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TRANSCRIPT
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