Checking the vacuum on your old car is an important part of a thorough, but most of the average guys don’t bother, or don’t have the gauge. Most often if you do have the gauge, you might not be exactly sure what the reading might be. For the recorded I’ve never had a car where the readings were stable. Normally you’ll get a fluctuating reading that move around. Below are a few ‘ranges’ and what they may indicate.
Standard Vacuum Gauge
These are for V8’s only:
– A good reading will be between 15 and 22 in. hg (a steady needle). Closing the throttle you should see a 5 notch drop and reopening it should settle back previous span (15-20).
– If you get a reading where the needle has a swing 4-5 in. hg ethier side of normal, you are looking at a possible simple carb adjustment.
– If the reading is higher than normal you need look at replacing your air filter or check to ensure your choke is not stuck.
– Low reading would be fairly steady reading around 5 hg will have you looking for a vacuum leak on the intake/carb area. Look for poor connections at the carb or any “T” or “Y” or straight though connection. Baring that look for cracked hose. You know I hate vacuum leaks. I recently restored a 1984 corvette with the Crossfire injection set up. Rather than chase down a single spot, I just replaced them all. That was a lot of vacuum hose and tucked in some stuff spots. But you are working on something bit older, like my 1970 Mustang, replace it all, it’s going to go back sooner or later, especially with the kind of temps we have out here in the Southwest.
– If you get a reading that between 8 and 14 you are looking at a timing issue at the least to leaking piston rings at the worse. Do a compression check for the leak and use your timing light before you tear it apart.
– If the needle is swinging (unsteady) between 14 and 19 hg that indicates valve guide problems. That going to be a valve guide problem.
Thanks for reading
Tim
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yep that was the old school way…use to even drive by vacuum gage….
I’d like to add that a 15hg reading a sea level is low on a stock cam engine. High lift cams will result in low vacuum readings but that’s not a bad thing on those engines. Also if you have one installed inside the car you can drive for economy by always trying to keep the reading as high a possible. Back in the 60s Pontiac included the vacuum gage with the optional instrumentation package. The vacuum gage is your friend come tune up time. I was able to get 22hg at idle where I live (2,300 ft) with a very smogged 82 Cougar with the 200 six. If you don’t have one, get one. When measuring vacuum always check at a carb point below the throttle plate or manifold vacuum. Ported vacuum sources (above the throttle plate won’t give an accurate reading.
Very good info Tim.
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