Note: This is the second of several articles on marine engines written by Bill Hancock, a member of ACBS and the Sunnyland Chapter since 2007. 

Engine Health 101-Vacuum Gauge Diagnostics


In this age of electronic diagnostic devices, one of the best devices to check your engine’s health is still the trusty old vacuum gauge. The vacuum gauge has been around forever, and while decidedly low tech and inexpensive, these simple gauges offer a quick way to check our engines, which in many cases, are also old and gloriously unsophisticated.

Let’s begin by understanding what we are measuring. Depending on where you are located on the planet, the barometric pressure varies considerably. For our discussion we will use the standard figure of 14.7 Pounds per square inch or psi, which corresponds to 29.92 inches of mercury (in-Hg).

What does 29.92 in. Hg mean? if we are sitting on our porch at sea level, there is 14.7 psi of air pressure surrounding us. Another way of looking at this is to visualize a one inch square column of air reaching into outer space. This tall column of air would weigh 14.7 pounds. Hence in everyday life we deal with two pressures; Gauge and Absolute. Gauge pressure measures the pressure above atmospheric and is often referred to as PSIG. Absolute pressure measures the pressure above or below what we are experiencing and is designated as PSIA. Absolute pressure is used in scientific calculations and as a reference for devices such as an altimeter or barometer.

When we open our mouth and expand our lungs, the atmospheric pressure surrounding us forces air into our lungs. It is the same for our engine. Learn to think of the intake stroke in an engine not as producing a vacuum but rather creating a lower pressure void inside the cylinder. Air is pushed from the atmosphere into the cylinder to fill this void. Most people refer to the intake stroke as creating a vacuum. Whatever you decide to call it, we can utilize a vacuum gauge to measure and the pressure fluctuations, and from there, draw some conclusions which will lead us down the diagnostic trail.

Let’s begin by examining the equipment. In this case we have a simple and inexpensive vacuum gauge setup which measures vacuum inside the intake manifold plenum. These gauges are relatively inexpensive and can be found online, at auto parts stores and at discount tool stores. I would recommend a combination vacuum and pressure gauge shown in Fig 1  so you will have the capability to check not only the manifold air pressure (MAP) or vacuum as the non-engineers call it, as well as the pressure of your fuel being delivered to your engine. When you purchase a gauge, make sure you get a kit which contains a flexible hose as well as an assortment of connectors which will allow you to connect the gauge to the intake manifold or fuel pump.

Fig 1. A compound vacuum/pressure gauge. These can be found at auto parts stores or tool specialty stores.

Suffice it to say, if your engine is worn, it will not create a proper vacuum nor will it correctly seal the compression and combustion pressures. This wear will manifest itself in lowered output and generally poor performance. The beauty of a vacuum gauge is that it will allow you to more easily pinpoint which component/s are faulty or at least suspect. In most cases, unless there is an early individual component failure, such as a broken piston ring, we are looking at an overall depreciated output due to wear. In the case where a single cylinder may have broken a ring or a burnt valve, it can be more difficult to isolate the location. If you have been reading this series, you will remember that this is where we may need to use a cylinder leakage checker or compression checker to further isolate the problem.

The output of our engine is modulated by the throttle. The throttle position determines the amount of air admitted into the engine. Think of the throttle as a trap door which swings open to a room. At idle, the door is only open a few degrees and admits only enough air to sustain idle rpm with no load. Once we pull away from the dock, we open the throttle, letting in more air, which, when combined with proportionally more fuel from the carburetor, produces more power and ultimately allows the boat to go faster. Let’s hook up a vacuum gauge to the intake manifold and start examining the pressure inside the intake manifold.

Once we have our gauge, select a vacuum line which will allow you to use a “Tee” fitting and tap in to the plenum of the intake manifold to measure the vacuum signal. Pick a vacuum line entering the area of the manifold which is near the carburetor or throttle body. Do not pick up a vacuum signal from one of the individual passages leading to a aprticular cylinder in order to avoid measuring only one cylinder. Do not disconnect the original vacuum line, but rather use a Tee fitting and tap into the line so you don’t disable the function of the vacuum line as designed. Be sure to go as close to the inlet manifold plenum as possible. Try not to connect your Tee near some component, which in some cases, may be receiving a refined or modified vacuum signal. If your engine has a positive crankcase ventilation (PCV) system, block off the PCV valve so it does not create a leak and influence your test.


Positive Crankcase Ventilation. PCV was introduced on cars in the late 60’s as a way to control crankcase emissions. PCV works by essentially using engine manifold vacuum to draw the crankcase fumes, or blowby as it is often called, from the engine and then route these fumes through the intake system and into the cylinders where the harmful blowby is essentially  burned again during the power stroke, and then exhausted through the tailpipe. By doing this, much lower levels of harmful emissions are achieved, thereby reducing air pollution.

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Once we have attached the vacuum gauge to the intake manifold plenum we begin by warming up the engine to operating temperature. At a steady idle, you should show a vacuum reading of between 17-21 in-Hg. If the reading is smooth but significantly lower, you probably have worn rings. While the engine is still idling, grab the throttle and quickly open and close the throttle. The pressure should jump from 25-to 2 in-Hg and back to normal. If the vacuum only goes from 22 to 0 in-Hg and back, this will point toward worn rings.

Next, at a steady idle, a steady reading of between 8-14 in-Hg would point toward incorrect valve timing. Vacuum leaks or poor compression can also produce a low vacuum reading. If you see between 14-16 in-Hg, check the ignition timing.


Ignition timing-often called spark advance, is measured with a timing light. When connected to the #1 spark plug wire, this special battery powered light flashes (see Fig 2) when it is triggered by current passing through the spark plug wire. Timing is checked by aiming the timing light at the harmonic damper or flywheel of the engine where the timing marks are located. During the flash, the timing marks appear to stand still and you can read the value. This becomes a big problem if the timing marks have either moved or have been incorrectly marked when the engine was assembled. Sometimes, simply using the incorrect parts, such as a damper or timing cover, will result in a timing error. If need be, have a knowledgeable mechanic check your timing marks at Top Dead Center-TDC to verify they are correct. Dampers, especially on older engines have been known to slip the outer ring and lose their alignment when the rubber portion gets old and brittle. Luckily few of our marine engines use automotive dampers.

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Fig 2- A battery powered timing light made by Snap-On Tools.

Our next test will be a simple non-firing test. Disable the ignition so the engine will not start. Typically, we do this by disconnecting the coil wire from the coil. Remove the air cleaner element then back off the throttle stop or idle screw adjustment until the throttle blades are completely closed. Close the idle air bleed screws if so equipped. Remember to count the turns on both the idle stop screw as well as the idle air mixture screws so you can restore the settings after the test. Next, if your engine has a PCV (Positive Crankcase Ventilation) valve, block the valve so it is inoperable otherwise it will influence your readings. Remove the oil dipstick and plug the dipstick hole with a tapered rubber plug to prevent any leaks.

Engage the starter and spin the engine over, while watching the vacuum gage. On a good engine, you should see approximately 10 in-Hg. of vacuum. The reading should be steady and not fluctuate.

If your engine produces less, look for a leak such as a loose fitting or leaky intake gasket. Leaky vacuum lines or fittings, or incorrect valve timing will cause this value to be low. In the case of improper valve timing, check the timing marks on your camshaft gears to determine if your camshaft is properly installed. If the cam is installed misaligned with the crank by one tooth, the engine will typically run, just not very well. Worn throttle shafts in the carburetor can also create a leak. It is also important to have a freshly charged battery and be able to spin your engine over at a normal starting RPM.

If the vacuum reading fluctuates or pulses, you probably have a problem with an individual cylinder or possibly a few cylinders. This could be a stuck valve, a burned piston, broken rings or worn valve guides.

Here we should note that on 6 cylinder and 4 cylinder engines, you will experience some pulsing due to the fewer number of cylinders. A V8 will produce a relatively stable vacuum reading.

Before the next test, you will need to restore the engine to running condition. Once you have restored the settings and reconnected the coil wire, start the engine and adjust the idle mixture screw/s and reset the idle RPM then let the engine warm up. After the engine is warm, read your vacuum gauge. You should see a steady reading of somewhere between 15-22 in-Hg. at idle speed.

If the needle is rapidly fluctuating between 14-19 in-Hg, it indicates worn valve stems or valve guides.

If the needle fluctuates or drifts by a few PSI, increase the engine speed to somewhere between 1500-2000 RPM. If this cures the condition, look for carburetor adjustment at idle. The other area which can affect the vacuum is ignition or spark timing problems. Check to make sure all the cylinders are firing properly. Use the timing light and check the spark timing. An inexpensive hand held infra red thermal gun (Fig. 3) aimed at the individual exhaust pipes can give you a quick indication of a misfiring or weak cylinder which will produce a markedly lower temperature.

Fig 3-This digital infra red thermometer with a laser pointer works well to pinpoint temperatures of various components, and locate a misfiring or dead cylinder. They are relatively inexpensive and can be found at auto parts stores or at home improvement stores.

If the fluctuations are larger, (5-7 in-Hg for example) look for a more serious issue such as broken or weak valve springs. If the pulsing gets shorter and more rapid, check for carburetor or manifold leaks or sticking valves.

If your engine uses a vacuum advance distributor, it is simple to check the vacuum to the distributor. Use a tee fitting and plug it in the line just before the distributor. If there is little or no vacuum at idle, you may have a plugged or broken line. This will limit your vacuum advance and greatly affect your performance. Once you have confirmed the vacuum reading, remove the gauge and install a timing light and start the engine. As you rev up the engine watch the timing move as it advances along with the RPM. This will confirm that your timing is advancing.

Our next test, while not frequently a problem with marine applications is nonetheless important. With the boat in neutral, slowly run the engine up from idle to 2000 RPM. The vacuum should be higher at 2000 than at idle. If the vacuum is lower at high rpm, then you may have a restricted exhaust system, which will produce increased exhaust back pressure.

Next, we will check for a blown head gasket. At idle, the gauge needle will fluctuate back and forth, if the head gasket is blown between two adjacent cylinders, the decreasing vacuum drop will be much more noticeable. Again, you will want to verify and pin point the location with a cylinder leakage test.

These simple inexpensive tests will serve to pinpoint engine internal problems in a simple cost effective way. They can be performed in under an hour and for very little expense.  If nothing else, you can have some idea of what you may expect to hear from your mechanic. If you do not have the specialized repair equipment, find a trusted mechanic who is set up to do the work. Remember, it is usually cheaper to have somebody do it right the first time than try to pay somebody to repair a poor or mangled attempt at a repair.


    • Nice article, but the definition of absolute pressure is incorrect. Absolute pressure (PSIA) is not pressure above or below what we are experiencing. That is what Gauge pressure (PSIG) measures. Absolute pressure is the pressure measured from an absolute vacuum – i.e. from a pressure of absolute Zero. (PSIA is to pressure as degrees Rankine is to temperature.) Hence, absolute pressure will always measure as a positive number. Gauge pressure may measure as either positive – a pressure above the pressure of the atmosphere we are experiencing; or negative – a vacuum, a pressure below the pressure of the atmosphere we are experiencing. Zero gauge pressure (0 PSIG) means we are measuring an absolute pressure equal to the local atmospheric pressure occurring at the gauge.

      • Thanks for the correction. You are ABSOLUTELY (LOL) right. I don’t know how that one slipped by. Thanks for catching it.


  1. Your article did nm.jit mention differences in altitude. Can you please tell me what a goond vacuum shuould be at 4500 feet above sea level.

  2. WOW! What a lesson in gauges, vacuum, and Hg. The only thing missing
    with this boat is that it most likely has not had a recent Vessel Safety Check. If it had, it would have covers on the battery terminals,
    necessary to prevent arcing with the other pole, and possibly causing a fire in the hole.
    Vessel Safety Checks are always available, especially in Sunnyland, offered by the US Power Squadrons and the Coast Guard Auxiliary. It’s free and only takes a few minutes. It might save your boat, and a life.
    I spend my winters in Central Florida, and do Vessel Checks at local
    marinas. Call me if you need help in finding a qualified inspector near you. Thanks and keep up the good work with the antiques.
    Art Cournoyer, P.IN
    USPS Certified E219990

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