Troubleshooting Headlight Vacuum
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Remove and plug off ALL vacuum lines from intake manifold and carb. Start the engine and measure vacuum at idle. This will be the highest level of vacuum you will ever see from the engine. Note: It is possible that vacuum might go a bit higher with different ignition timing adjustment...but not much.
Now re-connect ONE vacuum line and measure vacuum again. If it has not dropped significantly (more than 1-2" Hg), re-connect another line. Continue that process until you see that the vacuum level dropped significantly. If it has, THAT vacuum line and the components on it are suspect for leakage. Chase the culprit[s] down and eliminate them. Then continue with the re-connection process.
When all is completed, your engine should make nearly as much vacuum with everything re-connected as it did with NOTHING connected. THAT is how you eliminate vacuum leaks.
P.S. The carburetor and intake manifold could be sources of vacuum leakage, as well. And you can't "disconnect" them. But, with all vacuum line disconnected and engine idling, you could spray some [volatile] carb or brake cleaner in possible ares of leakage (throttle shafts on carb: anywhere along intake to heat gasket edges on intake manifold, etc) to see if the engine RPM increases (indicating that additional 'fuel' was ingested where it should NOT be able to enter the intake charge).
Now re-connect ONE vacuum line and measure vacuum again. If it has not dropped significantly (more than 1-2" Hg), re-connect another line. Continue that process until you see that the vacuum level dropped significantly. If it has, THAT vacuum line and the components on it are suspect for leakage. Chase the culprit[s] down and eliminate them. Then continue with the re-connection process.
When all is completed, your engine should make nearly as much vacuum with everything re-connected as it did with NOTHING connected. THAT is how you eliminate vacuum leaks.
P.S. The carburetor and intake manifold could be sources of vacuum leakage, as well. And you can't "disconnect" them. But, with all vacuum line disconnected and engine idling, you could spray some [volatile] carb or brake cleaner in possible ares of leakage (throttle shafts on carb: anywhere along intake to heat gasket edges on intake manifold, etc) to see if the engine RPM increases (indicating that additional 'fuel' was ingested where it should NOT be able to enter the intake charge).
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
Remove and plug off ALL vacuum lines from intake manifold and carb. Start the engine and measure vacuum at idle. This will be the highest level of vacuum you will ever see from the engine. Note: It is possible that vacuum might go a bit higher with different ignition timing adjustment...but not much.
Now re-connect ONE vacuum line and measure vacuum again. If it has not dropped significantly (more than 1-2" Hg), re-connect another line. Continue that process until you see that the vacuum level dropped significantly. If it has, THAT vacuum line and the components on it are suspect for leakage. Chase the culprit down and eliminate them. Then continue with the re-connection process.
When all is completed, your engine should make nearly as much vacuum with everything re-connected as it did with NOTHING connected. THAT is how you eliminate vacuum leaks.
P.S. The carburetor and intake manifold could be sources of vacuum leakage, as well. And you can't "disconnect" them. But, with all vacuum line disconnected and engine idling, you could spray some [volatile] carb or brake cleaner in possible ares of leakage (throttle shafts on carb: anywhere along intake to heat gasket edges on intake manifold, etc) to see if the engine RPM increases (indicating that additional 'fuel' was ingested where it should NOT be able to enter the intake charge).
Now re-connect ONE vacuum line and measure vacuum again. If it has not dropped significantly (more than 1-2" Hg), re-connect another line. Continue that process until you see that the vacuum level dropped significantly. If it has, THAT vacuum line and the components on it are suspect for leakage. Chase the culprit
When all is completed, your engine should make nearly as much vacuum with everything re-connected as it did with NOTHING connected. THAT is how you eliminate vacuum leaks.
P.S. The carburetor and intake manifold could be sources of vacuum leakage, as well. And you can't "disconnect" them. But, with all vacuum line disconnected and engine idling, you could spray some [volatile] carb or brake cleaner in possible ares of leakage (throttle shafts on carb: anywhere along intake to heat gasket edges on intake manifold, etc) to see if the engine RPM increases (indicating that additional 'fuel' was ingested where it should NOT be able to enter the intake charge).
good tips. I appreciate that
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
I got a new override switch today. My wiper one had a broken pull down tab and even though I don’t need the wiper one because I have a long hood, I wanted it to look clean. I will take the headlight one and move it to the wiper one as the prop and use the new at the headlight hook up. Still chasing this thing down.
Le Mans Master
Joined: Mar 2004
Posts: 7,470
Likes: 1,491
From: Little Rock AR
Last edited by stingr69; Sep 11, 2021 at 11:53 AM.
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
I got them to go up and down although very slowly by ting the 2 hoses to the actuators and bypassing the tank. I could only get them to work though by connecting my mighty vac to the hose from the relays to the override switch. I’ll spend some more time on it tomorrow. Urgh!.!.!.!.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
so try the mighty vac on the hose going from the engine compartment into the pass compartment to hit the headlight or override switch. see if t you can feel vac on hose going to the relays,
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
good suggestion. I’ll try that later. I’ve done everything to this car myself and have gotten tested many different times and some more than others but this vacuum system is kicking my ***
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
it’s probably something so fundamental I’ll call myself a dmbass once I find it. If I find it because I’m still tempted for the electric motors.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
I think I may have found the culprit. It may be a slight leak in the damn check valve. I didn’t think it was a big enough leak but since my old engine had more vacuum I’m thinking the higher vacuum compensated for a leaky check valve. I ordered it and will have it next week but when I measured the vacuum to the relays it’s only getting like 6 inches. From the manifold I’m getting 12. When I used the mighty vac I can get more vacuum and the relays moved up and down no problem! We will see. Went ahead and ordered a new filter while at it.
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
I believe it was 12 but I’m going to double check that. If it’s more, and I’m getting 12 down stream then that would be a sign of leak, correct?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Yes. That's why you need to KNOW what maximum level of vacuum the engine will make at idle.
If you do not have a high performance cam in the engine, a normal Chevy V8 will create about 17-20" Hg, if it is not heavily worn internally. Since you are getting 12" Hg with 'things' still connected, you must have a high-performance cam....or you have significant vacuum leaks.
If you do not have a high performance cam in the engine, a normal Chevy V8 will create about 17-20" Hg, if it is not heavily worn internally. Since you are getting 12" Hg with 'things' still connected, you must have a high-performance cam....or you have significant vacuum leaks.
Last edited by 7T1vette; Sep 21, 2021 at 05:11 PM.
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
Yes. That's why you need to KNOW what maximum level of vacuum the engine will make at idle.
If you do not have a high performance cam in the engine, a normal Chevy V8 will create about 17-20" Hg, if it is not heavily worn internally. Since you are getting 12" Hg with 'things' still connected, you must have a high-performance cam....or you have significant vacuum leaks.
If you do not have a high performance cam in the engine, a normal Chevy V8 will create about 17-20" Hg, if it is not heavily worn internally. Since you are getting 12" Hg with 'things' still connected, you must have a high-performance cam....or you have significant vacuum leaks.
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
Well mission fail here. Back to the drawing board. New valve didn’t do it. Although the old one did have a leak and new one is good. My vac at the source is 15 inches. When I plug in the check valve and measure it at either nipple and plug the other I get like 6 inches. What hell?? Looks like the check valve restricts the vacuum down and not enough to work the system correctly
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Well mission fail here. Back to the drawing board. New valve didn’t do it. Although the old one did have a leak and new one is good. My vac at the source is 15 inches. When I plug in the check valve and measure it at either nipple and plug the other I get like 6 inches. What hell?? Looks like the check valve restricts the vacuum down and not enough to work the system correctly
I had to put my original one back on…..
Jebby
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
When you put the check valve in the line and want to check vacuum level on the output, you connect the gauge to one nipple and you must block off the other one. The check valve cannot 'limit' vacuum level; it can block vacuum or it can pass vacuum. The easy way to test a check valve is to suck on each nipple (yeah, I know....) to see if it passes air. If it's good, it will pass air when you suck on the single nipple side, and it will block airflow when you suck on either of the others.
And, I guess, you still do not know what your engine's baseline vacuum level is at idle. Not sure how you're going to proceed when you have no 'goalpost' to aim at.
And, I guess, you still do not know what your engine's baseline vacuum level is at idle. Not sure how you're going to proceed when you have no 'goalpost' to aim at.
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
I’m away from the car for a day or two now but when I took it out of the package tried that it did seem a little restrictive. I’ll double check later but now you got me wondering if it’s chinesium
Thread Starter
Drifting
Joined: Dec 2020
Posts: 1,586
Likes: 956
From: Central Florida
2024 Corvette of the Year Winner - Modified
2023 C3 of the Year Winner - Modified
2021 C3 of the Year Finalist - Modified
When you put the check valve in the line and want to check vacuum level on the output, you connect the gauge to one nipple and you must block off the other one. The check valve cannot 'limit' vacuum level; it can block vacuum or it can pass vacuum. The easy way to test a check valve is to suck on each nipple (yeah, I know....) to see if it passes air. If it's good, it will pass air when you suck on the single nipple side, and it will block airflow when you suck on either of the others.
And, I guess, you still do not know what your engine's baseline vacuum level is at idle. Not sure how you're going to proceed when you have no 'goalpost' to aim at.
And, I guess, you still do not know what your engine's baseline vacuum level is at idle. Not sure how you're going to proceed when you have no 'goalpost' to aim at.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Could be a bad valve. But also look at the condition of the hoses that connect to the valve. There may be a small split at the end of the line...and that can leak more than you can imagine. You can also put a light wipe of RTV on the nipples before you connect the hoses so that a better seal might be achieved.
Is the check valve connected to anything when you test it at the output? If you plug off one output nipple and hook gauge to the other and it checks 6" Hg, there is definitely a problem with the check valve. The only other possibility is that the hose going to the vacuum gauge has a leak!!!
Is the check valve connected to anything when you test it at the output? If you plug off one output nipple and hook gauge to the other and it checks 6" Hg, there is definitely a problem with the check valve. The only other possibility is that the hose going to the vacuum gauge has a leak!!!