Engine Misfiring - What Can Cause It - Tracking Down The Problem

What is engine misfiring and what are the main causes.

Engine misfiring, is bad, so it’s best to find the problem and fix it as soon as possible.

Because, engine misfiring reduces gas mileage and increase emissions, which can cause you to fail an emissions test.

More seriously, Engine Misfiring can cause damage to other engine parts, like the Oxygen (O2) Sensors or catalytic converters.

Engine misfiring occurs when, ONE or more of these factors is wrong, missing or just not working together:

• A too lean or too rich air/fuel ratio.
• Bad ignition spark / wrong timing of the ignition spark.
• Low compression / air/fuel ratio is leaking out.
• The timing of the inlet/outlet of the air/fuel ratio is wrong.

Common Engine Misfiring Causes

If one or more of the above factors aren’t in proper working condition, you’ll run the risk of misfiring. There are a variety of different misfire causes that can occur. And, most of them, will keep a cylinder from combusting properly. Sometimes it takes a combination of issues, in order to make a cylinder misfire. Other times, misfiring isn’t consistent, and sometimes the cylinder will fire properly. So, it all depends on the type of issue, and how severe the issue is.

Faulty Or Fouled Spark Plugs

Spark plugs provide the spark for the ignition of the, air/fuel mixture in the combustion chamber. But, spark plugs can go bad for a number of reasons, and are one of the leading causes of misfiring. Spark plugs can get fouled from oil leaking into the combustion chamber, or from carbon buildup.

They can also wear down more quickly, from overheating and improper plug gap. The bottom line is, that if you’re experiencing misfiring one of the leading causes, is bad spark plugs. It’s easy to check and easy to fix.

Cracked Distributor Cap ( If You Have One )

The distributor is what controls, the timing of the spark. When the distributor is functioning properly, the spark to each spark plug is sent, at just the right time. And, will ignite the air/fuel mixture, right when the piston is ready. A cracked or damaged distributor cap, can cause the signal to be lost. As a result, causing one or more of the spark plugs, to misfire.

Lean Air/Fuel/ Mixture

The air/fuel ratio is a pretty delicate balance. If the mixture gets out of balance, it can cause a misfire. So, what are some ways that the, air/fuel ratio can get out of balance? If things like the fuel pump or the fuel injectors get clogged. Then, not enough fuel will be able to get to the combustion chamber, and can readily cause a misfire.

Lack Of Compression, From Leaking Valves Or Head Gasket

If a cylinder lacks proper compression, then the spark from the spark plug, won’t ignite the air/fuel mixture. Most of the time, by leaky exhaust valves or a blown head gasket. In either case, you’ve got a pretty major problem. So, there is a way to tell whether your head gasket, or a leaky valve is causing the problem. Check to see if two cylinders, next to each other are misfiring.

If so, then you most likely are dealing with, a blown head gasket, instead of a leaky valve. In addition, if an engine overheats or runs hot and if you’re noticing that you’re also, mysteriously losing coolant. Then, you most likely are dealing with, a blown head gasket.

Quick Diagnosis With A Engine Misfiring Code

On OBD-II vehicles, the OBD-II system will not only identify misfires, but also coil and injector problems.

Consequently, if the Malfunction Indicator Lamp (MIL) is on, you find a code for a misfiring cylinder. And, a second code, indicating an injector fault, for the same cylinder. Bingo, the engine probably has, a bad fuel injector.

Likewise, if you find a misfire code for a cylinder. And, also a code indicating a coil fault for a multi-coil, Distributorless Ignition System (DIS) or Coil-On-Plug (COP) ignition. Then, you can probably bet, on a bad ignition coil.

In cases where there’s a cylinder misfire code, but no other codes. The ignition or fuel delivery system, may be borderline. But, not yet bad enough, to set a code of its own. A shorted or open fuel injector solenoid, or a shorted or open coil will usually set a code. Consequently, a dirty or weak fuel injector or a weak coil, probably won’t set a code.

Bad spark plug wires, are a common cause of misfire codes. So, after 50,000 or so miles, the (OEM) wires may be leaking current to ground. Sometimes, it can be, other wires shorting the spark, before it can reach the plug. Check resistance and if it exceeds specs, replace the wire set.