How to find mechanical engine misfires

obd 2 reader
obd 2 reader

Mechanical engine misfires can contribute to low compression (bad cylinder, piston rings, sticky or leaky valves or even valves out of adjustment) will affect the amount of air coming into the cylinder which will cause a cylinder to misfire and could set a P0300.

You have done your diagnosis and determined it is a Mechanical Engine Misfire.


Engine Misfires
Engine Misfires

So, The first thing you are going to have to get is a ( Diagnostic Scanner Fault Code Reader ). There are many types to choose from with tons of options and as you can guess vary in price depending what your needs are. To understand fully what an engine misfire is we need to understand what it means.

Furthermore, If the engine is running perfect and all of the cylinders are “firing” you could say there is no misfire. So, Once the air fuel ratio inside the combustion chamber in any of the cylinders gets interrupted the engine will misfire.  Depending on the situation one or more cylinders can misfire.

You will see Engine shaking when idling,  Hesitation when accelerating  or Loss of power.

These are the most common trouble codes that can cause your Check Engine light to come on. 

The list below is for information purposes only and is only intended to give you a starting point to find misfires.

P0300 Random/Multiple Cylinder Misfire Detected
P0301 Cylinder 1 Misfire Detected
P0302 Cylinder 2 Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0304 Cylinder 4 Misfire Detected
P0305 Cylinder 5 Misfire Detected
P0306 Cylinder 6 Misfire Detected
P0307 Cylinder 7 Misfire Detected
P0308 Cylinder 8 Misfire Detected

When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of hydrocarbons (HC) in the exhaust. What causes a cylinder to misfire? Basically, it’s one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents voltage from jumping the electrode gap at the spark plug. Causes include worn, fouled or damaged spark plugs, bad plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

Some things to consider-Misfires

Consequently, Low compression cause by (bad cylinder, piston rings, sticky or leaky valves or even valves out of adjustment) are most common. As a result, Any of theses will affect the amount of air coming into the cylinder which will cause a cylinder to misfire.

A simple Compression Tester will be able to verify if a certain cylinder is low on Compression. If you find that a Cylinder has low Compression and would like to know why, using a Cylinder Leakage Tester will be able to tell you how much air is leaking out of the cylinder and where it is leaking out from. Consequently,  It could be the intake valve, exhaust valve, piston or piston rings, cylinder wall, cylinder head or even the cylinder head gasket.

vacuum testing
vacuum testing

One good fast trick I like to use is checking engine vacuum with a hand held Vacuum gauge.  Most engines pull in upwards of about 19 or 20 inches of vacuum and nice and steady at idle.  If your engine has a cylinder misfire due to a mechanical issue, the needle on the vacuum gauge will bounce around. This could indicate a mechanical failure with the engine itself that will need to be addressed.

So, If you have a major compression loss or if you just want to see inside of the cylinder, combustion chamber to inspect the piston itself or even the cylinder wall for nicks or scratches without removing the cylinder head or you could use a tool called a Bore Scope.  This neat little device will do just that.  It will be a bit expensive but may save you more money down the road in repairs.

Carbon can hold valves open causing compression loss-Misfires.

Carbon is going to form, but you can limit its creation by using gasoline that contains a fuel system cleaner. I would also limit idling time and cold starts, employing high-quality oil and keep the carburetor/fuel injection system tuned properly. Once these protocols are invoked, a semi-annual chemical cleaning will keep your engine carbon-free and in top running condition.


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