What Makes a Car Hesitate When Accelerating?

by Richard Rowe
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"Hesitation," noun, from nominative Latin "haesitatio," meaning "irresolution, uncertainty or indecision." Automobiles are decisive instruments by nature, traversing the Earth in, as much as possible, a straight line from Point A to Point B. You press the go-pedal, you've committed to going forward. Life should be that simple. But it isn't -- especially when our vehicles decide of their own accord that indecision is the appropriate response to a decisive request to go.

The Nature of the Problem

Many things can cause an engine to lose power: bad timing, airflow obstructions going into or out of the engine, carbon buildup on spark plugs, bad gasoline and improper fuel burn in the cylinders are just a few. But these things don't often cause engine hesitation -- at least, not initially. Hesitation by nature is a transient phenomena, where everything in the engine might be working perfectly fine one moment, and malfunctioning three milliseconds later. This means that whatever's going cyclically wrong in the engine has to be breaking and un-breaking very quickly. As a rule, hardware failures in the engine don't un-break themselves, and airflow variations aren't fast enough. That leaves two likely sources of power fluctuation: electricity and fuel pressure.

Fuel, Air and Misfire

Engine hesitation, almost by definition, usually goes back to a misfire in one or more of the cylinders. When one or more cylinders stops firing, the power cuts, and the vehicle hesitates under acceleration. The misfire is causing the hesitation; you just need to find out what's causing the misfire. Often, it will come back to a "lean" condition," meaning that there's either not enough fuel or too much air going in. Clogged fuel filters, failing fuel pumps or regulators, and dirty or malfunctioning fuel injectors are the most common causes of a failure of fuel delivery. The accompanying "lean misfire" will trigger a check-engine light. On the other side, you could have a massive air leak from a bad vacuum line or intake manifold gasket. Failed intake sensors -- throttle position, mass airflow, intake air temperature and manifold absolute pressure sensors included -- will often cause the computer to think there's less air going in than there really is. So, as far as its concerned, there's a massive vacuum leak present.

Electrical and Sensor Faults

Especially on modern engines, with their plethora of sensors and electrical components, electrical faults are a prime suspect in hesitation issues. A bad sensor or rlectrical faults in the wiring will usually display two telltale signs. The first is often that the hesitation is very rapid, even violent; electricity can shut on and off very quickly, which means engine power can flick on and off like a light switch. Second, electrical and sensor faults tend to cause random, spastic hesitation without any regular pattern. That doesn't mean regular, rhythmic hesitation can't be caused by an electrical problem, especially in the case of a misfire. It only means that if you see sudden, irregular, jerking hesitation without much consistent pattern of behavior, it's likely electrical in nature.

Reading the Codes

Electrical-sensor and fuel-air faults will often throw two very different kinds of diagnostic codes. Fuel-air problems tend to be very regular, affecting either the entire engine at once, or specific cylinders on a regular basis. With these, you'll most often see "Misfire on Cylinder No. __" and "Lean Condition" codes. Electrical-sensor faults are often much less predictable, because any amount of fuel could go through any cylinder unburned. Electrical-sensor faults, particularly in the ignition, are often signified by "Random Misfire" or "Rich Condition" codes. If you get either of these, the problem is most likely electrical in nature, probably to do with the ignition system. A single-cylinder code, or a single, consistently misfiring cylinder usually goes back to a bad fuel injector, spark plug, plug wire or electrical connection for that cylinder.

Specific Engine Cases

Your entire engine runs on the crankshaft and camshaft positioning sensors, so any fault with either of these could cause practically anything; typically, you'll see random misfire and rich condition codes. These failures are very common -- almost as much so as a bad throttle position sensor. A bad TPS can easily cause jerking hesitation, since the computer thinks you're rapidly opening and closing the throttle. If everything in the world seems to be going wrong at once, check your engine and frame's ground straps and the battery connection. A bad ground will cause everything to act up, including the computer. On modern vehicles, oxygen sensor failures don't typically cause hesitation; the computer should realize the sensor is bad, and will likely ignore its data and detune the entire engine until you get it fixed. Impeded exhaust flow from a bad catalytic converter can cause jerking hesitation, but an overall power loss is more likely.

Transmission Troubles

Hesitation doesn't always come from the engine. Electronically controlled automatic transmissions are subject to the same kinds of sensor and actuator problems engines are, and have a lot of complex hardware to control. Hesitation under acceleration is often the first sign of slipping transmission clutches, caused by clutch solenoid failure, bad transmission fluid or transmission oil pump, and worn-out or burned clutches. If your vehicle only seems to hesitate in top gear, or over about 45 mph, you may have a problem with the lock-up clutches in the torque converter. Of course, low transmission fluid can also cause hesitation, both because of fluid pressure fluctuations in the transmission, and because of an empty torque converter churning up air instead of transferring power. Modern vehicles will typically self-diagnose the transmission just like the engine, but these manufacturer codes may not be readable with a generic scanner. If you get hesitation, but a generic code scanner doesn't revealed any trouble codes, take your vehicle to a shop that specializes in your make of automobile. They'll have a scanner to check the manufacturer's codes.

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