue to use that some process to locate the leak if pump stops (for a minute or more) with the first line block with thumb.

Disconnect the lined further along and if pump stops (or cycles less), leak is beyond that line you just disconnected. Keep going and if pump does not stop (or cycles less) when you hold thumb over end of vacuum line, you have found the leak (or one of the leaks). Fix that leak and then continue disconnecting lines for possible additional leaks if pump does not stop once that leak is repaired and reconnected.

It is usual that the leak can be found at locations where fittings are easy to disconnect (Some of those connections are nearly impossible to disconnect....go past them as it would be unlikely the leak will be at that connection). With this method you can jump in anywhere and determine which direction to go if pump stops (or cycles less) to locate additional leaks. This will also locate the leak if found at the drivers side check valve which blocks the vacuum path to the plenum when plenum vacuum is low.

Once ALL significant leaks are found, the pump will stop cycling after initial vacuum is reached for a minute or more. At that point you are in business :thumbsup:

Before you pull the Plenum, check the pump and line going under the Plenum. Also check the Drivers Side vacuum line going into the Plenum at center of Plenum. The leak could be that Plenum connection or the check valve under the Plenum.

Checking for Vacuum Leaks TIPS

Re: Vacuum leak? Marc has me confused now!
 

Keep in mind, there is a vacuum switch inside the pump that can be adjusted.

RIP has me confused now!
 

How do you adjust that pressure/vacuum switch inside the vacuum pump?

What do you set the vacuum pressure at for proper vacuum?

Re: Vacuum leak? Marc has me confused now!
 

Welcome onto the boards there. Post snappics of your Z there.


RIP - you see the adjuster here?

I did give a passing thought to what the controller mechanism was...

https://lh4.googleusercontent.com/-D...0/HPIM7531.JPG

Re: Vacuum leak? Marc has me confused now!
 

When you draw a vacuum the small white piston in drawn in, separating the points and shutting the pump off. With a hand operated vacuum pump you should be able to see the points open and close. You could try to use your mouth. Suck, baby, suck.

Re: Vacuum leak? Marc has me confused now!
 

If the contacts are corroded they can be cleaned up with alcohol and covered with dielectric grease. If the pump runs for
ever it might be that
the diaphragm (in the round section of the pump) has a void in it.
It looks like this and is repairable using a rubber from a kitchen glove.

http://zr1.net/forum/picture.php?alb...pictureid=1445

Re: Vacuum leak? Marc has me confused now!
 

seems most of the system is under the plenum..can someone tell me where to get fsm's?i need diagrams etc..thx..

Re: Vacuum leak? Marc has me confused now!
 

ebay. Three manuals.

1. Field Service manual covers both standard and ZR-1.

2. LT-5 Supplement. Covers the LT-5 engine.

3. Electrical manual. Covers electrics.

Expect to pay $100 to $150 for all 3.

Re: Vacuum leak? Marc has me confused now!
 

Now, Scott...
:)
...don't you know alcohol is for drinking and "contact cleaner" is for cleaning electrical connections.

Re: Vacuum leak? Marc has me confused now!
 

Guys, guys...maybe we're "overthinking this"?

The Record Run took place on 1-2 March 1990. Ever been to central TX in March? At night, it's freakin' cold...in the high-20s. Plus, during the 24-hr period the RR was staged, it snowed and rained. At start time on 1 March it was 35°F. Cooling was not an issue because 1) the low ambient and 2) since the engine was running at only part throttle, cooling load was reduced.

Limiting RPM to place the LT5 at peak torque, regardless of what Heart of the Beast may say, was not an issue, either. Besides, with a throttle stop, the engine was not making peak torque, anyway.

The main reasons for the throttle stop were 1) fuel mileage and range, 2) durability and 3) ensuring drivers would not exceed a TPS value which would compromise 1 and 2.

Re: Vacuum leak? Marc has me confused now!
 

I think the car in question is a 94.
The Factory Service Manual set for a '94 has only two volumes covering both VIN P and VIN J (LT5) engines. I've seen them on eBay but, also, Zip Products stocks Service Manuals, and they're the real thing not reprints or scans. In recent years, I've been getting my manuals from Zip. See this web page

Re: Vacuum leak? Marc has me confused now!
 

RE: peak torque at the Record Run (RR):

Well, of course the motor wouldn't be making max torque with the throttle stop, compared to WOT. But, rather I was referring to the shape of the torque curve, i.e., where the torque azimuth occurs, assuming no significant change in the curve, regardless of throttle being at 70% vs. 100%.

I refer to one of many graphs I can produce which share the same general torque curve shape as the one (below). Notice the torque reaches azimuth around 5200 rpm for this particular motor - which forms the basis of my comment. (Note: This happens to be a modded motor, but I can produce several graphs - all having very similar curve characteristics if need be.)

http://i185.photobucket.com/albums/x...ps79a8b9ce.jpg

http://i185.photobucket.com/albums/x...ps837f03ef.jpg

Considering the 3.07 differential affect on speed vs. rpm for a stock diff gear: 3.07/3.45 = rpm factor = 0.8899. By multiplying the rpm for a stock ZR-1 (5th gear) required for speed close to the average of the Record Run (6000 rpm ≈ 177 mph from the chart (c/o Haibeck Automotive)), the result is (6000 x .8899) = 5339 rpm w/ the 3.07 differential gear. That's purdy dang close to the peak torque value. Just a coincidence? :icon_scra If Young didn't quote someone for his "HEART OF THE BEAST", he at least did the math, I recon.;)

Re: Vacuum leak? Marc has me confused now!
 

What's a "torque azimuth"?

In your calcs, did you figure tire size?

The RR speed (175.885) was an average so the speeds on the track were faster. About 190 at corner entries or about 5500RPM in fifth gear. The car had slowed to about 170 at corner exits.

Re: Vacuum leak? Marc has me confused now!
 

"Apex" would have been a better choice than azimuth, my bad; peak torque, to be clear...is the point.

With that cleared up, my point is/was choosing a 3.07 differential gear would obviously put the LT5 rpm right in the sweet spot of the torque curve; with the LT5 passing through the peak of the curve on every lap and remaining near the peak most of the time.

As for tire size, if the tires were not the stock 315x35x17 size, then the results would be skewed more or less, depending on the amount of departure the tires used on the Run were from the stock tire size. (What were the tire make/model/size used on the Run?)

So, it strikes me that choosing the 3.07 ratio for the differential, resulting in the motor operating near peak torque the whole time (in 5th gear) was not accidental, but a major consideration - along with limiting motor output for all the reasons you mentioned.

Re: Vacuum leak? Marc has me confused now!
 

The tires were not stock.

They were Goodyear Racing Eagles which were made specifically for the Record Run. I can't remember the size. I'll have to do some digging to see if I still have that information.

And yeah, the 3.07 axle was selected so the car could run 190 mph in fifth gear. As for if that was the optimal ratio? Maybe not but the 3.07 was the tallest ratio available for a D44 in a Corvette. The 2.73 and 2.59 ratios for D36 gearsets.

Re: Vacuum leak? Marc has me confused now!
 

I'm intrigued by the details, as you seem to be too... (Probably because my favorite TV program was "Mr Wizzard", or maybe because I have a 'low entertainment threshold', or as some have said ... "You're easily amused" - referring to me: never quite sure how to take that:icon_scra).

But, yeah! Actual tire metrics would be interesting, and I agree the 3.07s were available, perhaps more so than what would be exactly perfect for the "task at hand". Any way ya slice it, the RR was a fascinating moment in automotive history, regardless.

:cheers:

Re: Vacuum leak? Marc has me confused now!
 

Since 1990, I've written several articles on the Record Run for different magazines and web sites.

The tires size was not in the last two stories I did, but a search of pre-2000 manuscripts and, finally, found it.

The tires were special race radial race tires done by Goodyear just for that program and the size was 25.5x12.0-17. That tire was used at all four corners on both cars.

Re: Vacuum leak? Marc has me confused now!
 

(small)progress update..my fsm's have arrived, i've done a bit of reading there and here..have opened the first connection in the pump suction line (which is just a coupling..why is it there, iwonder..) and with it thumbed off the pump stops instantly, telling me there is a leak downstream which is under the plenum unless its map hose ..so i'm starting to think i'm gotta remove the plenum..
symptomatically :key on engine off pump runs forever..there are times when i punch it and really feel a power surge at 3000 rpm or so, and there are times when i dont..also there are times when i punch it and i think the secondaries are open, and then it peters out and wont get over 5500 rpm so i guess the secondaries have gone closed as vac drops and pump cant keep up..