1970 Corvette Vacum Problem

[Bill Anderson]

I would appreciate any assistance in fixing a vacum problem that I am having with my 1970 Corvette. When I bought the car the headlight vacum system was working fine, but the wiper door would not open. I replaced the check valve that comes of the manifold and leads to the wiper and headlight system.
I then replaced the wiper system vacum relay and the system began to work.
The problem now is that when I start my car the headlamps come up until I rev the motor for a bit and then they close and never come back up until I open them with the switch. Then when I open my wiper door it stays open and will not shut when I use the switch or bypass switch to close it. I am 100% sure I have all the hoses in the correct order.
Thanks in advance for any assistance.

[Pat Bush #35083]

Re: 1970 Corvette Vacum Problem

Bill:

Couple of thoughts on your problem:

1) Peruse the archives on this topic. There are hundreds of posts on vacuum troubleshooting. Your problem is the RULE not the EXCEPTION! You may find a bit of additional guidance there on this particular problem.

2) The only way I have successfully been able to nail this problem is step by step, hose by hose, component by component. There are way too many things going on to try and troubleshoot the whole thing at one time. You can do this with a bit of methodical determination, patience, and a little time. Try the following:

A) What I usually do is start at the manifold with a vacuum gauge and start taking readings. It is not so important what the actual readings are although somewhere in the 25-30Hg area usually is acceptable. I do this with another wise old sage's recommendation of using a "mule car" to provide a vacuum source so you can hear any leaks. The entire system should be able to sustain this vacuum even when the engine is turned off (for several minutes anyways). If not, you have a vacuum leak somewhere.

B) Isolate the systems and troubleshoot each one individually if it ain't something obvoius. To do this, make a DRAWING (or get the routing kit info from Dr. Rebuild) of all the hose locations and connections. Then isolate the systems and troubleshoot each one looking for leaks or bad components. I usually have cheater hoses to route around in the engine compartment.

3) Based on what you have said, I would think the actuators for both systems are OK since they do open and close. I would check the vacuum resoivoir cannister to see that it does not have a leak and all the hose routing is correct first. VERIFY THAT IT IS ALL CONNECTED PROPERLY USING A RELIABLE DIAGRAM. Simply because you hooked it up "right" the way you bought it does not mean some other "mechanic" wasn't there before you and fiddled with it!

It sounds like a leak -- and also don't forget a vacuum leak anywhere else can cause problems in this system as well. Check everything on the carb, distrbutor, AIR, power brake booster, and PCV set-ups as well. It is possible that you inadvertently disconnected something while doing your repair.

4) If all that stuff checks out, then take a good hard look at the relays. You have 3 -- 2 in the front for the headlights and one in the engine compartment for the W/W.

5) If everything else checks out OK, then you have to look under the dash. There are a couple of tees, several hoses, another wiper relay (electric/vacuum), the emergency switches, and the headlight switch to look at. If it is an AC car, also the venting control feeds. I usually focus on "things coming through the firewall first". I have not found too many problems under the dash simply because the hoses and relays are not subjected to the kind of heat and temp variations you see in the engine compartment but it is a 32 year old car.....

Let me know how you make out and if I can help any more, send me an email.

Good luck -

Pat

[Wayne W.]

Re: 1970 Corvette Vacum Problem

The most comon problem is the canister seals, both on the headlight and wiper door canisters. Check them first. The next problem with the wiper door is the delay valve that is located under the wiper grille on the RH side.

[Jack H.]

Plus,

poster should NOT forget to replace the MOST COMMON wear item in the system--the in-line filter between the check valve and the intake mani. You'll find GM called for this to be replaced about once a year and most owners NEVER service it! You can generate 'winking' headlight & 'dancing' wiper door syndrome with a filthy element in this filter pretty easily....

[Jim T.]

Re: Plus,

Jack is the inline air filter still available from GM?