发信人: bangyi (hhh), 信区: Automobile
标  题: a good article on "no-start diagnosis" zz(3)
发信站: BBS 未名空间站 (Mon Aug 22 18:32:17 2005), 转信

Spark

If the cranking circuit has passed the appropriate tests, it's time to conce
ntrate on why the engine won't run. There are four areas of concern-spark, f
uel, compression and timing. The easiest test to start with is usually the o
ne for the ignition system. On older systems, it's convenient to remove a co
il wire, then use a spark tester to test for proper spark. An adjustable spa
rk tester is preferable to a fixed design because it allows you to adjust th
e tool to a smaller gap, which will tell you if you have no spark, or just w
eak spark. Weak spark will send your diagnosis in a different direction than
no spark.

A diagnostic procedure is most efficient when possible causes can be elimina
ted as quickly as possible. On ignition systems, a weak spark should rule ou
t most primary input sensors. Of course, this depends greatly on the vehicle
on which you're working, but it can generally be said that a weak spark is
not caused by cam and crank type inputs. On the other hand, no spark can be
caused by these inputs. You can see how this distinction will affect the dir
ection of your diagnostic procedure.

The relationship between crank and cam timing events can also cause no spark
, especially on Chrysler products. On these vehicles, the PCM won't trigger
the ignition coil if the cam and crank are out-of-sync. Cam and crank inputs
may appear normal when tested with a digital storage oscilloscope (DSO) or
graphing multi-meter (GMM). Using a scanners bi-directional controls, the co
il output and primary wiring circuits will appear normal as well, which may
lead you to believe that replacing the PCM is the answer. Testing the cam an
d crank relationship may lead you to a slipped timing belt, the root cause o
f a no-spark, no-start condition.

Timing

Incorrect timing is the most overlooked and misdiagnosed cause of a no-start
. Perhaps you automatically think of ignition timing as the position of the
crankshaft when the No. 1 plug fires. While this is important, it isn't the
only timing condition that can influence a no-start. Valves must open, the i
njectors must open and the ignition must fire-all the proper time.

Ignition timing on a no-start can be confusing, but it can still be tested b
y connecting a timing light, then watching the timing marks while an assista
nt cranks the engine. While this may or may not be base timing as specified
by the manufacturer, it does let you know where the spark is occurring. With
a little common sense, this information can get your diagnosis headed in th
e right direction.

Valve timing can be a critical event, as well. Hondas are notorious for runn
ing rich when the cam timing is retarded from specification. This may cause
a no-start condition if the start is attempted in cold weather. The result w
ill be a flooded engine that you're able to start after cleaning the plugs,
which may cause you to miss the root cause of the problem.



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※ 来源:·BBS 未名空间站 mitbbs.com·[FROM: 68.35.]

发信人: bangyi (hhh), 信区: Automobile
标  题: a good article on "no-start diagnosis" zz(4)
发信站: BBS 未名空间站 (Mon Aug 22 18:34:39 2005), 转信

Fuel

Fuel system diagnosis can be broken into two parts-the fuel delivery system
and the injector electrical circuit. Just as the ignition system has primary
and secondary components, the fuel system has command and fuel delivery.

Command, or the injector electrical circuit, is usually diagnosed with a sca
n tool and a DSO. A noid light, which is plugged into the wiring harness in
place of one of the injectors, is not a good test. While convenient, this to
ol does not put the proper load on the circuit and does not test voltage and
ground. A DSO, on the other hand, tests the circuit while under load and ca
n give you an indication of the location of the problem within the circuit.
DSO tests measure voltage and current. Voltage will tell you about the suppl
y circuit integrity as well as the command status. Current measurements can
also tell you how well the circuit is functioning.

Test the fuel delivery system components by measuring fuel flow and pressure
. The best way to measure flow is to test the circuit while it's loaded. For
return-type systems, it's easy to test with a flow gauge such as the one sh
own in the photo on page 32. Returnless systems present problems of their ow
n, as do variable-speed fuel pumps. Diagnosis of each type of system should
be based on its specific characteristics.

For example, early Subaru vehicles utilized a two-speed fuel pump system, wi
th a slower speed during idle and a faster speed above idle. Two distinct fl
ows can be seen as the system alternates from one to the other. Later models
, such as Ford's returnless/variable-speed fuel systems, can be quite diffic
ult to diagnose if you're unfamiliar with how they operate. To speed diagnos
is, use a scan tool to retrieve the parameter identification (PID) that disp
lays fuel pressure. This PID is valuable, but only if it's accurate. A manua
l fuel pressure gauge connected to the system will confirm that the pressure
transducer is indeed functioning properly.

An often overlooked fuel delivery problem can be caused by the theft-deterre
nt system. Some manufacturers include a PID in the data stream, which lets y
ou know if fuel is enabled or disabled. More on that later.



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※ 来源:·BBS 未名空间站 mitbbs.com·[FROM: 68.35.]

发信人: bangyi (hhh), 信区: Automobile
标  题: a good article on "no-start diagnosis" zz(5)
发信站: BBS 未名空间站 (Mon Aug 22 18:37:25 2005), 转信

Compression

You can test engine integrity with a compression gauge, but the starter's cr
anking speed will give a trained ear an indication of low cylinder compressi
on. If you hear a fast-cranking engine, test the compression to see where th
e problem is. Low compression does not always indicate an internal engine pr
oblem.

After identifying low compression, a leakdown test is a good follow-up step.
On some engines, it may be convenient to check cam timing before engine com
pression. This depends on your familiarity with the vehicle on which you're
working.

A leakdown test tells you where the compression is going. If you determine t
he compression is leaking by the rings, add some oil and recheck. On current
vehicles, look for anything that could cause too much fuel to enter the cyl
inders, resulting in flooding. Incorrect cam timing, especially on speed-den
sity systems, is a common cause. Honda, Mazda and Chrysler products come to
mind for flooding caused by timing belt problems.

Poor calibration of the PCM's cold cranking parameters may also be to blame.
Some of these vehicles will start once you clear the excess fuel from the c
ylinders, but they'll return on the hook sooner or later if you don't determ
ine the root cause of the excess fuel. The next cold morning a flooded condi
tion will result, as the slower crank speeds and higher injector pulse width
for cold starts results in too much fuel.

At higher altitudes, some Nissans will overfuel during cold cranking. For 19
93-95 Altimas, TSB 96-061 prescribes replacement of the engine coolant tempe
rature (ECT) sensor. The new ECT affects cold-start injector pulse width by
fooling the PCM into thinking the engine is warmer than it really is. For 19
91-94 Sentras, TSB 97-023 describes the installation of a subharness and alt
itude switch, to address the same high-altitude/cold-weather starting proble
ms. This modification has no effect on vehicles starting at altitudes below
5000 feet. Use your vehicle repair information sources to locate any applica
ble TSBs for the vehicle you're servicing.

On later models that exhibit the same symptom, or those not covered by a TSB
, the injector circuit may not be the cause of the overfueling. It could als
o be due to a lack of air. Poor airflow around the throttle plates (and thro
ugh the IAC, when there is one) can cause overfueling on Nissans and many ot
her models. This condition may be corrected by cleaning the throttle plates
and adjusting the minimum air rate. On late-model Nissans, make sure all of
the basics, such as spark plug condition, engine oil and timing, are up to s
pecification as well.

Theft-Deterrent Systems

Theft-deterrent systems go by many different names. They've been around for
years but can still cause many no-start complaints. Sometimes, this complain
t may occur after you've repaired the vehicle. Replacing a theft module with
out proper initialization will result in a no-start on many makes. Still oth
ers may require multiple controller replacement in certain situations, such
as when a master key is lost.

Antitheft system activation without cause will result in a no-start that can
lead to faulty diagnostic conclusions, not to mention wasted time. A common
condition is a start-and-stall complaint. If you encounter this complaint o
n late-model cars, the first step of your diagnosis should be to disarm the
theft system. Jeeps, for instance, will start and stall until you lock and u
nlock the passenger door or the rear hatch, depending on the model year.

Another important consideration for theft deterrent is the security light st
atus. Most systems employ a dash light that will give an indication of where
the problem lies. Each system is different, so consult the repair manual. A
flashing light during cranking on one system may indicate a security malfun
ction, while the same may not be true for the identical make and model of a
different model year.

How the system behaves is variable as well. Some keep the engine from cranki
ng, some disable the fuel pump, while others disable spark and fuel. As I me
ntioned, its not uncommon for the theft-deterrent system to allow the engine
to start, then immediately cause it to stall. Product knowledge is very imp
ortant under these conditions.

Although systems have evolved to make no-start diagnosis much more difficult
than in previous years, its important to not lose sight of what's needed to
make a spark ignition engine run. We still need spark, fuel and compression
, and we need them all to occur at the correct time. Concentrating on this b
asic concept will help keep you focused on the task at hand, and achieve a c
orrect diagnosis in a decent amount of time. M

Provided by ProQuest Information and Learning Company.



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