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Diagnosing an Engine that Won’t Start


8/18/2010
By Larry Carley

In today’s world of high-tech electronics, onboard diagnostics and scan tools, you would think engine diagnostics would be easier than ever.
 
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.












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