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Engine Misfire Video

On this page, you’ll find a short video outlining four common causes of your engine misfire and you’ll find links to tools and some tips to help you figure out what’s going on.

A misfiring engine is common and the range of causes as you can imagine is wide. I’ve covered the four most common causes in the video but I’ve added a ton more on this page also.

But just before we get into the list let’s quickly cover what a misfire actually is.

What is a Misfire?

Most engines are four strokes, they are described as four-strokes because of the four distinct phases or strokes each cylinder passes through. Bear in mind each cylinder is simultaneously on a different stroke but all cylinders pass through the same four strokes.

Here’s a fast rundown of what each of the four strokes does.

Induction stroke

Induction stroke – Retreating piston sucks gas and air(fuel) into the cylinder through the open intake valve.

Compression stroke – The piston moving back up the cylinder compresses the fuel into the combustion chamber.

Compression stroke
Power stroke

Power stroke – As the piston passes TDC (Top Dead Center) the spark plug fires causing the fuel mix to ignite in the combustion chamber.

The explosion sends the piston lower powering the crankshaft as it does.

A misfire is said to occur when a cylinder’s combustion chamber fails to ignite at the appropriate moment (Piston just past TDC)

Exhaust stroke – The piston moving back up the cylinder pushes the spent gases out through the open exhaust valve and into the exhaust system.

And the cycle starts over once again.

Exhaust stroke

Misfire Symptoms

A misfire is a pain in the ass especially an intermittent misfire and it’s a very common fault. Anyhow, here’s a list of common symptoms before we look at the list of common causes.

  • Engine fault light on also known as MIL on
  • Common misfire fault codes logged P0300, P0301, P0302, P0303, P0304, 0305, P0306, P0307, P0308, P0171
  • Engine won’t start
  • Engine hard to start
  • Engine stalls
  • Engine hesitates
  • Engine vibrates
  • Loss of power
  • Engine in limp mode (limited performance)
  • Vehicle shakes

Causes Of Misfire

The causes of a misfire generally fit into one of three buckets:

  1. Fuel mix (air & gas)
  2. Ignition system
  3. Mechanical issue

We’ll look at each in a little more detail.

Bucket 1 – Fuel Mix

An engine won’t run on gas alone, we need air (oxygen) in the mix and so when we talk about the fuel mix we are really talking about the mix of air and gas. It’s known as AFR (Air Fuel Ratio) and an engine loves a ratio of 14.7 parts air to one part gas.

If this ratio is off significantly, the engine misfires.

Fuel misfire classification

A misfire caused by a fuel mix issue may be further broken down into two smaller groups. They are as follows:

  1. Lean condition – A ratio greater than 14.7 parts air to one part gas means the engine is running lean.
  2. Rich condition – A ratio of fewer than 14.7 parts air to one part gas means the engine is running rich.

So changing the amount of either ingredient can create the same effect, ie if we added more air to the mix it would cause a lean condition, equally, if we took away some gas from the mix we could also cause a lean condition. That’s a useful way to think about a misfire problem when diagnosing.

Lean Condition Common Causes

  • Bad gas
  • Faulty oxygen sensor
  • Faulty fuel injector
  • Faulty fuel pump
  • Faulty fuel regulator
  • Blocked fuel filter
  • Vacuum leak
  • Faulty head gasket
  • Faulty MAF sensor
  • EGR valve

Rich Condition Common Causes

  • Bad spark plug
  • Bad ignition coil
  • Faulty MAF sensor
  • Faulty fuel injector
  • Exhaust system leak
  • Faulty ECT sensor
  • Boost pipe leak

Bucket 2 – Igition System

No fire without a spark, although pretty robust ignition coils are a common cause of misfire. Here are some other causes.

  • Bad ignition coil
  • Bad spark plug
  • Faulty Knock sensor
  • Timing issue
  • CKP sensor fault

Bucket 3 – Mechanical

Under this heading we’ll include all the oily bits that can go wrong which will cause a midfire. The items in this bucket tend to be more expensive to correct than the other two buckets and in some cases depending the age of the vehicle may not be worth fixing.

A lack of sufficient compression in one or multiple cylinders will cause a misfire and that’s a common demonator with many of the common causes in this bucket.

  • Head gasket fault
  • Valve or valve seat fault
  • Stretched timing chain issue
  • Piston or rings fault
  • EGR valve stuck

Tools and Parts

Here’s some tools to help diagnose an ignition system fault:

The Autel MaxiCOM is a shop-level tool. It is capable of reading, clearing fault codes, coding a new battery, placing EPB module in service mode, and calibration after the repair. It is also capable of activating ABS modulator solenoids when performing a full brake bleed procedure. Picture links to

The Topdon is an inexpensive scan tool capable of scanning and clearing Powertrain flat codes. I wrote a review of it here “Topdon Vs Autel”. Picture links to

OTC make great tools, you don’t use a compression test kit every day but when you need it you want it to be reliable. That’s especially true when diagnosing major engine issues like lack of compression. Picture links to

Combustion leak test kit. Head gasket failure isn’t always so obvious. In those cases use this chemical leak tester. Check for combustion chamber leaks into the coolant system. Comes with a test tool and chemical, enough product here for about a dozen tests. Picture links to

Bar’s leaks is one of the best in the business. I wouldn’t use this on a fresh engine, I’d replace the gasket. But head gasket repair isn’t economical on older vehicles and Bar’s is the way to go. There’s a different potion for each type of leak you have, see the chart on the sales page. Try and match your type of leak to their chart for the best chance at success. Picture links to

Perfect coolant system and rad cap pressure test kit. Coolant leaks out and air sneaking into your coolant system are common causes of overheating, poor heat, and intermittent heat. Very often the issue is simple, a bad coolant cap or loose hose clamp. This little tool while not premium quality is perfect for the occasional user. Picture links to