Engine Misfires: Learn What Can Cause Them, And The Damage

So, Engine Misfire Causes, Can Usually Be Grouped, Into Three Categories:

1. Fuel
2. Ignition
3. Engine Mechanical

So, the first thing you need to do is, figure out which of the cylinders, is causing the misfire. I always start by removing the spark plugs, so i can read them. Also, a diagnostic scanner, can usually point you in the right direction. And, usually tells you which one is being problematic. Once you have that narrowed down, you know where to shift your focus.

So, There are a few suspects that occur more than others. However, The primary villains are simple, spark or fuel. And, are usually manifesting in spark plugs, plug wires, the coil(s) or the fuel delivery system.

But, there are also other more dire causes:

• Computer or wiring problems.
• Breakage in the rotating mass (pistons, rods, crank bearings).
• Valves and the heads can fail or distort.
• Cooling difficulties might permit overheating.

And, finally, any number of gaskets, could have blown or may be leaking.

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Random And Single Cylinder, Engine Misfires

So, a random misfire means, your engine is misfiring. But, the problem is not isolated, to one or two cylinders. It is jumping around, in a random way, from one cylinder to another. Also, a Random Misfire Code (P0300), usually means the “air to fuel” mixture, is running lean.

But, the cause might be anything from:

• Hard to find vacuum leaks.
• Dirty fuel injectors.
• Low fuel pressure.
• A weak ignition coil.
• Bad plug wires.
• Compression problems.

So, even a dirty (MAF) Sensor, can cause a lean code and or misfire to occur. The engine may be stalling, because it isn’t getting enough throttle opening. As a result, the cause is often a problem, in the (IAC) valve.

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The First Thing To Check, Is The Intake Manifold Vacuum

So, on most vehicles a normal reading is, 17 to 21 inches Hg. But, if the needle is lower and is jumping up and down or steadily dropping, you have a vacuum problem. Look for possible vacuum leaks, by checking vacuum hose connections. Also, the throttle body, intake, (PVC) valve and plumbing. An (EGR) Valve that is leaking, can also act like a vacuum leak. And, can cause, a random misfire.

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The Next Thing To Check, Is The Fuel Pressure

If the fuel pressure is not within specs, (refer to a service manual for proper specs). So, fuel pressure is critical, for proper engine performance. However, the problem may be, a weak fuel pump, low voltage to the pump (check the relay and wiring). Also, obstructions in the fuel line, (like a clogged filter). A bad fuel pressure relay, can also leak pressure. And, prevent an otherwise good fuel pump, from delivering full pressure to the injectors.

Dirty injectors, can also restrict fuel delivery, and cause a lean fuel condition. Many regular grades of gas do not contain, adequate levels of detergent, to keep the injectors clean. Frequent short trip driving, accelerates the buildup of injector deposits. Try cleaning the injectors with a good quality, fuel tank additive. And, if that does not work, having them professionally cleaned, can solve this problem.

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Ignition, Engine Misfires

An ignition system problem, is one of the most common reasons, for an engine to misfire. Spark plugs, ignition cables, distributor cap and rotor, and ignition coil wear over time. Hence, their ability to transfer the needed spark to ignite the “air to fuel” mixture, becomes compromised. In the early stages, the spark will only be weaker and the actual misfire, will be subtle.

But, as the ignition components continue to wear, the misfire will intensify. As a result, the combustion process, can be interrupted completely.

Consequently, this will cause a severe jerk or shock, in the operation of the engine. And, the engine may even backfire, through the air intake system, producing a loud “pop”. So, its possible the problem at the moment, may just be affecting one of the cylinders. But, you may as well change out, all of the spark plugs at the same time.

When there’s a misfire code for a specific cylinder, always remove and inspect the spark plug. NOTE: Cylinder three is common on a jeep. most often from heat.

Oily deposits on the plug, would tell you, oil is being sucked into the combustion chamber. So, it may be going past worn valve guides or seals, or worn or broken piston rings. There’s no easy fix for this kind of problem, short of a valve job or overhaul. Installing a spark plug with a slightly hotter heat range, may help resist fouling. Coolant leaking into a cylinder, can also cause spark plugs to foul.

NOTE: On engines with waste spark (DIS) systems. Misfire codes for any cylinders that share a common coil, would tell you the coil is the likely problem.

If a Coil On Plug (COP) Ignition system has a misfire that only affects one cylinder. Then, swapping coils between two cylinders, is a quick way to see if the misfire, changes cylinders. When the misfire follows the new location of the coil, it confirms the coil is bad. And, what if there is no change at all. Then, the misfire is being caused by a bad plug, a compression problem or a lean fuel condition.

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Lean, Engine Misfires

So, a lean misfire, is another common reason, for an engine “miss”. Usually, this is due to an unbalanced, “air to fuel” ratio. Normally, too much air and or too little fuel. Since an engine needs a richer (more fuel) mixture for a smooth idle. This problem may be more noticeable, when the vehicle is idling.

The lean misfire, may decrease or disappear, as the engine speed increases. Because, the efficiency of the volumetric flow, into the combustion chambers, increases dramatically. This is one reason why a vehicle gets, better mileage on the highway than in the city.

You Could Have A Lean Misfire Caused By:

• Stuck Open (EGR) Valve.
• A leaking, Intake Gasket.
• Defective, Mass Air Flow (MAF) Sensor.
• Weak Or Failing, Fuel Pump.
• A Clogged, Fuel Filter.

A misfire caused by uneven ratio of “air to fuel”, is almost always caused by, too much (O2). This makes for a weak ignition, that will leave your car struggling for power. The most common cause will be, a (O2) or (MAF) Sensor malfunction.  This is an easy fix, that you can do yourself. Less common, is too much fuel in comparison, with the amount of air causing the misfire. This will most likely be caused by, a fuel injector leak. And, will usually cause all of the cylinders to misfire.

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Mechanical, Engine Misfires

Mechanical problems, can also cause an engine to misfire. Common causes are:

• Worn piston rings.
• Valves.
• Cylinder walls.
• Lobes on a camshaft.
• A leaking head gasket or intake gasket.
• Damaged or broken rocker arms.
• Defective fuel injectors (and or the electronics that control them).
• A slipped or incorrectly installed timing belt or timing chain.

Generally, this type of misfire has, more of a “thumping” feel to it. It is usually noticeable regardless of engine speed. In fact, it may even intensify, as the engine speed increases.

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Powertrain, Misfire

Sometimes, the engine has nothing to do with a misfire. So, there is one common cause for “jerky” performance, that feels like a misfire. And, this is a problem in the transmission, and its ability to properly up or down shift. If the misfire occurs during higher speeds. Then, it could be a problem, with the overdrive gear or a chattering clutch, in the lockup torque converter. If the vehicle jerks or feels like it is “missing” during deceleration, it could be due to harsh transmission downshifts. Furthermore, badly warped rotors, out of round brake drums, and or sticking brake pads or brake shoes.

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Conclusion

So, nothing gets your attention like an unexpected change, in the performance of your vehicle’s engine. If you are aware of this issue, have it fixed as soon as possible. While it may be inconvenient, it’s far better than having your engine, suddenly quit or suffer catastrophic damage.

BY DANNY BENDER