There have been several questions about vacuum, I ran across the following awhile back about using a vacuum gauge. I thought this might be helpful.

VACUUM GUAGE DIAGNOSTIC CHECKS:

A vacuum gauge provides valuable information about what is going on in the engine for the small investment of purchasing this instrument. You can check for worn rings or cylinder walls, leaking head or intake manifold gaskets, vacuum leaks in the intake manifold, restricted exhaust, stuck or burned valves, weak valve springs, improper valve timing, and ignition problems. Vacuum gauge readings are easy to misinterpret, however, so they should be used in conjunction with other tests to confirm the diagnosis.

Both the absolute readings and the rate of needle movement are important for accurate interpretation. Most gauges measure vacuum in inches of mercury (in-Hg). The following references to vacuum assume the diagnosis is being performed at sea level. As elevation increases (or atmospheric pressure decreases), the reading will decrease. For every 1,000 foot increase in elevation above approximately 2000 feet, the gauge readings will decrease about one inch of mercury.

Connect the vacuum gauge directly to intake manifold vacuum, not ported (throttle body) vacuum. Be sure no hoses are left disconnected during the test or false reading will result. NOTE: Do not disconnect engine sensors or vacuum solenoids to connect the vacuum gauge. Disconnected engine control components can affect engine operation and produce abnormal vacuum gauge readings.

Before you begin the test, warm the engine up completely. Block the wheels and set the parking brake. With the transmission in Park, start eh engine and allow it to run at normal idle speed. Warning: Carefully inspect the fan blades for cracks or damage before starting the engine. Keep your hands and the vacuum gauge clear of the fan and do not stand in front of the vehicle or in line with the fan when the engine is running. (Would not apply to C4’s).

Read the vacuum gauge; an average healthy engine should normally product about 17 to 22 inches of vacuum with a fairly steady gauge needle at idle.

1) A low steady reading usually indicates a leaking intake manifold gasket. This could be at one of the cylinder heads, between the upper and lower manifolds, or at the throttle body. Other possible causes are a leaky vacuum hose or incorrect camshaft timing.
2) If the reading is 3 to 8 inches below normal and it fluctuates at that low reading, suspect the intake manifold gasket leak at the intake port or a faulty fuel injector.
3) If the needle regularly drops about two to four inches at a steady rate, the valves are probably leaking. Perform a compression check or leakdown test to confirm this
4) An irregular drop or downward flicker of the needle can be caused by a sticking valve or an ignition misfire. Perform a compression check or leakdown test and inspect the spark plugs to identify the faulty cylinder.
5) A rapid needle vibration of about four inches at idle combined with exhaust smoke indicates worn valve guides. Perform a leakdown test to confirm this. If the rapid vibration occurs with an increase in engine speed, check for a leaking intake manifold gasket or head gasket, weak valve springs, burned valves, or ignition misfire.

 

Good info, a vacuum gauge is an often overlooked diagnostic tool. Can also be used to time older design engines if your timing marks are suspect and also for other tuning. Thanks for posting.

 

great info! , but what is you have a cammed car with overlap ? like a Katech 110

 

Thanks for posting, John.
Have suggested posting as a 'sticky' to mod.

 

Lsx Revival, I don't know how to answer your question. I would think than the gauge would indicate general areas of concern. The cam you refer to is pretty wild compared to the cams that are most often used in our C4's. With the overlap I would think you would have less than standard vacuum readings. My gut feel is that if you were having issues with your car that had this cam, you would not be trying to figure out the issue yourself but going to a shop know for building race engines. The vacuum gauge is just a tool that is often overlooked that can assist you narrowing down a problem. Most of these gauges will double to test vacuum and pressure. Just another tool to add to your chest with a compression tester, multimeter, a good data logging scanner or computer and torque wrench.

Nothing like bringing up a 11 year old thread