Digging Into The Internal Engine Void: Handheld Vacuum Gauges
Wayne Scraba
A time tested means to determine the internal condition of an engine is by way of a good old fashioned vacuum gauge. Vacuum gauges aren’t that expensive (there are all sorts of options out there for as little as $30) but the information gained from it can prove invaluable in troubleshooting.
For the uniformed, here’s the basics: A vacuum gauge reads the level of vacuum that has built up inside the intake manifold while the engine is running. A number of engine and vehicle accessories operate off vacuum and this includes things such as power brakes, distributor vacuum advance and in some older cars, even the windshield wipers (vacuum wipers are definitely forgettable). The vacuum gauge connects to the intake manifold or between the manifold and one of the accessories that uses vacuum to function.
All vacuum gauge read engine vacuum in inches of mercury. Inches-of-mercury is typically abbreviated as “ins. Hg” or simply “Hg”. Ins. Hg measurement was initially developed by placing liquid mercury in a glass tube and marking the tube off in divisions. A vacuum applied to one end of the tube caused the mercury to rise or fall in the tube a specific distance. A barometer works on this same principal. That’s how it was developed, and there are all sorts of different vacuum gauges, but most inexpensive jobs available today use some form of bourdon tube or diaphragm that changes position in response to changes in pressure. But essentially, any good vacuum gauge will be clearly marked and easy to read. Using a vacuum gauge for troubleshooting is most often accomplished while the engine is at operating temperature and at idle.
When working with a vacuum gauge, a normal reading will be quite steady at idle however valve overlap (valve timing) will show some needle movement. “Pedestrian” cars with mild cams and tune ups will more likely have a reading of 18 ins. Hg or higher. Not so with big cam combinations. They might be 8 to 10 inches lower at idle. We can’t give you an exact baseline vacuum reading number because all cars and engine combinations will be different.
Back to the basics: A leaking valve will often show a reading of 1 to 7 (or so) ins. Hg. This will occur at regular intervals each time the leaking valve attempts to close at idle. On the other hand, a sticking valve can be identified by a rapid, intermittent pointer drop each time the sticking valve is intended to close
If the needle drifts slowly back and forth above the normal reading when the engine is idling the air-fuel ratio mix is often too rich. A lean mixture will cause the gauge to drop slightly at irregular intervals. If there’s a manifold leak present (vacuum leak), the gauge pointer will drop to low levels (below the normal for your particular engine) and remain steady. This reading could be as much as 10 ins. Hg lower than your baseline norm.
When setting idle mixture in a carburetor, common practice is to use a vacuum gauge as a setting “tool”. Typically, most folks set one mixture screw to the highest vacuum reading, then set the opposite mixture screw (and if it has four corner idle, repeat here too) and finally enrichen the mixture on each screw a very slight amount. Usually, this process results in a slightly higher idle speed, so you’ll have to adjust the idle speed screw to compensate.
Should you encounter a low, steady vacuum reading at idle, this can indicate two things: Retarded ignition timing or the valve clearances (lash) are too tight. If you advance the ignition timing in this situation, and the gauge reading increases it’s a good sign the timing the was set incorrectly. If the reading doesn’t change with an increase in ignition timing, then there’s a good chance the valve timing is out of whack.
That’s not the end of it either. There are numerous other things you can diagnose with a vacuum gauges, including work valve guides, defective rings, bad valve springs, exhaust system restrictions, blow head gaskets and more. Going through all of them is beyond our scope here, and in some cases, there are other tools better suited to the specific diagnosis.
So what’s available when it comes to vacuum gauges? As pointed out elsewhere, there’s a wide range of vacuum gauges out there. Price points vary but for the most part they’re all affordable. They’re a must have for any racer or hot rodder. For a closer look, check out the accompanying photos and captions.
