An intermittent engine problem can be a nightmare to diagnose. Problems that come and go or only occur under certain driving or operating conditions can be very difficult to troubleshoot, unless you have a diagnostic strategy to outwit these kinds of problems.
One thing all intermittent problems share in common is they are not a steady-state condition. If an engine dies and refuses to start, you can check for spark, fuel and compression to start isolating the cause. But when you have an engine that runs fine one minute, then dies, bucks, hesitates or misfires, then runs fine again, it is a different story. Something is obviously occurring that is interfering with normal combustion. But what? Is it the ignition system, fuel system, computer or something else? That is where a diagnostic strategy comes in.
The worst kind of intermittents are those that occur infrequently, say once a week or less. Your odds of correctly diagnosing an infrequent intermittent are probably the same as winning the lottery. Unless you are lucky enough to catch the problem when it is occurring, you will have few clues to go on.
You can always guess at a diagnosis based on a hunch or previous experience, but the odds of hitting the target every time are pretty slim. That is how parts changers "fix" cars. They replace the part they think might be causing the problem, and when that fails to cure the problem they replace something else and so on. Eventually they may replace the right part that was really causing the problem. But in the meantime, they have bought parts they did not really need.
There are better ways to tackle intermittent engine problems. One is to wait until the intermittent has become a more frequent or continuous problem. It is always easier to diagnose a part that has failed than one which is only misbehaving. But if you cannot depend on your car and don't want to risk a breakdown, you need to fix it now.
One time-saving step that may allow you to zero right in on the cause is to check for any technical service bulletins that might be out on the problem. It may be a situation where there is a pattern failure and the vehicle manufacturer has already done the diagnostic homework and figured out the solution for you. In many cases, taking a few minutes to check for a possible TSB can save you hours of frustration and wasted diagnostic time. The cure may not be to replace a part but to reflash the computer to change its operating instructions.
The next thing you should always do is hook up a scan tool and check for trouble codes even if the Malfunction Indicator Lamp (MIL) is not on (the lamp may be defective). Depending on the system and scan tool you use, you also should take a look at any history codes or pending codes that may be in the Powertrain Control Module (PCM) memory. Also, look at some of the data stream perimeters like long- and short-term fuel trim, oxygen sensor cross counts, TPS and MAP sensor signals. See anything that looks out of range? It might be a clue as to what is causing the problem.
Another basic check that should always be made is the battery voltage and charging voltage. A low battery, weak alternator or overvoltage condition can all play havoc with onboard electronics. Solenoids and relays all require minimum voltages to function properly, so if the battery or charging system is not within normal specifications, you may have found the root of the problem.
If a problem has left no tracks (no trouble codes or odd readings to steer you in a particular direction), your next task is to establish a pattern - if there is one. Does the problem only seem to occur during certain operating conditions?
The two most common types of intermittent problems are an engine that cranks but may not start under certain conditions, and an engine that runs fine but occasionally experiences some kind of driveability problem such as stalling, surging, hesitation, stumbling, bucking, misfiring, knocking, rough idle, idles too fast, etc. In both cases, the intermittent may only occur during some kind of environmental or operating condition (only during wet or rainy weather, only when hot, only when cold, only when accelerating, and so on).
Ah hah. We now have the beginnings of a diagnostic strategy for dealing with intermittents. Once you have established the conditions that are associated with the problem, you have a clue as to what might be causing the problem.
If an intermittent starting or driveability problem only occurs when the engine is hot or cold, only during warm-up, only when the engine reaches normal operating temperature, or only when the ambient temperature is high or low, you know that temperature is affecting something. The question is what?
Temperature-related intermittents often mean a circuit is shorting out or opening up as a result of thermal expansion or contraction. Heat may be causing a loose or corroded connector or ground to break contact. Microscopic hairline cracks in circuit boards, soldered connections, wiring connectors and even integrated circuits may open up as operating temperatures rise. An injector solenoid or ignition coil that shows normal resistance at room temperature may short out or open up when it gets hot. The same goes for relay coils and contacts. Sometimes diodes and transistors can become flaky at high temperatures and/or voltage loads, too.
Temperature effects on electronic components can be simulated with a blow comb or hot air gun. By directing heat at suspicious connections, modules or other components, you can sometimes get the part to misbehave when it gets hot. If so, this would confirm the problem and complete your diagnosis. The next step would be to replace the faulty component.
Changes in operating temperature also affect the way the PCM controls spark timing, the fuel mixture and other emissions functions. If an intermittent problem only occurs after the vehicle has been driven several miles, it may be occurring when the PCM goes into closed loop. The underlying cause might be a bad oxygen sensor signal, airflow sensor signal or MAP sensor signal that is upsetting the air/fuel mixture.
If a problem seems to occur only when the engine is running in closed loop, that would tell you it is probably a sensor or PCM-related issue. The strategy here would be to look at some of the key sensor inputs with your scan tool to see if readings are within normal limits. Some problems may occur too quickly for the normal data stream to detect a fault, so you may have to hook up a digital storage oscilloscope to detect a momentary glitch.
Temperature also can cause mechanical things to stick as a result of thermal expansion when a part gets hot. Valves and lifters can stick if an engine overheats. EGR valves can stick from heat or a buildup of accumulated carbon deposits. Relay contacts may be affected by changes in temperature, too.
One thing to check here is the operation of the cooling system. A low coolant level may prevent the thermostat from opening and closing normally. An inoperative electric fan or a clogged radiator also may allow unwanted fluctuations in engine temperature that affect the way it runs.
Intermittent problems that only occur when traveling at a certain speed, when driving on a rough road, when hitting a bump, when accelerating or braking, etc., are a pretty good indication that something is loose. The underlying problem may be a loose wiring connector, ground strap, a wiring harness that is chaffing or rubbing against something, or a circuit board with hairline cracks or fractures.
Ford's key on engine off "wiggle tests" are one way to check for loose wires and connectors that may be shorting or opening as a result of movement or vibration. A visual inspection of the wiring and connectors combined with some continuity checks while wiggling or moving wires will often reveal the bad connection.
Motion-related intermittent problems also can occur when harmonic vibrations in the exhaust system, driveline or suspension feedback through the powertrain or chassis. This may affect the operation of certain parts or make you think your engine is running rough or making unusual noises.
Problems that only occur during damp weather, when it is raining, after driving through a puddle, etc. would tell you water is acting as a conductor and shorting something out.
When humidity is high, moisture can condense out of the air and form droplets of liquid on any cold surface. If the surface happens to be a distributor cap, ignition coil or spark plug wires, the ignition voltage may find a shortcut to ground instead of following the normal path to the spark plugs.
Moisture is especially damaging to PCMs and electronic circuit boards. Moisture can cause corrosion that shorts out circuits and causes all kinds of weird electronic problems. That is why flood-damaged vehicles are so unreliable. Sooner or later, they usually need to have the PCM and/or other electronic modules replaced.
A totally random intermittent is the worst of all to diagnosis. God help you because the only thing you know about the problem is that it doesn't depend on any specific driving or environmental condition. It may occur at any time, at any temperature or any driving situation.
If there is no apparent pattern to a problem, how can you duplicate it? The answer is you often can't. The only way to catch these kinds of problems is to keep driving your car until it acts up, then hope you can find some clues as to why the problem is happening.
One alternative to the "wait-and-see" diagnostic approach is to take your car to a repair shop that has what's called a flight recorder. This plus into the OBD II diagnostic connector under the dash and records engine data while you drive your car. When the problem occurs, you push a button and the recorder stores a snap shot of the engine data at that moment. The shop can then look at the data on their scan tool or with a PC to hopefully get some insight into what happened and why.
Scenario #1: An engine starts normally, idles smoothly and runs okay under light load, but misfires erratically when it comes under load or accelerates. The engine is obviously misfiring under load but does not do it all the time. The most likely causes here would be a defective ignition coil, bad plug wires or worn out or fouled spark plugs.
When the ignition system comes under load, the spark finds an easier path to ground causing ignition misfire. The coil primary and secondary resistance may both be within specs, but the coil may have hairline cracks or internal shorts that only show up under load.
A visual inspection of the ignition system and observing the snap voltage and ignition patterns on a scope can help you identify and isolate the parts that need to be replaced.
Scenario #2: An engine starts normally one day, but not the next. It cranks but refuses to start the first or second time, but may start normally after it sits awhile. The condition does not seem to depend on temperature or humidity. Something is obviously disrupting spark or fuel, but what?
The trick here is to catch the engine when it is acting up. If there is no spark, check the crankshaft position sensor or distributor pickup and module. If there is spark, check the injectors and fuel system. Is the fuel pump working? Are the injectors buzzing? No buzz means no injector driver signal from the PCM. The underlying cause might be a bad injector relay, no trigger signal to the PCM or a bad driver circuit inside the PCM. If there is no spark and the vehicle is a Chrysler with an ASD (auto shutdown) relay, it is probably a bad relay because the relay supplies voltage to both the fuel pump and the ignition coil.
If the coil, fuel pump and PCM are all hot but there is no injector signal from the PCM (which you can check for at the injectors with a noid light), the driver circuit in the PCM may be bad - or one of the injectors may be shorted and pulling down the PCM driver circuit. Check the resistance of each injector before you condemn the PCM.
If there is no fuel and the pump is not running, the pump may have a loose or corroded wiring connection, or a bad relay. If the vehicle has a safety inertia switch that cuts off the fuel pump in case of an accident, check that too.
Scenario #3: An engine starts normally when cold, but has hot starting problems. Check the residual pressure of the fuel system when the engine is shut off. If the system is not holding residual pressure, the fuel inside the fuel rail may be boiling from engine heat. The cause here is a leaky check valve in the fuel pump, a leaky pressure regulator, or a leaky fuel injector.
Scenario #4: An engine suddenly quits running, but then starts again and runs normally. The cause may be the loss of an important sensor signal such as the crankshaft position sensor signal (distributorless ignition systems) or the ignition trigger signal (engines with distributors). Most of the old Ford Thick Film Integrated (TFI) modules that would quit working when they got too hot have been replaced, but there are still a lot of them on the road. Older GM HEI ignition modules also were prone to this disorder - especially if someone replaced the module in the distributor and forgot to apply the heat sink grease underneath to prevent the module from overheating.