In today’s world of
high-tech electronics, onboard diagnostics and scan tools, you would
think engine diagnostics would be easier than ever. Yet some
technicians struggle when confronted with an engine that cranks, but
refuses to start. Most no-starts do not generate any fault codes, so
how do you proceed to find out what’s causing the problem?
For any
engine to start, three things are necessary: fuel, compression and
spark. If any of these components is lacking, the engine won’t start.
NO FUEL, NOT ENOUGH OR TOO MUCH?
The
fuel system must be generating normal fuel pressure, and the air/fuel
ratio must be correct for the ambient temperature and engine
temperature. If the air/fuel mixture is too lean, it won’t ignite and
the engine won’t start. If the air/fuel mixture is too rich, it can
flood the engine and also prevent it from starting.
No fuel pressure
is one of the most common causes of no-starts on fuel-injected engines.
When the ignition key is turned on, the powertrain control module (PCM)
is supposed to energize the fuel pump relay and turn on the fuel pump.
The pump is energized for a few seconds to build pressure in the fuel
system. But if the engine fails to start as it’s cranked, the PCM may
switch off the fuel pump as a safety precaution and to reduce the risk
of engine flooding.
If the fuel pump doesn’t buzz and generate
pressure when the ignition key is turned on, it doesn’t necessarily
mean the fuel pump has failed. The problem could be a bad fuel pump
relay, a blown fuel pump fuse, a loose or corroded wiring connector
anywhere in the fuel pump electrical circuit, or even a fault in the
PCM itself. All too often, these other causes are overlooked and the
pump is blamed for the no-start. And when the pump is replaced and the
engine still doesn’t start, the technician scratches his head and
wonders why it isn’t working.
If you suspect a no-start condition is fuel-related, there are a couple of quick checks you can do:
•
Remove the air inlet tube from the throttle and give the engine a
couple shots of aerosol starting fluid. If the engine starts, runs a
couple of seconds, then dies, you know it has spark and compression but
is not getting fuel.
Check the fuel pump fuse, relay and wiring.
Check for voltage to the relay and the pump. No voltage at the relay
indicates a computer fault or an open in the wiring harness upstream of
the relay. Refer to a wiring diagram for the vehicle to trace the
circuits. Voltage to the relay but not to the pump would tell you the
relay needs to be replaced, or there is an open circuit between the
relay and pump.
• Hook up a fuel pressure gauge to the service
fitting on the fuel rail (or tee into the fuel rail supply line if
there is no service fitting) to check fuel pressure when the key is
turned on. If pressure is within specifications, the no-start condition
is not linked to the fuel pump. It might be no spark, not enough
compression, or a fuel mixture that is too lean or too rich. If fuel
pressure is less than specifications, the problem might be low voltage
to the pump, a weak pump, plugged filter, obstructed fuel line, no fuel
in the gas tank or a leaky fuel pressure regulator. If fuel pressure is
greater than specifications, the engine may have a stuck fuel pressure
regulator or a plugged fuel return line.
A fuel mixture that is too
lean to start the engine can also be caused by dirty fuel injectors or
a large vacuum leak (PCV valve, EGR valve, any vacuum hose on the
engine or the intake manifold gaskets).
Vacuum leaks have the most
effect on the idle mixture, which you can see with a scan tool by
looking at fuel trim. If the long-term fuel trim value is more than
about 8, the engine is running lean.
Vacuum leaks can be found by
visually checking all the vacuum hoses for cracks or loose connections,
by listening for whistling or sucking noises while the engine is
idling, and/or by using propane vapor or throttle cleaner to spray
suspected leak points. If the idle quality suddenly changes or improves
after spraying an area, you’ve found a leak. A smoke machine can also
be used to feed smoke into the intake manifold (with engine off) to
find leaks.
Another overlooked cause of a fuel-related no-start
condition may be bad gas. This includes too much alcohol in the fuel,
water contamination or even diesel fuel accidentally put into the gas
tank. Gas also gets stale over time as the more aromatic elements
evaporate away and the heavier elements turn to varnish. A vehicle that
has been in storage or was sitting for months without running may be
hard to start because of bad gas in the tank. The fix for bad gas is to
drain the tank and refill it with fresh gasoline.
NO COMPRESSION OR LOW COMPRESSION
If
the engine has spark and fuel but won’t start, it might not have enough
(or any) compression in the cylinders. The most common cause of a
sudden loss of compression would be a broken OHC timing belt. If the
timing belt has failed, the camshaft doesn’t turn and the valves don’t
open and close as the engine is being cranked. Consequently, it
develops no compression and doesn’t start. And if the engine is an
“interference” design with no clearance between the pistons and valves,
it will probably lock up as soon as a piston encounters an open valve.
Timing
chains can also break, but much less often than timing belts. The
recommended replacement interval for timing belts ranges from 60,000
miles up to 100,000 miles or more depending on the year, make and model
of the vehicle. So if the engine has a lot of miles on it, a broken
timing belt is likely. Removing the timing cover and inspecting the
belt can confirm a broken belt.
Low compression can also make an
engine hard to start, but as long as it has some compression it will
usually start and run. Normal cranking compression for most engines is
typically 140 psi or higher. If compression is down to 80 or 90 psi in
one or more cylinders, it means the valves and/or rings are worn and
the engine will soon need a valve job or overhaul.
Compression can
be checked “the old fashioned way” by removing the spark plugs and
using a compression gauge to measure cylinder pressure in each cylinder
as the engine is cranked. Or, it can be measured electronically with an
engine analyzer or scan tool that has a power balance or cranking
compression test function.
Compression may also be an issue if the
engine is cranking too slowly because of low battery voltage or a bad
starter. Most engines need about 200 to 300 rpm to start.
The engine
also needs a good hot spark to ignite the air/fuel mixture, and the
spark must occur at just the right moment as each piston approaches top
dead center on its compression stroke.
On engines that have spark
plug wires, a quick check for spark is to pull off a plug wire and see
if there is any spark when the engine is being cranked. To do this you
need to connect a spark plug tester or an old spark plug to the end of
the wire, and ground it to the engine. Or, you can stick the tip of a
Phillips screwdriver into the plug boot and place the metal shaft of
the screwdriver near a metal surface on the engine. A good spark should
be capable of jumping about 1/4 inch or more.
If there is no spark
while the engine is cranking, the problem could be a bad ignition coil,
bad coil wiring connections, no voltage to the coil, no coil switching,
a bad high-voltage wire from the coil to the distributor (if the engine
has a distributor), a bad distributor pickup or crankshaft position
sensor, or a bad ignition module. If there is a spark, but it is very
weak, the fault may be low voltage at the coil or a bad coil.
Hot
starting problems due to a no-spark condition are frequently caused by
bad ignition modules, but can also be caused by a bad crankshaft
position sensor or distributor pickup.
On engines with distributors,
a magnetic pickup, Hall effect sensor or optical sensor inside the
distributor generates a pulse signal that goes to the ignition module.
The ignition module, in turn, switches the coil on and off by grounding
the negative side of the coil (the positive side is always hot when the
ignition key is on). When voltage flows through the primary windings
inside the coil, it charges it up. And when the coil is switched off by
the ignition module, the magnetic field inside the coil collapses and
causes a high-voltage surge in the secondary windings that creates a
spark at the spark plugs.
On engines without distributors, the PCM
and/or ignition module controls the coil(s) using a trigger input
signal from the crankshaft position (CKP) sensor (and camshaft position
(CMP) sensor on some engines).
There are two types of crank
sensors: magnetic and Hall effect. The magnetic type can be checked
with an ohmmeter, while the Hall effect can be checked with a
voltmeter. Hall effect sensors typically have three wires: positive,
ground and a signal output. You can also use a scan tool to look for an
rpm signal while cranking the engine. No rpm signal often means a bad
crankshaft position sensor.
COIL CHECKS
Quick checks for the ignition coil include:
•
Checking the positive (+) coil terminal for voltage when the key is on.
No voltage means a problem in the ignition circuit (loose/corroded
wiring connector, bad ignition relay or fuse, bad ignition switch or a
PCM fault).
• Measuring coil primary and secondary resistance with
an ohmmeter. Primary resistance is measured between the coil positive
(+) and negative (-) terminals, and is typically very low (0.2 to 2
ohms). Zero resistance would indicate a shorted coil, while a high
resistance reading would indicate an open coil.
Secondary resistance
is measured between the positive (+) terminal and high-voltage output
terminal. Secondary resistance values can range from 5,000 to 25,000
ohms, so always refer to the vehicle manufacturers’ test specifications
for the exact value.
If primary or secondary resistance is not within specifications, replace the coil.
If
an engine has an intermittent starting problem or a hot starting
problem, retest the coil(s) when the engine is warm. If resistance is
higher, replace the coil.
• Check the coil for cracks or carbon
tracks. If the engine starts, observe the coil(s) with the hood
partially closed or after dark to see if the coils are sparking or
arcing (replace the coils if they’re leaking voltage).
FUEL, COMPRESSION & SPARK, BUT NO START
If the engine seems to have fuel, compression and spark, but still won’t spark, what do you do? You obviously missed something.
It
may have fuel, but the pressure may be low or the engine may be
flooding with too much fuel. You need to check fuel pressure and/or the
fuel command going from the PCM to the injectors.
The PCM should
provide a richer-than-normal air/fuel mixture for a cold start, but if
the driver is pumping the gas pedal or holding it down, or the throttle
position sensor is misreading, the PCM may go into the “clear flood”
mode and turn off the injectors — making starting impossible.
Also,
don’t forget that just because the engine has fuel, it doesn’t mean the
fuel is good fuel that will burn. As we said earlier, bad gas could
cause a no-start condition as well as a rough idle, stalling and
hesitation problems.
As for spark, the engine may have spark, but is
the timing correct? On older engines with a distributor, the
distributor may be retarded or over-advanced.
If the engine cranks
slowly or hesitates as it cranks, that might be a clue of over-advanced
timing (which you can check with a timing light or a scan tool). If the
engine cranks easily but fails to start, the timing may be too late
(retarded).
If the engine is backfiring and popping as it’s being
cranked, that may be a clue that somebody switched around the plug
wires and the wires are not in the correct firing order.
Or, the
distributor cap has cracks or carbon tracks that are causing crossfire
between the cylinders. It could also mean there is a weak or broken
valve spring that is leaking compression.
On engines with a
distributorless ignition, the ignition module and/or PCM controls spark
timing. There are no adjustments, but internal faults in either one can
throw off the timing. Use a scan tool to compare the ignition timing to
specifications.