vacuum system

[Joseph Y.]

my 68 435 hp when i shut the engine off i have no vacuum to close headlights or wiper door could it be a problem in the storage tank leaking i have bee told the 435s vacuum is lower than a small block and this could be normal for a 435

[Roger O.]

Re: vacuum system

A leak anywhere in the system can drain the storage tank. One common problem is the check valve. Try this: with the engine running put a clamp on the hose between the check valve and the intake manifold and then shut the engine off.Operate the headlights,if they work better then the check valve was leaking. There are many more tests for leaks but that should get you started.
Many parts suppliers have vacuum troubleshooting guides that you can buy. There are also online sites like Corvette101 that has a vacuum troubleshooting guide but that site starts you out with an incorrect diagram of the vacuum system.
I see troubleshooting the vacuum system as fun so if I can help I will be more than happy to.

[Michael W.]

Re: vacuum system

One of the design quirks of the C3 system is that EVERYTHING is plumbed to the same common source- headlight doors, w/wiper door, even the HVAC. There are no check valves to isolate one area from another. This means that a tiny leak in the heater controls could prevent the headlight doors from raising, as strange as it seems.

Many owners use a shotgun approach to fixing the system which can get very expensive very fast, not to mention frustrating. Test each component separately and be prepared for more than one to be faulty.

[Roger O.]

Re: vacuum system

One of the design quirks of the C3 system is that EVERYTHING is plumbed to the same common source- headlight doors, w/wiper door, even the HVAC. There are no check valves to isolate one area from another. This means that a tiny leak in the heater controls could prevent the headlight doors from raising, as strange as it seems.

Many owners use a shotgun approach to fixing the system which can get very expensive very fast, not to mention frustrating. Test each component separately and be prepared for more than one to be faulty.

Mike,
I don't know about a tiny leak in the heater controls causing the headlights "not" to raise. The tiny leak would have to drain the working side of the system (large hose side) and if it did the trigger side (small hose) would already be in the "open" or "up" mode.
I do agree with you about the entire system being plumbed together , working side and trigger side. I've thought about taking the check valve out and using 2 single hose checks to divide the working and trigger sides and see how that works. The idea seems OK but this system works on balance and with the trigger side being small hosed (tiny volume) a tiny leak and not being balanced by the large volumn of the working side the headlights and wiper door might be popping up and down all the time.For instance turning on the headlights while going up a hill would also make the wiper door come up. I think dividing the system and adding more check valves could mean having to add more reserve tanks. Seems most times I thought I could out think the engineers and improve the system something would always suffer.
( I "have" done OK using a single check valve inside the white filter and I've done OK with putting in hose restrictions to slow down some things.)

[Michael W.]

Re: vacuum system

I was speaking from personal experience. The leak on the HVAC side was one of many, but sufficient in size to push the system over the edge into non-functionality.

[Dick W.]

Re: vacuum system

I used mulitiple pairs of hemostats to crimp off various vacuum lines, testing each circuit individually. Time consuming but works for me

[Roger O.]

Re: vacuum system

A leak is a leak is a leak and they all create issues in the system. The HVAC Ts in usually on the large yellow feed for the wiper relay and is one of the most elusive leaks.

[Roger O.]

Re: vacuum system

I used mulitiple pairs of hemostats to crimp off various vacuum lines, testing each circuit individually. Time consuming but works for me

Isolating different parts of the system is wise as is individual component checks. Here is $8 well spent for isolating leaks. http://www.harborfreight.com/cpi/cta...emnumber=97578

[Kenny C.]

Re: vacuum system

Roger
Your a sick man if you think troubleshooting the vacuum system of a C3 as "fun" I urge you to seek help before you start thinking re packing trailing arm bearing is a good time!

[Roger O.]

Re: vacuum system

Roger
Your a sick man if you think troubleshooting the vacuum system of a C3 as "fun" I urge you to seek help before you start thinking re packing trailing arm bearing is a good time!

I used to chase young women for fun , getting old requires compromises.

[Jack H.]

Re: vacuum system

One of the design quirks of the C3 system is that EVERYTHING is plumbed to the same common source- headlight doors, w/wiper door, even the HVAC. There are no check valves to isolate one area from another.

Hum, I don't believe that's true (no check valves whatsoever)...

The source is the intake manifold. A common vac line runs from the intake to an in-line filter. From the filter, vac runs to a one-in/two-out check valve.

The small diameter outlet runs to the cockpit to branch and service the Control side components of the vac system. The large diameter outlet runs to the vac storage reservoir that services the Servo side of the vac system's components.

If there weren't some kind of check valve in the system, then vac pressure would jump around WILDLY in lock-step to the engine's native vac pressure in the intake (one pressure characteristic for steady state RPM, another for WOT acceleration, and a third for foot-off-the-gas deceleration)...

[Jim H.]

Re: vacuum system

Joesph;
On my '69 a lot of the electrical and vacuum system was messed up. To solve this I got the diagnostic/troubleshooting paper and vacuum schematic from Dr. Rebuild. In addition I have the electrical schematic and a really cool paper from Wilcox Corvettes on how to bench test components (I can email this to you). With these tools and a hand vacuum pump (like you use to bleed brakes) Dr. Rebuild walks you through each component of the "Messed-up" wiper/head light system. My '69 systems work perfect now, but w/o the above resources I'd still be working on the system.

On my car the connections at the headlights were brittle (cut hose about 1/2" off) made new connections; vacuum connections off the intake manifold were incorrect; wiper solenoid burnt (in the center console); over ride switch needed repair; grounds were bad - what a nightmare, but it works now.
Good luck,
Jim

[Roberto L.]

Re: vacuum system

my 68 435 hp when i shut the engine off i have no vacuum to close headlights or wiper door could it be a problem in the storage tank leaking i have bee told the 435s vacuum is lower than a small block and this could be normal for a 435

Sometime ago I made this article, perhaps the images could help.

http://webspace.webring.com/people/ib/bobluis/vacio.htm



Best regards

[Michael D.]

Re: vacuum system

I used to chase young women for fun , getting old requires compromises.

Agreed. Now you have to chase older women for fun, the younger ladies are too quick.

[Kenneth H.]

Re: vacuum system

Roberto,

Your article looks good, and I'd like to use it to troubleshoot my vacuum issues over the winter, but I can't read spanish. Has the article been translated to english anywhere?


Thanks.