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12/23/16 4:34pm - Original Message: 'Vacuum goes to zero as soon as engine is turned off'
BillHanna
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Cana, VA - USA

Vette(s):
1975 Stingray

Joined: 7/3/2016
Posts: 70
Since there is a vacuum reservoir, I would have thought that even if there were a significant vacuum leak that it would take at least a few seconds for the vacuum to drop to zero after turning the engine off, but it immediately drops to zero (I checked this with a vacuum gauge hooked up to to the vacuum lines).
I don't hear any vacuum leaking (although I don't have a vacuum pump to test with), so I don't understand why the vacuum drops so quickly.
I have replaced the vacuum check-valve and filter.
All of the vacuum operated controls seem to work fine with the engine running.
Any ideas of what to look for?

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12/23/16 11:02pm - Reply: 'Re: Vacuum goes to zero as soon as engine is turned off'
73shark
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Overland Park, KS - USA

Vette(s):
1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.

Joined: 7/9/2003
Posts: 381
Could the vacuum reservoir be leaking?  Mine is just a large tin can.

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1973 L-82 4 spd

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12/24/16 12:16am - Reply: 'Re: Vacuum goes to zero as soon as engine is turned off'
BillHanna
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Cana, VA - USA

Vette(s):
1975 Stingray

Joined: 7/3/2016
Posts: 70
73shark said: Could the vacuum reservoir be leaking?  Mine is just a large tin can.

This is a 75, which uses the tubler front frame piece as the reservoir.  I would think that a leak large enough to drain it in a second when the engine is shut off would be enough to keep the vacuum components from working even when the engine is running.




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12/24/16 2:06am - Reply: 'Re: Vacuum goes to zero as soon as engine is turned off'
73shark
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Overland Park, KS - USA

Vette(s):
1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.

Joined: 7/9/2003
Posts: 381
My bad,I was thinking of the earlier reservoir. You could use a Mityvac to check the tank and sections of hose.

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1973 L-82 4 spd

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12/24/16 9:13am - Reply: 'Re: Vacuum goes to zero as soon as engine is turned off'
Redwoman Gold Member
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Bellingham, MA - USA

Vette(s):
1980 loaded red on red. 68,000 original miles. Second owner. Well documented with all original paperwork. NCRS Founders Award 9/27/14. NCRS Top Flight 10/12/14.

Joined: 6/23/2014
Posts: 171
The engine has to be running to get vacuum from the intake manifold. The fraction of a second you turn off  the engine it creates a negative pressure. If you have the car running with headlights on and forget to close them when you shut the car off they stay up. You need to restart engine to have vacuum to close them. That is how they operate. To get a better explanation I googled does a corvette engine need to be running for vacuum. Then scroll down to superchevy site. http://www.superchevy.com/how-to/vemp-0212-corvette-vacuum-systems-guide/

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12/24/16 10:53am - Reply: 'Re: Vacuum goes to zero as soon as engine is turned off'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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BillHanna said: (I checked this with a vacuum gauge hooked up to to the vacuum lines).


The question is, WHERE did you have the gauge connected? Anywhere other than in the headlight circuit, the vac WILL go to 0 with the engine off. The headlights(and wiper door on early models) are the only components(other than the brake booster, which has it's own separate vac source & check valve) that retain vacuum with the engine off. The headlights should be able to operate for a short time after the engine is off from the vac reservoir. Thumbs Up



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12/24/16 11:20am - Reply: 'Re: Vacuum goes to zero as soon as engine is turned off'
BillHanna
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Cana, VA - USA

Vette(s):
1975 Stingray

Joined: 7/3/2016
Posts: 70
Adams' Apple said:
BillHanna said: (I checked this with a vacuum gauge hooked up to to the vacuum lines).


The question is, WHERE did you have the gauge connected? Anywhere other than in the headlight circuit, the vac WILL go to 0 with the engine off. The headlights(and wiper door on early models) are the only components(other than the brake booster, which has it's own separate vac source & check valve) that retain vacuum with the engine off. The headlights should be able to operate for a short time after the engine is off from the vac reservoir. Thumbs Up



Ah, that makes a lot of sense.  I checked it just past the check valve off of the intake manifold.  I need to get a "Y" fitting so I can check it at the vacuum tank.
I didn't know there was more than one check valve.
Thanks!



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