Diagnosing Problems with a Vacuum Gauge
Mark Simpson
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A vacuum gauge is a valuable tool in diagnosing problems with your car. From poor performance to bad braking, the vacuum gauge can help you find the root of many problems. Mark Simpson demonstrates how to perform an accurate engine vacuum test.
A gasoline engine is essentially a big air pump. It sucks in air it combusts fuel and then expels it through the exhaust. And in this process of acting like a big pump it creates a vacuum in the intake. And the vacuum source on a car can give you a good indication of the health of the engine. A good steady vacuum of 18 to 21 inches of mercury will indicate that you have a good solid motor while other indications might tell you something's wrong.
So with this car it kind of had a little bit of trouble with the braking and because our hydrovac unit on our braking system is operated by vacuum, we're actually gonna start checking the braking system by actually doing a vacuum check. We've got our vacuum hoses and our vacuum gauge hooked up. What we did is we actually drew our vacuum off the hydrovac the power brake booster that is a good source of vacuum. So we know we'll be getting a good test reading here with the gauge. After we got that hooked up, we then did a visual inspection of all the hoses and plugs on the carburetor and any other possible sources of a vacuum leak.
And with a good visual inspection you'll see right away if you have any issues. The next thing we can do then is start the engine and then see what our numbers are. Go ahead and start it up. Very good shut it down. Okay as you can see we had a good solid 19 to 20 inches of mercury.
Right where we wanna be. Even though this motor has got a hundred thousand miles on it never been opened up never touched, pulling good vacuum, good solid motor so we shouldn't have any problem with our braking system. And, we know that we've got a good source of vacuum for that. The vacuum system in a car can tell you a wealth of things about how the engine is. Whether your timing is late, too advanced.
If you have stickier burnt valves. I've put together a chart that shows basically what each of the readings indicate. You can get that at classiccarrestorationclub.com.
Good video and nice chart. Downloaded the chart and printed it out. Going to have it laminated.
What about a flathead Mercury motor, master cylinder is under floorboard?
Shouldn't the needle be steady? It looked in the video as if it was shaking a bit. I am in the process of testing mine and at the beginning mine was shaking as well, but I had most of the vacuum hoses connected. Then my friend recommended to test it again after caping some of the vacuum ports and I got a steady reading.