Diagnosing drivability problems can be really challenging, but it doesn't have to be when you have a systematic plan of attack. Most common drivability problems can be traced to faults in the ignition system, faults in the fuel management system, or faults in the engine mechanical system. Diagnosing the problem involves determining which system is faulty, then pinpointing why it's faulty.
The best way to diagnose drivability concerns is to systematically attack the problem by figuring out what could possibly cause the problem, then taking some measurements and data to confirm or discredit your suspicions accordingly.
The process gets easier with experience, but if you're just starting out, there are a few things to know and understand to "get you up to speed."
Even just a few years ago, ignition system problems were easy to identify because faulty ignition components acted up when it rained (a spray bottle used to "mist" the ignition system was a common diagnostic tool).
This rule isn't always true of newer ignition systems. Not only are seals and weatherpacks a bit more durable, the components that caused many of the problems — such as spark plug wires and distributor caps — have been eliminated. But there are still ways to identify an ignition problem.
From experience, the no-start conditions caused by faulty ignition systems tend to occur suddenly, as opposed to the problems that start small and get worse over time — such as a complaint that all of a sudden the car just won't start, even though it was fine yesterday.
To make diagnosis as quickly as possible when you're dealing with this type of a no-start condition, it's really important to determine whether the vehicle has a "cranks, but no start" condition or a "no crank, no start" condition. In other words, does the engine even crank over? It may seem simple, but it's an important check that saves diagnostic time.
If the vehicle doesn't even crank ("no crank, no start") it means that battery power isn't getting to the starter. You need to figure out why. Visually inspect the system, locate the components in the path to the starter and make sure the battery and connections are OK. If they are, start to systematically check for power until you find out where the problem is.
This isn't as hard as you'd think. Using a multimeter (rather than a test light), check for battery power at the ignition switch, making sure that power is both coming in and going out. If the switch checks out OK, move along to the Park/Neutral switch (or clutch switch, if it's a manual transmission vehicle). Again, check for power going in and coming out. Continue along to the starter itself if needed.
When you find the location where there stops being power, check between that spot and the last place you checked that still had power. Trace it backward until you find the problem. Hopefully, it'll be easy to locate and easy to repair.
The problem may even be as simple as a blown fuse (it happens, especially if someone's tried to boost the battery incorrectly). However, if you make it all the way to the starter without finding the problem, then suspect the ground for the starter, a cracked or damaged cable (especially at the connection), or the starter itself. It's worth mentioning that one TV news show loosened a battery cable, then sent the vehicle to various shops to find out whether shops were overcharging for simple repairs. Very few shops found the actual problem.
One more important note: Remember when people used to tap starter motors with a hammer to see whether this would make the car start? On modern vehicles, hitting the starter motor is a bad idea. Depending on the style of starter motor, tapping it may knock permanent magnets loose and ruin it — possibly causing two problems (the one the vehicle came in with, and the one you just caused).
While a "no crank" condition is diagnosed by figuring out why there's no power at the starter, a "cranks, but does not start" condition involves a different diagnostic procedure. Because you can be relatively sure the battery and starter are OK (there's at least enough power and ground to turn the engine over!), to diagnose this condition, you'll need to look for problems elsewhere.
In these cases, verifying the Check Engine Light (CEL or MIL) and checking for DTCs is usually the best (and easiest) place to start. The CEL is supposed to light up briefly for a "bulb check" when the ignition is turned on. If it doesn't, check for blown fuses that supply power to the control module, a bad control module or a wiring problem as the likely cause.
If the light does come on, check for DTCs. Hopefully, the stored data will point you in the right diagnostic direction — but you'll still need to prove your suspicions to avoid condemning the wrong parts. Here's where you need to start doing some thinking.
For example, say you find a DTC P035- ("-" represents the cylinder number where the fault occurred) — Igniter Malfunction. While this may indicate a faulty ignition coil, you need to verify your hunch before ordering any parts. The easiest way to verify your suspicions is by simply swapping the coil with another one on the same engine, clearing the code, then attempting to start the vehicle. If the DTC moves to the new cylinder (where you installed the suspected bad part), you've found the problem. You can order a new one with confidence.
However, things often aren't this straightforward when diagnosing drivability problems. If you check DTCs and find igniter codes for every cylinder, look for the one that has freeze-frame data. From experience, that's the one that most likely started the problem.
Here's another basic drivability diagnostic tip: Look for what multiple problems have in common. In some cases, ignition coils are commanded to fire individually by the ECM, but the confirmation signal back to the ECM can be linked together in a single wire. When one coil malfunctions, it can cause problems in the whole system because of that common signal wire. In some cases, it's a malfunctioning ground common to several components or a pass-through point on the body that wiring harnesses share. Look for what the systems have in common, and you may well find the problem.
Sadly, for both wiring harnesses and the rodents that snack on them, injector and ignition coil wiring seem to be an attractive choice for our little rodent friends to chew on or through. One Toyota technician who works near farms and fields told me he's seen rodent-modified wires short together, causing the driver in the ECM for that particular coil to be permanently damaged. In addition to the wiring repair, the vehicle needed a new ECM, too.
If for whatever reason you can't check DTCs, the first thing to do is check for spark — good, strong, hot spark from a clean, dry, correctly gapped plug with no cracks or leaks.
This can get a bit tricky, but what you'll need to do is remove the ignition coil or spark plug wire lead (but still leave it connected), install a spark tester into the end of it, and evaluate the spark while cranking. The color of the spark indicates its strength: orange spark is weak; blue spark is strong. You want to see strong spark. Weak spark may not be igniting the mixture properly, causing drivability problems.
If you determine there's no spark, you may have a problem with the ignition coil itself, or the vehicle may not be commanding the coil to fire. You need to find out which one it is. Again, this isn't a big deal if you have a systematic plan of attack.
First of all, check the ignition coils for battery power, ground and good wiring and connections so that you can eliminate those possibilities.
Here's another diagnostic trick: I've had good luck hooking up a timing light and using it to check coil operation by touching the inductive clamp portion of the light to the coil while cranking. If the coil's firing, it causes the timing light to flash.
Fortunately, most vehicles today store incredibly helpful DTCs to aid the diagnostic process — not to mention the freeze-frame data and stored history. If you're diagnosing a drivability concern that's turning on the CEL and setting DTCs, checking and writing down the codes is the best place to start.
If the stored codes indicate a fault in the fuel management system, inspect the wiring and terminals inside the connectors involved with the circuit. A friend at GM tells me that they've been instructed that if a connector has been unplugged and reconnected three times, that terminal is considered a weak point in the circuit and the fit needs to be checked — and that the terminals involved may need to be replaced. There's even a special tool to do this if necessary. "Drag testing" the terminals to determine whether the pins are making a good, solid connection in all conditions is a trick used by some of the best diagnosticians when trying to fix elusive intermittent concerns, and it does help locate some strange problems. Again, systematically attack the problem by figuring out what could possibly cause the problem, take some measurements and data, and confirm or eliminate your suspicions accordingly.
It's worth mentioning that wiring harness connectors that are exposed to the road and the elements tend to attract salt, dirt and corrosion, and cause drivability conditions associated with wiring problems. Inspecting these areas closely can often help locate the cause of the problem. Unfortunately they're often a real pain to fix.
However, if the problem isn't setting DTCs, start by checking the things that can affect fuel delivery, such as the fuel pump (pressure and volume), injectors, check valve and pressure regulator (along with the related wiring, connectors and grounds). It's also worth checking that there's actually fuel in the tank. Fuel level indicators have been known to fail (the sending units go bad) and indicate various incorrect fuel levels — when in fact the tank is quite empty. One important note, though, be really careful handling aging plastic components like sending units, filters and connections. They may be brittle and easy to crack, and expensive to replace.
One more observation from up here in the snow belt: Fuel just doesn't seem to keep for a very long time. If the vehicle has been sitting for a few months, the gas may have gone bad. Along those lines, using the wrong grade of fuel can cause no-code drivability concerns. Switching fuels may solve the problem. And if you live in an area that offers E85, don't eliminate the possibility your customer has filled the tank with a fuel that's not compatible with the car.
Mechanical engine problems are responsible for a surprisingly large number of drivability concerns on newer vehicles, and the reason for this may be surprising.
In addition to the common reasons why base engine problems cause drivability concerns (such as timing belt issues, valve issues, and restricted or gunked-up intake and exhaust systems), using the wrong engine oil can cause drivability issues, too. Ford trucks are known to develop strange drivability conditions after an oil change because the wrong oil was used. Draining and refilling the crankcase with the correct oil fixes the problem. Be suspicious if the drivability condition developed after the vehicle was just serviced.
For that matter, be suspicious if the vehicle has rarely been serviced. Vehicles with VVT-i need regular oil changes, and if the oil is left too long between replacement intervals, it may thicken up and actually block the small passages that go to the oil control valve (the valve controlled by the ECM that regulates oil flow and pressure to the VVT-i gear). This can set DTCs — usually ranging from P0012 to P0022, but look it up and be sure.
To run properly, vehicles need spark, fuel, air and the compression cycle in the right amounts, at the right time. If any of these are missing, removed or affected by something, drivability problems result.
Diagnosing drivability problems involves planning a systematic attack — consisting of forming an idea of what could cause the fault, then carefully checking, measuring or evaluating those components or systems to find out whether they're functioning as they should to confirm your suspicions. It's not hard, you just need to be thorough and not take anything for granted. It can be challenging, but a systematic plan of attack will help you find the problem and fix it fast — and right the first time.
Vanessa Attwell is a Master Technician for two major manufacturers, and has also worked on the bench of an independent shop. She has developed and delivered training for both vehicle manufacturers and independents, and helped develop government training and regulations standards. She drinks too much coffee and spends her spare time sitting in traffic.
