MILEPOSTS Garage - The Online Classic Car Magazine
Everything from Hood Ornaments to Tail Lamps When you properly diagnose and repair vacuum leaks, you get the benefits of:

- A smoother running engine
- Better acceleration and performance
- Improved fuel economy
- Vacuum systems that work as designed
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Vol. 4, No. 8
September 19, 2008

Diagnose and Repair Vacuum Leaks (Part Two)
by Sam Richards
Automotive Mileposts

Image: MILEPOSTS Garage

Image: Troubleshooting/Tech Tips Series

(This is a continuation of Vacuum Leaks article.)

The easiest way to determine if your vacuum system might be causing a performance or other problem is to disconnect it. Locate the vacuum connection that feeds all the vacuum lines and disconnect it. Be sure to cap off the port on the manifold or carburetor so it doesn't leak, and please use real vacuum caps for this, not a piece of vacuum hose with a screw in it or a wad of chewing gum or something, OK? That doesn't really provide a good seal. Don't forget to plug any vacuum going to the power brake booster, as it can certainly leak just like any other vacuum component can.

Once you're certain all vacuum has been disconnected, check to see if the rough idle, hesitation, or overheating has been resolved. If it has, you can be certain there's a large enough vacuum leak to cause the air/fuel mixture to be off. Now, the problem is to find exactly where the leak is located, and the best way to do this is by process of elimination.

Start with the brake booster. Connect vacuum to it again and see if the problem returns. If so, send it out for a rebuild. Often bad brake boosters will be easy to identify because the engine will idle rougher when the brake is applied, and smooth out somewhat when the brakes are released. This is not always the case, but frequently it is. Once the booster has either been eliminated as a suspect, or the line has been capped off again if bad, go to the main vacuum connection that you plugged off and reconnect the vacuum system. Is the engine running better now? You may have just found your biggest vacuum leak, but there could still be others, which you can locate by isolating individual vacuum components to determine if they are operating as designed.

There are a couple of things you're going to need to perform these tests:

1. Vacuum pump with gauge (hand operated)
2. Assortment of various sizes of vacuum caps
3. Vacuum gauge (separate from the pump)
4. Supply of 1/8", 3/16", 1/4", 1/2," and 3/8" (internal diameter) rubber vacuum tubing*
5. Factory vacuum diagrams (sometimes included in shop manuals)

*Keep in mind you may not need any vacuum tubing at all, but the most popular sizes are listed.

The hand held vacuum pump (like a MityVac brake bleeder) will help you troubleshoot most of the system. For larger reservoirs (like the headlight cover reservoir), you'll need a vacuum gauge that can be placed inline so you can let your powerful engine pump things up for you. You just need something with a gauge on it so you can readily see if there's a leak in the component you're testing. Before you get started, test your vacuum pump itself to make sure it will hold vacuum. Just cap it off, pump it up to 18 psi, and lay it down for 5 minutes. The needle shouldn't move at all during this time. If it does, either your pump has an internal leak or you have a bad cap.

With a good vacuum pump, you'll want to isolate each vacuum component in the system to identify any leaks. Start with the vacuum reservoirs. For large ones, like the headlight cover reservoir, you'll need to use a separate vacuum gauge so you can allow the engine to pump up the reservoir for you. Ford's written policy on normal vacuum leak down rates was no visible loss of vacuum indicated on the gauge for a period of one minute. That's it, just one minute. And this applies to individual components, not the entire vacuum system tested at one time. In theory, vacuum components should hold vacuum much longer, and remember that some of them, like spring loaded door or vent motors, will use some vacuum constantly to maintain the position of the door or vent, which makes them more difficult to test.

Trico, which was the original equipment manufacturer (OEM) for many vacuum components for Ford, indicated in their original quality control documentation that a vacuum component can show a sustained reduction of 5" of mercury over a 60 minute period to pass quality control standards.

Once you've checked all the vacuum reservoirs to determine their performance, test the check valves. Normally, you can just blow through them or suck on them to determine if they're still one way. If you can blow or suck in both directions, (even if it's just a tiny little bit) it's no good and must be replaced. The original life expectancy of check valves was only a couple of years, so if your original one has made it this long, you've done well.

Next, start checking vacuum connections: at reservoirs, inline tees, three way connectors, four way connectors, check vacuum caps, connections at vacuum switches (door lock, trunk release, rear vent, etc.), to make sure they're tight. Try to twist the rubber vacuum line on the connector. If it twists, it's too loose and needs to be fixed. Sometimes, cutting back an old, dried up hose an inch or so will be good enough, that is if there's enough extra hose that you can afford to shorten it a bit. Sometimes, this trick will only work for a short time. If at all possible, especially under the hood where heat is the enemy, replace the entire section of vacuum line with new. It will save you time in the end.

Be sure to inspect all plastic vacuum connectors for tiny cracks, including the ones that may be up under the instrument panel, inside doors, behind the back seat, etc. Plastic can dry out with age and exposure to heat.

By this time, you've no doubt discovered a couple of leaky connections at least, and more than likely a bad vacuum motor or check valve. This is a time consuming and tedious process, and you might need to spread it out over a couple of days or a weekend or two. Just remember, that with every leak fixed, that's just that much less vacuum your engine must compensate for. When you have a vacuum system that works as designed, you'll find your engine runs better, you'll know your gas mileage isn't suffering due to vacuum leaks, and when you want to use a vacuum accessory after the car has been shut down, you'll be thrilled to discover it will still work!

Note: While most vacuum systems such as the door locks and trunk release should hold vacuum for a week or longer before running out through normal leak down, the Ford shop manual specifically states that the normal leak down time for concealed headlight covers is overnight (approximately 12 hours). Cars with just one headlight cover vacuum motor may last a bit longer, as might cars without the internal springs in the headlight motors, but it's been our experience that if your headlight covers still have vacuum holding them down after about 18 hours or so, your system is doing pretty well. Automotive Mileposts has documented that when these classic cars were new, it was not unusual for some of them to retain enough vacuum to keep the headlight covers closed for a week before the springs started to open them.

If you have advice, tips, technical ability, or just know a secret or two about old cars, and you'd like to contribute, click here and tell us about it. We'll help you write it, and give you the credit for it! It's the perfect way to help out your fellow enthusiasts in the old car hobby.
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