nelson71

I have owned my 71 coupe less than a year. All of the vacum components were replaced new by the previous owner. Monday when I drove the vette to work, the wiper door popped up as soon as the engine started. Before it would go up at WOT and go back down again, but this time it came up and stayed up. When I got ready to come home the headlights would not pop up. They eventually did pop up a couple of minutes later. I know very little about the vacum system. Do any of you know what would cause these symptoms?

Jack71

Hi there,

It sounds like you need to roll up your sleeves and dig into the vacuum system because there are obviously some routing problems. I would suggest buying a vacuum hose diagram, or there is good book on diagnosing vacuum problems that most of the vendors sell. Actually, its more like a 15 page paper stapled at the top... but its good.

The vacuum system isn't really that difficult once its explained clearly. Its really two systems, one for controlling when things happen and another for moving the parts.

WOT should not impact operation, that sounds like a vacuum line connected to the carb instead of the intake or something.

Don't be afraid to dig into it! If I can do it you certainly can too!!! Good luck...

Tom Johnson

A leaky check valve will cause the head lamp or wiper door to pop open when you shut off the engine. I would also check the filter. They are located on top of the engine, drivers side. They are connect to each other by a vacuum hose going to the intake manifold.

stingry

Do not buy a book. There are several web sites with great articles on the vaccumn system. Doc Rebuild catalogs also have pictures of the routing. Check for leaks one area at a time. Look under your dash also as tehre are lots of connections for the vaccum under the column. (they are connected to the override switches and the headlight switch)

Grinchia

Try this vacuum diagram:

http://www.stingray.nu/index2.htm

got to Tech Tips and scroll down to the vacuum diagram link.

...the whole site is pretty interesting actually, have a look around while you're there!

-Roy


[Modified by Grinchia, 4:41 PM 4/14/2004]

Martin72

Give this a try....it's for '68 to '72 models

pws69

Mine does the same thing - but I know what it is. There is a "vacuum valve" that is mounted under the wiper door on the passenger side (in the above picture, it is just above and to the left of the Vacuum "motor" or canister). When the wiper arms are "parked", the arm pushes on the valve plunger - this tells the vacuum system that the arms are, in fact, parked and it is OK to shut the door. There is an adjustment on the valve to move it closer to the arm, but my valve is worn so I am out of adjustment. When the door pops up and stays up, I just push the wiper arm down against the valve and the door shuts (quickly, I might add - watch your fingers!). I have a new valve - just need to do it!



[Modified by pws69, 8:00 AM 4/15/2004]

gerry72

The wiper door going up when the engine is started or turned off and at WOT is caused by a faulty checkvalve. You'll find this valve in the in the main vacuum line off the manifold. It has one large nipple on one end and two smaller branch nipples on the other end. The reason your vacuum motors are doing this is that there is no stable vacuum in the vacuum tank.

Your system is running on apparent vacuum which changes based upon the throttle position and engine load. The checkvalve allows the vacuum tank to maintain a high vacuum and act as a buffer regardless of throttle. What's happening is that when you start the car, the tank has no vacuum and draws a vacuum once the engine fires.

There are two separate systems with the vacuum system. The control and the operating system. So now the system is coming under vacuum pressure and the control system (the control system is the command part of the process and tells the motors what position they are supposed to be in) is responding to this condition. The way the control system works is as a fail-safe. It opens the wiper and headlight doors when there is little or no vacuum in the system. When you first start your car, there is very low vacuum in the system (the tank has to be pumped down) for a brief period. There's enough to make the door motors function but not enough to position the actuator valves properly. The door motors have a very large vacuum chamber while the actuators have very small chambers so the motors are more sensitive to prevailing vacuum conditions that the actuators are. So the actuators see not enough vacuum to make them behave in the off condition (high vacuum closes the doors, low or no vacuum opens them, remember the control system opens the doors in fail-safe with little or no vacuum) while the vacuum motors have enough vacuum to operate.

If you replace the checkvalve, most of these problems will go away. You want to re-establish the vacuum tank as the system buffer.

Now, if the headlights are staying up full time that tells you that there is no vacuum going to the headlight actuators. That could be a hose problem or perhaps you pulled the override.

nelson71

Thanks for all of the help! I plan to replace the check valve in the near future. Yesterday I was checking for vacum leaks and after working the overide buttons back and forth a couple of times the wiper door went down and the headlights started functioning properly. When I pull down an overide button, should I hear a hissing vacum sound or not? I hear this with one button but not the other.

My goal is to have everything on the car work. I have already rebuilt the leaky power steering. Next stop....... parking brakes.
:steering:

jpatrick636

An easy way to "fish" for vacuum leaks is with a can of ether. Spay it on the tubes with the engine running, if there is a leak, the engine will rev.


[Modified by jpatrick636, 9:35 PM 4/16/2004]