Weather forecasting has long been an imprecise

science. Before radar and weather satellites, meteor

ological predictions were based on things like thecolor of the sky in the morning and the length of a

woolly booger's coat. More scientific approaches to

forecasting have included the use of a device called

a barometer, used to measure changes in atmospher

ic pressure. Generally, low pressure means the

weather will suck.

In the automotive world, however, low pressure

in a spark ignition engine's intake manifold has

always been considered a good thing, an essential

element in a forecast of good engine performance.

Engine vacuum has traditionally been measured by

a mechanic's barometer, known as a vacuum gauge.

Generally, low pressure means the engine can suck.

By the way, have you seen your vacuum gauge

lately?Probably not. Odds are it's in the same drawer

with your grease gun, a half used tube of valve lap

ping compound, and your brake adjusting spoon.

And that's not so surprising if you think about it.

Vacuum gauges were never all that popular to begin

with, even though editors kept writing articles and

book chapters about vacuum gauge use. These arti

cles were composed of a tedious and boring series of

vacuum gauge mug shots. These shots of the gauge

face showed the tiny white vacuum gauge needles

strategically posed at various positions. Accompa

nying the mug shots, editors commonly listed all

the possible causes for erratic movement of the vac

uum gauge needle.

You remember...high vacuum shows advanced

timing...low vacuum shows retarded valve timing.

EEK.

So what do YOU think. Is there a good reason to

dig out your vacuum gauge? I mean, unless someone

changed the operation of the internal combustion

spark ignition engine while we were sleeping last

night, good engine vacuum is still important, essen

tial even.

But what about the vacuum gauge itself? Is it still

useful in a world filled with engines that may never,

ever need a valve job, adjust their own ignition tim

ing, correct their own curb idle, monitor their own

misfire conditions, and correct fuel delivery to com

pensate for small vacuum leaks?

There really is one very good reason to drag out

your vacuum gauge. Here goes: Vacuum testing is

the fastest, easiest to perform test in the book. You

don't need to hunt down special fuel gauge fittings

as you do for many fuel pressure tests. You won't

need to remove spark plugs as you do for a compres

sion test. Just find a good place to tee in a vacuum

gauge and hook it up.

The gauge can then be dragged through an open

car window or taped to the windshield and watched

during a test drive. Vacuum gauges are cheap,

portable, and easy to use. And they make great

stocking stuffers.

So let's hook up our own gauge to several vehi

cles, both good and bad running vehicles. But

instead of concentrating on the gauge, let's concen

trate on the vacuum it measures. Let's think about

how changes in vacuum affect performance, how

changes in performance affect vacuum, and how

those changes affect PCM sensor inputs.

Relax, this is a low pressure session.

—By Ralph Birnbaum

January 1996 IMPORT

1

Connecting your vacuum gauge is a snap in most

cases. Remember that manifold vacuum sources are

located below the throttle plate. This brake booster

line has a check valve in it and a capped vacuum

nipple. And even if you don't have a nice capped

port at the ready, teeing into a line isn't all that dif

ficult.

3

Your vacuum gauge can be especially useful fordiagnosing no-starts caused by low vacuum. You

know the sound of a cranking engine with no com

pression, commonly caused by a broken timing beltor chain. But what about an engine with good com

pression that has low cranking vacuum? Can't hear

a loss of cranking vacuum—can you.

5

Cranking vacuum is especially important to carbu

reted cars, and we still have a lot of those to repair.

First, the choke must close fully during cranking to

pull extra fuel from the carb jets. High vacuum in

the manifold is also very important during cold

start and warm up as it helps atomize the fuel to

keep it from puddling in the manifold.

2

Technoids will adore the vacuum/pressure trans

ducers which convert a DVOM to a digital gauge.

The long electrical lead makes these nice on a test

drive, and they really get the oohs and aaahs from

customers. In a purely practical sense, your black-

faced analog gauge is just as effective, however.

Pay your money—make your choice.

4

The Rule of Thumb? Look for 3-5 inches of cranking vacuum. If the battery is dead or the starter is

drawing as much current as an electric water

heater, correct the cranking problem and retest at

the proper cranking speed. An engine cranking at 6

RPM isn't going to score straight A's on cranking

vacuum tests.

6

This helps explain why cars with carb base leaks

may not start, even after repeated fuel priming

from the accelerator pump. If low cranking vacuum

doesn't atomize the cold fuel/air mixture, the

engine can't burn it. You may get lucky and havethe engine "catch" and start, or you may end up

with wet plugs and a call for the wrecker.

IM January 1996

Low Pressure Area

7

This brings us to vacuum operated air intake doors.

Heated intake air works with engine vacuum to bet

ter atomize the fuel during cold operation. But a

leaking intake air door vacuum diaphragm spells

double trouble. Frigid air gets drawn into the air

cleaner of a cold engine, and the leaking diaphragm

lowers manifold vacuum.

9

Look at this rubber jungle at the rear of a Hondaintake manifold. Tracking down the exact location

of a vacuum leak here is similar to the proverbial

hunt for a needle in a haystack. Your vacuum gauge

may identify a low manifold vacuum condition,

but it's not much help in finding the exact location

of a leak in this thicket.

Sometimes, the four gas will send you to the gener

al area of a leak, but still fail to send you to the

exact location. This can be especially tough when

the leak is located at the underside of an inaccessi

ble intake manifold. We've used our electronic leak

detector in many cases to pinpoint the exact loca

tion of a loose hose or cracked manifold gasket.

10

8

Most carbureted Japanese vehicles have a number

of intentional vacuum leaks built into them.Vacuum-vent solenoids and bimetal valves control

various vacuum devices and trim the idle speed by

venting vacuum to atmosphere. A careless toss of

an air filter can damage an idle air compensator

and cause a lean miss.

10

In cases like this, you should recognize a low vacu

um gauge reading as a sign of a leak, but move on

as quickly as possible to to better, more precise

ways to locate the exact causes for low vacuum.

Your four gas analyzer is a good leak locator, and

responds quickly to a shot of propane or light oil

aimed at a vacuum leak.

12

Vacuum leaks aren't high tech, they aren't compli

cated, and they aren't pretty. This little hose was

causing lots of problems, but its frayed braiding

gave it away. We want its replacement to last just

as long as it did, so we're careful to use only the

best quality replacement hose and demand metric

I.D. hose for metric applications.

January 1996 IMPORT! SRvicze

Low Pressure Area

13

We all know what happens when an EGR valve

dumps its load of soot into the fuel/air mixture of

an idling engine. But an aging EGR valve can also

become the root cause of a constant manifold vacu

um leak. Wear between the shaft and body of the

valve has caused leaks on several cars we've seen

that were big enough to whistle at idle.

15

But guess what. PCMs in these cars compensated

for our tampering. Even when PCM corrections

didn't completely mask our tampering, idle quality

was good enough to fool most customers. Vacuum

gauge readings? High and steady. So don't bank on

vacuum gauge readings alone or you may be out

smarted by a PCM with fast reflexes.

17

Here's more proof of a smart engine in action. If we

watch the O2 sensor as we first pull the vacuum

hose, the O2 voltage drops low. But before the trace

crosses the screen, the PCM has richened the mix

ture, and O2 cross counts resume. Even with the

vacuum leak. We need to cross reference several

tests to outsmart a smart engine.

14

Be careful about those old rules of thumb for evalu

ating or adjusting base ignition timing using a vac

uum gauge. We advanced and then retarded igni

tion timing on several cars to see how it affected

vacuum at idle. Initially, the vacuum gauge would

drop as timing was retarded and rose with an

increase in timing advance.

16

Think we're kidding? Let's create a vacuum leak at

idle. As we first pull the hose, we see the vacuum

needle drop—and then climb right back to the

green zone! There's a very slight fluctuation in the

needle, but after a PCM "correction," most drivers

wouldn't notice the vacuum leak on this idling

engine.

18

Sometimes the PCM gets outsmarted. Feedback

fuel control systems don't know where the oxygen

is coming from, so a vacuum leak right next to the

intake port of one cylinder can lean that cylinder

more than the others. On some cars, the added oxy

gen in the exhaust can drive the system so rich that

it floods the other cylinders.

12 January 1996 IMi ERVICE

19

The poor PCM can really get snookered by a similar

vacuum problem when the vacuum hose to the

MAP sensor breaks or falls off. Low vacuum at the

MAP is seen by the PCM as high load so it calls for

more fuel. But added oxygen from the vacuum leak

can result in a low voltage from the O2. Ever heard

of the rich getting richer?

20

Some 1996 models will start monitoring their own

camshaft timing to meet OBD requirements. In the

meantime, don't forget the importance of correct

valve timing. And while the first thing that comes

to mind is late cam timing, don't forget the critical

importance of proper valve lash on millions of cars

that still have adjustable valves.

in progress!Modern cars have highly complex elec

tronic components and systems. Garages

need better equipment and better informa

tion on fuel-injection and ABS systems.

Information and equipment tailored to the

cars they're working on.

Car manufacturers keep developing new

systems, each requiring special equipment.

It's getting harder and harder for garages

to keep up with the pace of progress.

The Multi-Tester Plus is the ideal solution

for shops that work on several makes. It's

easy to update and upgrade, with memory

cassettes and adaptors for the fuel-injec

tion and ABS systems used on different

cars. It's user-friendly and easy to learn.

The Multi-Tester Plus improves trouble

shooting accuracy - and profitability - and

will give you more satisfied customers.

Contact your nearest dealer now for more

details on the Multi-Tester Plus.

Autodiagnos AB, S-145 84 Stockholm, Sweden.

Tel: Int. +46-8-680 85 00. Fax: Int +46*531 809 50.

Circle No. 104 on Reader Service Card

21

While a vacuum gauge can lead you to suspect

higher than normal exhaust backpressure, directtests of backpressure should always be performed

to be sure. This Honda Prelude is a real example. It

would cash out at about 3000 RPM at WOT. Our

own Sam Bell is shown connecting a vacuum gauge

prior to a test drive.

23

Oh rats! Looks like that shiny new customer-

installed muffler wasn't the deal he thought it

would be. There's almost 9 pounds of exhaust pres

sure showing on the gauge at WOT, and it's no

small wonder that the Prelude won't perform. The

customer's plastic pet rat sums up our feelings as

he turns his back on the proceedings.

25

On many vehicles, the air cleaner is a source of fil

tered air for pulse air systems. Reed valves allow

fresh air to pass downstream to promote oxidation

of HC and CO. But occasionally, the valves break,

turning the air cleaner into a large, full time EGR

valve. Look for this problem on cars that will idle

only with the air cleaner lid removed.

14

22

The vacuum gauge readings did not give any clues

about our problem, but Sam and I both had this

"plugged exhaust" feeling in the seat of our pants

as we drove the car. We pulled the O2 and in

stalled this adapter with a long length of silicone

hose which we dragged into the car. Sam drove

while I watched the gauge.

24

We also simulated different types of ignition mis

fires on several vehicles, from intermittents to full

blown dead cylinder syndrome. We discovered that

HC readings on the four gas started to indicate even

small misfire conditions long before the vacuum

gauge needle fluctuations gave us any clear-cut

indications of a misfire condition.

26

Finally. Just because you have good vacuum below

the throttle plate at idle, doesn't mean that changes

in vacuum are being properly metered by the air

flow sensor when the throttle opening changes.

This intake tube on a Volvo is wrapped like the

remains of King Tut and it still leaks. Remember,

engine vacuum is a closed loop system.

January 1996 IMPORT!

apt

Simple Keys to Suck-Sess

Let's admit it. Most of us aren't getting ANY useout of our vacuum gauges. Remembering all the

possible combinations of vacuum gauge readings is

like trying to memorize the menu at Denny's. And

that list has its own side menu of exceptions to

most rules you might cite. Let's also admit that

most of us never even look at that list unless we're

preparing for our ASE tests.

So don't expect this next section to help you passthe Ll. Instead we've boiled vacuum gauge testing

down to its barest essentials. We left out all the

bobbing needle tests and decided to concentrate on

two simple tests which will let us verify the

presence of GOOD vacuum before resort

ing to more exotic test procedures. By

keeping it simple, we're all more

to hook up our vacuum gauges in the

first place. (Then we'll learn all the

fine points by practice and associ

ation with real vehicle problems.

Fair enough?

Here are the tests:

1) Cranking vacuum

2) Vacuum readings on a

warm, running engine at vari

ous throttle openings

• at idle

• at part throttle, no-load

2000 and 3000 RPM

• during closed throttle

decel from a WOT stab

First, test cranking vacuum (usu

ally done on no-starts): "^^1) Connect a vacuum gauge to a manifold vacu

um source. Make sure the throttle is closed and that

the engine cranks at the recommended speed.

2) Make sure cranking vacuum is at least 3-5inches.

Second, test warm engine vacuum at idle,

part throttle, and during closed throttle

decel:

1) Measure engine manifold vacuum at idle.Vacuum should be "high" and steady at idle (17-21

inches of mercury).

2) Raise the engine speed to about 2000 RPM.Hold it steady and watch the vacuum gauge. After

an initial drop in the vacuum reading as the throttle

opens, it should return to the idle vacuum level

recorded in the previous step, or darned close to it.

(A thousand pardons, but we need to mention onepossible exception here. Some EGR's will kick in

no-load. Ifyou see a small drop in vacuum during a

steady throttle test, disable the EGR and repeat thetest)

3) Do the same test at 3000 RPM and look for the

same results.

4) Then allow the throttle to snap shut from a

snap WOT. Vacuum readings should rise above idle

vacuum (20-25 inches), and then fall slowly as the

engine speed falls. The vacuum needle should

come back to rest at the idle reading we started

with in step one, and hold steady there.

If the engine passes these tests, the odds are highthat it's mechanically sound—at least sound

enough to pump air at idle, part throttle, and closed

throttle decel, which is precisely what it's supposed to do!

Think about it. Steady vacuum readings in the

range of 17-21 inches at idle are GOOD. If

vacuum readings stabilize at the same lev

els at part throttle cruise, that's GOOD

too. (The engine doesn't have a bust

ed valve spring or two that stumble

over their own feet each time the

band shifts from a slow waltz to a

jitterbug.) And the valve/piston

synchronization stays correct or

the engine couldn't keep pump

ing at higher engine speeds.

Finally, internal engine parts

(valves and piston rings) seem to

be sealing well enough to jack up

the vacuum on closed throttle

decel. They keep right on sucking

against the closed throttle.

Hey, this is all GOOD! So look for

your problem elsewhere using tools

better suited for checks of specifics like

fuel pressure, secondary spark, and of

^^^" course, emissions.

What If Readings Aren't GOOD?

Fair question. Here are your options, and again,

we'll keep them few in number:

• If the cranking vacuum is low, or zero, look for

a major problem. A big, big, easy to find-because-it-

is-so-big type of problem. Something like a

camshaft that stopped dead in its tracks, or a vacuum leak the size of your fist.

• If idle vacuum is low but steady, check valvetiming first.

•For all the in-between problems, you can

always go back to one of those traditional vacuumgauge articles and try to stay awake long enough to

figure out if the bobbing needle is caused by an

ignition miss or extraterrestrials.

For our money, other tests: power balance, four

gas, secondary ignition, and fuel pressure tests are

all better ways to isolate specific problem areas. But

the vacuum gauge will point out low vacuum prob

lems before they're ignored and forgotten as possible causes for a problem.

IMPDRl January 1996 15