Reading a Vacuum Gauge

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There’s nothing morebasic than the fact thatan engine is just a bigair pump. It draws inair by creating a low-pressure area in the

intake manifold and cylinders, com-presses the air, mixes in a little gaso-line, lights a fire, generates heat andpressure and finally pumps out thespent exhaust. Our preoccupation to-day with things electronic sometimesmakes us overlook old-fashioned me-chanical symptoms of problems andthe mechanical test equipment usedto troubleshoot them. Vacuum gaugesare often in this category, but the in-sight that a vacuum gauge can pro- vide is as valuable today as it was 30,40 or 50 years ago.

Remember that engine vacuum is just air pressure lower than atmos-pheric pressure. The starting point toevaluate engine vacuum is the intakemanifold. When you connect a gaugeto a tap on the intake, you’re measur-ing manifold vacuum. Note that vacu-um will vary in different areas of theengine, such as above or below thethrottle valve and right at the intakeand exhaust ports.

Vacuum drawn from an openingahead of the throttle is called ported

vacuum. Throttle opening affectsported vacuum opposite to the way itaffects manifold vacuum. For exam-ple, at closed throttle, manifold vacu-um is at its peak. But there is no sig-nificant vacuum at a port ahead of the

throttle plate when the throttle isclosed. Vacuum appears at such aport only when the throttle opens.

It’s important to remember thatmanifold vacuum is used to power ve-hicle systems that need a steady sup-

BY KEN LAYNE

READING A

VACUUM GAUGE

47August 2001

An idle vacuum reading of around 20 in.-Hg is normal for most engines ingood mechanical condition. This engine is right there.

P h o t o s : J a c k

S p a r k s

a n d

E u g e n e

S a a r



ply of low-pressure air under all en-gine operating conditions. These sys-tems include power brake boosters,a/c vacuum motors and some emis-sions controls.

Ported vacuum is used to control vehicle systems in relation to engineload. These include old-fashioneddistributor vacuum advance di-aphragms and carburetor assist devices. They also include many emis-sions control devices and transmis-sion shift points. Under some engineload conditions, ported vacuum may equal manifold vacuum, but it cannever exceed it.

Get Out the GaugeMost vacuum gauges are graduatedin inches of mercury (in.-Hg) andmillimeters of mercury (mm-Hg).Some also show the modern metricscale of kilopascals (kPa). For com-parison, 1 in.-Hg equals 25.4mm-Hg,or about 3.4 kPa. For this review, we’ll stick to in.-Hg, or simply inchesof vacuum.

Because engine vacuum is basedon comparison with atmosphericpressure, it varies with altitude just asatmospheric (barometric) pressuredoes. The following table shows thatas altitude increases, vacuum de-creases about 1 inch for every 1000feet above sea level.

Inches ofAltitude Vacuum

Sea level-1000 ft. . . . . . . .18-22

1000-2000 ft. . . . . . . . . .17-21

2000-3000 ft. . . . . . . . . .16-20

3000-4000 ft. . . . . . . . . .15-19

4000-5000 ft. . . . . . . . . .14-18

5000-6000 ft. . . . . . . . . .13-17

Normal manifold vacuum at idlefor an engine in good condition isabout 18 to 22 in.-Hg. Manufacturersused to publish vacuum specs in ser- vice manuals, but this isn’t as com-mon as it was years ago. Still, thephysics of internal combustionhaven’t changed in a hundred years,

so the guidelines given here are agood starting point for vacuum gaugetroubleshooting. Your best analysisbased on vacuum readings will comefrom your own experience, however.

As you use a vacuum gauge on differ-ent engines, you’ll learn what’s typicalfor one model compared to another.Some engines have reputations aslow-vacuum motors; others are un-usually higher than average. Experi-ence is your best teacher.

Cranking Vacuum& Speed TestsYou can get a quick basic appraisal of engine condition by connecting a vac-uum gauge to the manifold and atachometer to the ignition to check vacuum and rpm at cranking speed. Warm up the engine first, then shut itdown and connect your test equip-ment. Close the throttle and disablethe ignition, or use a remote starterso the engine won’t start. Crank theengine for 10 to 15 seconds and ob-serve the vacuum and tach readings.

Note that different engines pro-duce different cranking vacuum read-ings. Some carmakers publish specifi-cations; others don’t. Again, experi-ence will be your best guide. What you’re looking for, most importantly,is steady vacuum and cranking speed.

If the cranking speed is steady (about 200 rpm) and vacuum also issteady (around 5 inches), the enginemost likely is in good mechanicalcondition. If rpm and vacuum are un-even, the cylinders aren’t pumpingequally. The engine probably hasleakage past the valves, rings or headgasket. If the vacuum reading is pret-ty steady but cranking speed is not, you’re probably looking at a damagedflywheel ring gear or starter. If thecranking speed is normal or high but vacuum is low and slightly uneven,the engine probably has low com-pression or retarded valve timing. A jumped timing chain or belt is a com-mon cause here.

The cranking vacuum test also canprovide a quick test for PCV restric-tions. Perform the test and note theaverage vacuum reading. Then pinch

48 August 2001

A low but steady vacuum reading at idle may be due to retarded ignition orvalve timing, low compression, an intake manifold leak or tight valves.



the hose to the PCV valve closed with your pliers and repeat the test. If thePCV system is clear, vacuum shouldincrease. If it doesn’t, check the PCVsystem closer for restrictions.

What Idle Tests Can RevealYou can zero in on several basic me-chanical problems by taking a quicklook at manifold vacuum. Warm theengine to normal temperature—get itreally warm—and connect your vacu-um gauge. Make sure you connect toa manifold vacuum tap and not toported vacuum. Connecting atachometer also is a good idea.

Just to be sure that the evaporativeemissions system doesn’t interfere with vacuum testing, disconnect andplug the canister purge hose and itsmanifold port. If you’re testing anOBD II car, check for evap-relatedDTCs when you finish testing to besure none set.

Run the engine at idle, low cruise(1800 to 2200 rpm) and high cruise(2500 to 3000 rpm). Note the vacuumreadings, and any fluctuations, ateach speed. Next, hold engine speedsteady at about 2500 rpm for 15 sec-onds and read the gauge. Now re-lease the throttle and watch thegauge as the speed drops. The vacu-

um reading should jump as the throt-tle closes, then drop back to its nor-mal idle reading. If vacuum doesn’tincrease at least a couple of inches

when you release the throttle, youmay be looking at worn rings, cylin-ders or valves.

Idle vacuum for most engines isabout 18 to 22 in.-Hg, but some may produce only 15 to 17 inches at idle.(Remember what we said about expe-rience.) If vacuum is steady and with-in these ranges, the engine and fueland ignition systems are operatingnormally.

If vacuum is steady at idle but low-er than normal, the ignition or valvetiming may be retarded. Low com-pression, an intake leak or tight valvesalso can cause low vacuum at idle.

If the vacuum reading fluctuates within the normal range—the gaugeneedle bounces around a lot—un-even compression (broken rings orleaking valves or head gasket in oneor two cylinders) is a likely culprit. Anuneven air/fuel mix, erratic ignitiontiming, a misfire, misadjusted valvesor a manifold leak near one or twocylinders also are possible causes.

If vacuum drops intermittently at

49August 2001

If a vacuum gauge gauge needle bounces around like this, uneven compression,an out-of-kilter air/fuel mixture, erratic ignition timing, a misfire, misadjustedvalves or a manifold leak near one or two cylinders are all possible causes.

Checking engine vacuum isas old as the internal com-bustion engine. In the last

few years, however, severalequipment companies haveadapted new technology for thisold diagnostic technique.

A few offer vacuum and pres-sure transducers that can beconnected to an engine (or a

transmission or a/c system orbrake system or...) that is asource of hydraulic or pneumaticpressure. These transducers con-tain a strain-gauge or piezoresis-tive sensor that’s similar to anengine MAP sensor. Pressure onthe sensor produces a voltagesignal that can be sent to a mul-

timeter, graphing meter orscope for in-depth analysis.

For vacuum-testing the elec-tronic way, Snap-on sells severalpressure/vacuum transducers forits Vantage graphing multime-ter. Each transducer provides adigital display of vacuum orpressure on the Vantage screen,along with a graph of the fluc-

tuating vacuum or pressure sig-nal. As with any digital testequipment, the Vantage lets yourecord the waveform and save itfor later analysis. It’s great forroad-testing.

Scope manufacturers offersimilar devices, so ask your tooldealer for information.

High Technology for Old Techniques



These vacuum graphs were produced by a transducer connected to a lab scope. The waveforms were synchronized to theignition through the scope’s external trigger. The spikes and dropouts (arrows) indicate intermittently sticking valves.

cylinders (steadily low vacuum)?Combine modern power balance test-ing with traditional vacuum analysisand you’ll have the answer.

The author would like to thank the staff members of The DMV Clinic inSanta Cruz, California, for their help with this article.

Visitwww.motor.com todownload a free copy of this

article. Copies are also availableby sending $3 for each copy to:

Fulfillment Dept., MOTOR Magazine,5600 Crooks Rd., Troy, MI 48098.

50 August 2001

Answers to PreviousAssessment Quiz

(Secondary Ignition, May 2001)

1-A 2-B 3-B 4-A 5-A

6-D 7-B 8-B 9-C 10-C

idle, one or more valves may be stick-ing open or dragging. Higher-than-normal vacuum at idle is a commonclue to overly advanced ignition tim-ing, while low vacuum can indicateretarded timing.

Low vacuum also can be an imme-diate clue to a plugged exhaust. Tocheck further, run the engine atabout 2500 rpm for about 15 seconds.If vacuum drops during this periodand does not increase when you closethe throttle, you’re almost certainly looking at a restricted exhaust.

Vacuum Fluctuations& Power BalanceSeveral of the guidelines in this arti-cle have distinguished betweensteady vacuum gauge readings andfluctuating readings, where the gaugeneedle bounces up and down errati-cally. This may seem secondary —al-most inconsequential—but it’s an im-portant distinction. A steady but ab-normal vacuum reading indicates aproblem common to all cylinders.Things like incorrect ignition timing

or an old, tired, high-mileage engineaffect vacuum equally for all cylin-ders. A bouncing needle, however,usually indicates that the problem islocalized to one or just a few cylin-ders. Here’s where power balancetesting enters the picture.

Compression testing on many late-model engines is flatly impracticalfrom a labor standpoint for a quickengine evaluation. That’s especially true on some of the weird vans for which removing and reinstallingspark plugs is a two-hour job. It’s rel-atively quick and easy, however, toconnect a vacuum gauge to the mani-fold and your engine analyzer to theignition system.

If your initial vacuum tests producegauge fluctuations, you have a definiteindication that the problem is limitedto just one or a few cylinders. In theseinstances, a power balance test canhelp you pinpoint those cylinders andthe condition they ’re in. Does the en-gine need a valve job (fluctuating vac-uum) or a complete engine exchangedue to universally worn rings and

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Reading a Vacuum Gauge