Vacuum 101 (or...Your Engine Sucks)

Tech information furnished by Jack Conrad

Your Engine Sucks…

…or at least it is supposed to. As the pistons descend during the intake strokes a vacuum is created in the cylinders and throughout the intake manifold. This is good. Vacuum is our friend. It literally sucks the air/ fuel mixture into the cylinders.

Additionally, Vacuum tapped off of the intake manifold is used to power or operate a wide variety of automotive systems. The most common use of vacuum on today's vehicles is for brake boosters and to actuate the advance mechanism on distributors. On older Cruisers it is used to shift the transfer case from two to four wheel drive. In days gone by most vehicles had vacuum operated windshield wipers. It is also used to activate most of the heating and ventilation in many cars and trucks. I almost forgot, vacuum is also used to power the many thousands of devices scattered around your engine compartment by the ever popular smog nazis. ( Remember now, a gun is not inherently bad just because it is used by a criminal.) Vacuum is our friend!!

Vacuum is measured in inches of mercury. It is expressed as the number of inches of mercury pulled up a tube at sea level. Fortunately for us, some rocket scientist figured out how to make an accurate vacuum gauge with a diaphragm and a spring. You will find two basic gauges on the market.

The first usually has three colored sections on the face of the gauge. It may or may not have any numerals. This one is marketed as a fuel economy or gas mileage gauge. The color bands are most often red, yellow, and green. A reading in the green indicates your best fuel economy. A red reading means you are sucking the gas really fast. Several automobile manufacturers, BMW for instance, have put these gauges into the dash as part of their stock configuration.

The second type of gauge is the one we will focus on. This one has graduations around the dial with numerals from zero to 30 thrown in so you can get an actual numerical reading. This gauge will also tell you about your relative fuel economy. The higher the reading, the better your gas mileage. With a careful eye on the vacuum gauge you can modify your driving style/habits and realize a significant increase in gas mileage. Keep that needle at 15 inches or higher and you'll be surprised at how many miles that Cruiser can squeeze out of a gallon of gasoline. I guess I should mention that a driving style that will yield a steady vacuum reading of 15 inches or higher is usually associated with little old ladies. It would make for a very long and boring trip to anywhere. You obviously aren't into "boring" or you wouldn't be driving a Cruiser. So, let's get on to the main course.

The vacuum gauge is one of the most versatile diagnostic tools you can own. I have had one on the dash of my 84 FJ60, Big Toy, since it was one week old. I prefer an in-cab gauge. An occasional glance at the vacuum gauge let's me know everything is fine, or that things are heading south. With it in the cab you can get used to your trucks normal readings under varying conditions. A gauge used strictly in-the-shop is great for diagnoses, but doesn't give you the long term knowledge that an in-cab unit provides.

The vacuum gauge will usually give the first indication of problems. Long before they become apparent to the ear, or by feel, the vacuum gauge will indicate impending trouble. It can reveal a wealth of information about the internal functioning of your engine. It is also one of the least expensive troubleshooting tools. For the price, around $15.00 to $25.00, it definitely gives you the biggest bang for the buck. The vacuum gauge is also the biggest mystery to the average "Shade-tree" ( typical Cruiser) mechanic. Stick with me gentle reader and we'll explore some of the more common readings and what they are telling you about your engine.

Let's start by defining the engines we are going to be talking about. These will be your typical grocery getter or four-wheeler gasoline engine. High performance, radical camshaft, nitro burning horse power monsters have a whole different set of rules. You won't find many of these in our Cruisers so they won't be discussed here. We're talking generalities here, YMMV! If you are running a hot cam check with the manufacturer. They should be able to tell you what to expect from a specific application.

Any year F or 2F engine with all systems "go" is going to show a steady reading at idle between 15 and 20 inches. With a perfect engine, like the one we all maintain in our Cruisers, the needle will appear to be glued in place. The farther that engine is away from perfect the more the needle will jump or float around.

I could go on, describing each reading and what it means, but I thought the table below would do even better. There may be other engine problems that can be diagnosed with a vacuum gauge. If you know of any please write, call, or e-mail so we can pass the information on to all readers.

NOTE 1: The information in the table below has been gathered over many years of reading, talking to other mechanics, and working with vacuum gauges. In many cases the exact numerical reading is not as important as what the needle is doing and how. Number 1 below lists the "normal" reading at idle. Your reading should be somewhere within this range. The exact idle reading isn't quite as important as the fact that the needle should be glued in place. There are many variables in the equation. Things like compression ratio, intake and exhaust manifolds, camshaft and ignition system all have an affect on your engine's normal readings. Only time and a close eye on the gauge will tell you what "normal" is for your rig.

NOTE 2: Notice number 4. The Vacuum gauge can help you set your idle mixture. As a general rule of thumb, and or fore-finger, the higher the vacuum reading at idle, the higher the low-end torque. Be careful with this. If the idle vacuum is too high your gas mileage can really suffer. I recommend tuning for an absolute max reading of 21 to 22 inches at idle.

NOTE 3: The readings listed in numbers six and seven below can be caused by a stretched timing chain. I know, they can't be in a Cruiser because it uses gears. Since these rules apply to vehicles other than Cruisers I wanted to let you know. These readings may also be caused by leaking piston rings.

readings are at idle unless otherwise stated. Gauge readings are in inches
1. Normal Steady at 15 to 21
Drops to 2, then rises to 25 when throttle pedal is rapidly depressed then released.
2. Intake leak Low steady reading less than 10
3. Head gasket leak Gauge floats between 5 and 19
4. Improper idle mixture floats slowly plus or minus 1 ˝ to 2 ˝ inches
5. Small spark gap or defective points slight float plus or minus 1 to 1˝ inches
6. Late ignition timing Approx 2 inches below normal idle
7. Late valve timing 4 to 8 inches below normal idle
8. Worn valve guides Oscillates plus or minus 2
9. Weak valve springs Violent oscillation (plus or minus 5 inches) as rpm increases. Often steady at idle.
10. Sticking valves Normally steady, will intermittently flick downward about 4 inches
11. Leaky valve Regular drop about 2 inches
12. Burned or warped valve Regular, evenly spaced down-scale flick approx 4 inches
13. Worn rings/ diluted oil Drops to 0, then rises to approx 18 when throttle is rapidly depressed then released
14. Restricted exhaust system Normal when first started. Drops to 0 as rpm increases. May eventually rise to approx 18

UPDATE! August 2006

I was reading your vacuum info section at http://www.ih8mud.com/tech/vacuum.php and discovered a major error. When a piston moves away from the valves and head, or down as we say, during the intake stroke, it creates a low pressure situation but does not create a vacuum, which would "suck the combustion mixture in." This pressure difference is what causes a vacuum guage to read "vacuum", but in reality we are dealing exclusively with positive pressure situations, both in the cylinder as well as in the intake and on both sides of the carburetor or throttle body.

For example, space is a vacuum because the ambient pressure is zero. In space, if you operated an internal combustion engine via the starter, you would not get a "vacuum" reading because no pressure that could push the theoretical combustion mixture into the cylinders exists. This is different from here on Earth, because we have roughly 15 psi pressure at sea level. This ambient pressure is what forces the mixture into the cylinders, not some imaginary suction force we all think of when we think of vacuum in an engine.

Another example of this phenomenon is with a water pump. The vertical distance that water can be drawn up into a pump is around 32 feet. This is because that is how much pressure the atomosphere pushes down on everything, while we are decreasing pressure inside the pump to nearly zero. If we could really create negative pressure, or vacuum, we could suck water much farther that 32 feet upward.
I hope this helps you reformulate your posting on your great site to help others understand the way things are.

Thanks for the great site.
Drop me a line if you want to.
Ben Hubbard

Posted July 3, 2002

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