vacuum question

[rick spears]

i have a 69BB, and am having trouble getting enough vacuum to operate both headlight doors and wiper door...i have looked for leaks, and replaced hoses to no avail. is it possible that this cam doesn't generate enough vacuum? how does timing affect vacuum, is it possible that i have it advanced too far? thanks, rick

[William C.]

Re: vacuum question

Firstly, what is the measured vacuum from a tap on the intake manifold at idle? That will give a start on sorting out the problem.

[Mark #28455]

if the vacuum at the manifold is ok

Another nuisance vacuum leak occurs when the big metal vacuum reservoir can in the driver's fender CRACKS (Aargh!)in the area of the rolled in reinforcing ribs - mine cracked on the backside that bolted to the skirt on the 1970. Prior owner "fixed" it with epoxy. I brazed it for a repair (and added extra brass) works fine now x 5 years. Found the same problem on one of my 1969's too.
Mark

[Joe L.]

Re: if the vacuum at the manifold is ok

Mark------

Yes, what you describe is not a rare occurrence. However, the tank can be a bear to remove to inspect, but it's easy to check with the tank installed in the car. The 68-69 tank has 2 vacuum fittings; the 70-72 has 3. So, all one needs to do is to temporarily, but securely, plug all but one of the fittings. Attach a vacuum pump to the remaining fitting and evacuate the tank. Then, see if it holds the vacuum over a reasonable period of time. If there is any sort of vacuum leak, the vacuum will bleed off rapidly. If it holds vacuum, it's fine and one doesn't need to go through the effort to remove it. If it doesn't, then the "fun" begins.

[rick spears]

Re: if the vacuum at the manifold is ok

thanks guys...bill you asked about my vacuum at the manifold..i just measured it, and i have 14in at low idle...almost 20in at fast idle. does this help trouble shoot at all? thanks, rick

[Joe L.]

Re: if the vacuum at the manifold is ok

Rick------

You should have plenty of vacuum to operate the system with the amount of vacuum you report.

[rick spears]

Re: if the vacuum at the manifold is ok

thanks joe, well, i guess i'll start lookin elsewhere. why is the resovoir so hard to remove? doesn't appear to be too buried....rick

[Joe L.]

Re: if the vacuum at the manifold is ok

Rick-----

I agree with you. It looks like it will come right out without any problems. Usually, it's not that easy, though. It's especially a problem with cars with power brakes. Without power brakes, it's not as bad. But, it's still not as easy as it looks.

The easy testing procedure eliminates the need, though, to remove the tank unnecessarily. You sure don't want to remove it if you don't have to.

[Jack H.]

Re: vacuum question

Have you checked your in-line vac filter? Few ever bother to replace these and the system was designed with expectations the filter would be replaced at least once a year... When the filter plugs, it restricts air flow and the time necessary to store sufficient vac in the reservior expands.

Remove filter from attached hoses, hold it up to a light source and sight through the nipples. If she's snow white, you're good to go. If she's black as coal, it's time to replace. If you can see right on through, it's PAST time to replace as the filter element has been sucked out (guess where it went?)...

[Guisseppe]

Re: vacuum question

Check this: http://www.corvette-101.com

[Nick K.]

Re: vacuum question

Ric

Have you put a Vac gauge on your engine and checked the reading? Could be if the engine has been modified with a more radical cam it will not produce as much vac at idle. Also check your vac advance to make certain you have the correct one. If at idle your vac advance is not functioning properly your timing at idle will not be advanced enough and could be causing a low vacuum conditon which could be interpreted as a vac leak.

Nick #33307

[D S.]

Re: vacuum question

Welcome to the club. I know that is no help but I, too, am experiencing the same dilemma and also a big block. The engine is a fresh rebuild and I have replaced actuators, switches, hoses, ran vacuum checks from the A/C control to the firewall to the front of the car. The right headlight refuses to pop up without assistance and the wiper door refuses to open.

[William C.]

Re: vacuum question

One more thing to check is put a vacuum gauge inline with the storage tank side of the ststem, run the engine and check the vacuum at the tank, it should be very close to that at the manifold. Then shut off the engine and check the rate of decay in the vacuum system A proper system should hold for a LOOOONG time. When I had my '72 it would set for a good half hour and still snap the lamps up and down with the engine off. If the vacuum holds well, look for a valve in backwards, or check the individual actuators to see if you have an actuator problem. If the vacuum decays rapidly, find the leaks.

[D S.]

Re: vacuum question

Bill, thanks. I am working hard at this with a vacuum pump and determination. All of the actuators, relays, and switches are new. It's only near 100 here in Texas and the shop isn't air-conditioned. I am half way though the vacuum system and working my way to the tank. I am also pinching off hoses at one end and performing the vacuum pressure test at the other to check for leaking hoses. Even uder the dash to those plenum actuators. We checked the reservoir last year and it held okay. Maybe this year it developed a leak but I have another reservoir just in case.

Scott

[William C.]

Re: vacuum question

You may be making it harder than it needs to be, my question is does the whole system hold a vacuum, if so, you have aproblem other than leaks. If the whole system does not hold, then you have a leak, valve installed backwards, or ...?