Engine Misfires – Finding And Fixing Them
Engine misfires can cause the driver to feel a jerking motion while driving.
Engine misfires can happen constantly or intermittently.
The engine stumbles for a moment and then regains its pace.
Just as soon as the rpm settle down, though, the engine misfire reappears, and you’re stuck with the sinking feeling that accompanies all automotive problems: “Something’s wrong.”
Engine misfires can be grouped into three categories:
- Engine mechanical
So, The first thing you need to do is figure out which of the cylinders is causing the misfire.
One of the first things I always do is remove the spark plugs and read them.
A diagnostic scanner can usually point you in the right direction by telling you which one is being problematic.
Once you have that narrowed down you know where to shift your focus.
There are a few suspects that occur more than others. The primary villains are simple – spark or fuel – usually manifesting in spark plugs, plug wires, the coil(s) or the fuel-delivery system. There are 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 any number of gaskets could have pushed.
Random and single cylinder Engine Misfires
A random misfire means your engine is misfiring. As a result, The problem is not isolated to one or two cylinders. It is jumping around in a random way from one cylinder to another. A random misfire code usually means the air/fuel mixture is running lean. But the cause might be anything from a hard-to-find vacuum leak to dirty fuel injectors, low fuel pressure, a weak ignition coil, bad plug wires, or compression problems. 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. The cause is often a problem in the idle air control system.
The first thing to check is the intake vacuum with a vacuum gauge.
On most vehicles a normal reading is 17 to 21 inches Hg. If the needle is lower, is jumping up and down or steadily dropping, you have a vacuum problem. Look for possible vacuum leaks by checking vacuum hose connections, the throttle body and manifold, and PVC valve and plumbing. An EGR valve that is leaking can also act like a vacuum leak and cause a random misfire.
The next thing you should check is the fuel pressure with a gauge. If it is not within specifications (refer to a service manual for specifics because fuel pressure is critical for proper engine performance), the problem may be a weak fuel pump, low voltage to the pump (check the relay and wiring), or obstructions in the fuel line (like a plugged 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 gasoline do not contain adequate levels of detergent to keep the injectors clean. Frequent short trip driving accelerates the buildup of injector deposits. Cleaning the injectors with a good quality fuel tank additive (or having them professionally cleaned) can solve this problem.
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/fuel mixture inside the combustion chambers becomes compromised. In the early stages, the spark will only be weaker and the actual misfire will be subtle.
As the ignition components continue to wear, the misfire will intensify and the combustion process can be interrupted completely. Consequently this will cause a severe jerk or shock in the operation of the engine (the engine may even backfire through the air intake system, producing a loud “pop”). Even though the problem at the moment may just be affecting one of the cylinders 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.
Oily deposits on the plug would tell you oil is being sucked into the combustion chamber. 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.
On engines with waste spark distributorless ignition 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, 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. If there’s no change, the misfire is being caused by a bad plug, a compression problem or a lean fuel condition in that cylinder.
Lean Engine Misfires
The lean misfire is another common reason for an engine “miss”—this is due to an imbalanced air/fuel ratio (too much air/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 freeway than in the city.
You could have a lean misfire caused by:
- Stuck open EGR valve
- A leaking Intake Manifold Gasket
- Defective Mass Air Flow Sensor
- Weak or failing fuel pump
- A plugged fuel filter
A misfire caused by uneven ratio of air to fuel is almost always caused by too much oxygen. This makes for a weak ignition that will leave your car struggling for power. The most common cause will be the O2 or air mass sensor malfunction. This is an easy fix that you can do yourself. Less common is too much gas in comparison with the amount of air causing the misfire. This will most likely be caused by a fuel injector leak and will cause all of the cylinders to misfire.
Mechanical Engine Misfires
Mechanical problems can also cause an engine to misfire. Common causes are:
- Worn piston rings
- Cylinder walls
- Lobes on a camshaft
- A leaking head gasket or intake manifold 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.
Sometimes, the engine has nothing to do with a misfire. One common cause for “jerky” performance that feels like a misfire is a problem in the transmission and its ability to properly up- or down-shift. If the misfire occurs during higher speeds, it could be a problem with the operation of 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, badly warped rotors, out of round brake drums, and/or sticking brake pads or brake shoes.