Vacuum pressure

[Les W.]

My LT-1 w/ac holds vacuum for about 40 seconds after shutoff. Therefore I assume I have a vacumm leak somewhere, but I would like to know what is the standard vacuum pressure supposed to be, before I start the hunt for the leak???

[Jack H.]

Re: Vacuum pressure

I don't think you'll find a factory spec on how long the system should hold vac and/or a pressure profile with respect to time... Most cars will bleed out sitting overnight, but I've seen a few (the exception) that will retain sufficient vac overnight to actuate, say, the headlights.

I agree with your presumption that 40-seconds after engine shut down is kind of short and you most likely have one or more leak points. Notice the 'or more' disclaimer. My experience is there's generally more than one point of vacuum leak on these older cars.

So, once you start to trouble shoot the system, don't stop with the first leak point you find! Step through the whole system...

[Les W.]

Re: Vacuum pressure

Thanks Jack, I'm working on the PV and the time is too close for the 30 sec. headlights down function. I am curious as to a range the manifolds vacuum output should be while the engine is running. I agree that one leak find may not be the end. I have sent for some info on the best way to start the search. Any advise on what make and model of a vacuum gage/pump along with acessories to have to do the testing?

[Domenic T.]

Re: Vacuum pressure

Les,
Look at the 1 way check valves in your search.

DOM

[Pat M.]

Re: Vacuum pressure

Les, as has been recommended on here before, if you can use another running vehicle some distance away to provide vacuum to your non-running Vette (connect one of the former's vacuum hoses to the Vette's manifold vacuum source hose) you can actually hear some leaks in your Vette's hoses, relays and/or actuators.

It's worked for me more than once, and was much more efficient than blindly checking everything.

[Michael W.]

Re: Vacuum pressure

Les,

Intake vacuum is usually in the 10-14 inHg range but that has little relevance to your problem. As Jack correctly pointed out, there are usually multiple leak points that cause the system to fall flat in so short a period of time. Random replacement of parts and guessing rarely gives good results.

Here's a good troubleshooting guide:

[Les W.]

Re: Vacuum pressure

Thanks everyone, I'm in the process of establishing A toZ method to find the leaks. I need the correct gages and pumps to do the tracing. Any suggestions on the right gage, pump and accessories that are needed?

Les

[Terry M.]

Re: Vacuum pressure

Thanks everyone, I'm in the process of establishing A toZ method to find the leaks. I need the correct gages and pumps to do the tracing. Any suggestions on the right gage, pump and accessories that are needed?

Les

Tools are not too tricky, the skill to use them is another matter.

I got an old Snap-on vacuum pump that was used for AC work, however back in the days of the old TDB I posted instructions on how to make an old window AC unit into a vacuum pump. The Snap-on pump came from a swap meet. It was "contaminated" with R12 and when the world went to R134a most AC shops divested themselves of the R12 equipment that can not be used with the R134 -- cross contamination is a no no. $20 bucks for the Snap-on pump pictured below. I made the window unit pump for about $5 (for the fittings) as I remember. Either will run nice and quiet so you can hear the vacuum leaks.

You can buy a vacuum gauge at any of the "big box" auto parts stores. Which ever is in your neighborhood, (called FLAPS here = Friendly Local Auto Parts Store) or look up "vacuum gauge" on-line. Most will come with a kit of adapters for different size hoses.

I found a pair of long nose pliers and a pair of long nose vice-grips useful along with an assortment of plastic plugs and caps of different sizes. The plugs and caps can come from the FLAPS like the vacuum gauge.

Now what to do with them is your challenge. That will take more keyboarding that I want to do, but the link posted above is a good start.

[Les W.]

Re: Vacuum pressure

Thanks Terry,

The Snap-On pump makes a lot of sense since I don't have the availability to a mule car for vacuum. Do I need to regulate the amount of Hg pressure when using a powered pump such as the AC evac unit? I sure don't want to over pressure the vacuum system. Or is that not an issue or concern??

Les

[Terry M.]

Re: Vacuum pressure

Thanks Terry,

The Snap-On pump makes a lot of sense since I don't have the availability to a mule car for vacuum. Do I need to regulate the amount of Hg pressure when using a powered pump such as the AC evac unit? I sure don't want to over pressure the vacuum system. Or is that not an issue or concern??

Les

I did not when I used it, but then the systems I was working on were anything but tight -- that's why I was there. Remember too there is a vacuum reservoir (driver's inner fender for 1968-1972 and part of the front bumper for 1973 up to some time in the later 1970s.) to draw down. These pumps do not move a very great volume of air (vacuum??) so I think over suctioning is not an issue.

As I was thinking about the widow AC unit re-purposing I think I printed the "how-to" in [i]The Restorer[/b] back in 1999 or 2000.

[Michael W.]

Re: Vacuum pressure

Les, the system sees up to about 15 in/hg in service so you'd need something in that range. Do the tests on the individual components using a hand pump before worrying about the entire system.