Classic Car Misfire : Simple Beginners Guide


Car engine

I’ve been working on classic cars for years and I’ve learned a few tricks for finding those missing horses. A misfire in your classic motor is a real irritant and can be difficult to find, but the good news, it’s usually something simple.

So, why does a classic car misfire? The ten most common reasons a classic car engine misfires include:

  1. Plug wires and distributor cap dampness
  2. Bad gas
  3. Low gas level
  4. Bad spark plugs
  5. Wet spark plugs
  6. Bad spark plug wires
  7. Bad distributor cap
  8. Bad rotor arm
  9. Bad points
  10. Bad coil

As a mechanic, finding misfiring engines on modern cars is simple, every misfire is logged in the ECU when it happened, and on what cylinder. Older classic cars don’t normally have ECU’s, so we have to do some detective work.

In this post, I’ll outline why car engines misfire, in simple terms, the most likely reasons your classic car is misfiring, and how you can diagnose and fix it.

Let’s just run over some of the simple things that you can check really quickly, any of these can cause a misfire.

  • Check that your gas tank has enough fuel
  • Check that all the plug wires are on securely
  • Check coolant temperature
  • Check the oil level

Washing Causes Misfires

Distributor cap

Washing a car engine, damp weather, driving through puddles will cause the plug wires and distributor to get wet, it’s a very common cause of misfires.

As it’s a common cause of misfires, check the distributor cap for dampness. Any moisture here will cause random misfires, across multiple cylinders.

You can dry the distributor with tissue paper, compressed air, and when dry spray with WD40.

While you have the cap removed, check it and the rotor arm for burn marks (tracking), and cracking, damaged caps and rotors are also common.

Your model may not have a distributor cap, either way, check the following components are secure and free from moisture:

  • Coil wires
  • Plug wires
  • Plug wire caps
  • Plugs

Why Do Engines Misfire?

Having an outline of why engine misfires will help you diagnose your classic car misfire problem. Stay with me here, I’m not going far into the weeds.

An engine needs three things in order to run smoothly:

  1. Spark
  2. Fuel
  3. Compression

All of these systems are calibrated quite precisely, any variation in the ratio between any of these components can cause a misfire, constantly or randomly over one or more cylinders.

Spark

In my experience, spark, or lack of, is the most common of the three to cause an issue, and the problem is usually wet plugs, bad plug wires, or damp/cracked distributor cap. More on spark later.

Fuel

After spark, gas is the next most common cause. Especially in classic cars, using the wrong type of gas can cause it to run poorly and misfire. Fuel is more than just the gas though, the fuel is a mix of both oxygen and gas, more on this later.

Compression

Last of all, and least likely to be your problem, is bad compression. Compression is important. If the cylinder can’t seal, firing the spark plug won’t ignite the mixture and produce any power to turn the crankshaft, that’s a misfire. More on this later.

Type Of Misfire

Coil

There are different types of misfires and it’s worth knowing what type of misfire you’re chasing, different misfire types will point to different causes. The type of misfires include:

  • Random/constant single cylinder
  • Random multiple cylinders
  • Random/constant adjacent cylinders

Random/constant single cylinder misfires

  • Bad distributor cap
  • Bad plug wire
  • Bad plug
  • Valve issue
  • Piston ring fault
  • Injector fault (fuel injected engines)
  • Head gasket fault
  • Coil over plug (later model cars)

Random multiple cylinders

Timing belt
  • Timing off
  • Distributor dampness
  • Bad gas
  • Fuel blockage
  • Bad rotor
  • Bad points
  • Bad coil
  • Bad coil wire
  • Faulty/worn distributor assembly
  • Engine overheating
  • Head gasket fault
  • Engine vacuum leak
  • Valve wear
  • Bad crankshaft sensor (later model cars)
  • Bad fuel pump
  • Bad fuel regulator
  • Faulty carburettor over/under fuelling

Random/constant adjacent cylinder misfires

  • Head gasket failure
  • Valve train issue
  • Plug wires mixed up

Now you have an outline of why spark, fuel, and compression components are important, and you also know that different type misfires may point you in a different diagnostic direction.

How To Diagnose Misfiring Engine

From years of experience on the job, this is the fastest way to isolate and diagnose the root cause of a single-cylinder misfire. At this point we’ll assume you have checked:

  • Gas quality and quantity
  • Oil level normal
  • Coolant temp is normal
  • Points (if fitted) and distributor cap plus rotor are OK
  • No vacuum leak in engine

Tools: For this, you’ll need insulated pliers, a spark plug boot pulling pliers works best but any insulated pliers do the job. A spark tester would also be handy but a known good spark plug will do the job too.

Warning! Metal-handled pliers won’t work, it will cause the coil voltage to ground through your body, it won’t kill you, but isn’t pleasant.

Method

Start the car and allow it idle with the hood open. Using insulated pliers, remove the number one plug wire and listen for the engine miss to get worse, place it back on the plug again, and move to the next cylinder.

Continue through all the plug wires until you remove a plug wire that seems to have absolutely no effect on how the engine idles. This will be the misfiring cylinder.

While the engine is still running, use your insulated pliers to refit the plug wire, wiggle it around, and see if that helps any. (If it does, replace plug wires)

Spark plug

Now shut the engine off and swap out the misfiring cylinder plug wire with any other cylinder plug wire that is long enough.

Start the engine again and check – Has the misfire followed the plug wire or is it misfiring on the same cylinder as before?

  • If the misfire has followed the suspected plug wire to another cylinder, go ahead and replace all plug wires, they’re faulty.
  • If the misfire has remained in the same cylinder, go ahead and remove the spark plug.
  • With the plug removed check for damage and the plug gap. The plug will likely be wet with gas, but if it’s oily, it suggests a mechanical issue or too much oil.

Go ahead now, fit a new plug, and test. If you need parts delivered to your door, check out the Amazon link below.

Amazon Ignition Coil

If the misfire persists, and your engine is carburetor feed, check cylinder compression. If compression is low, suspect head gasket, valve, or piston ring issue. More on this below.

rail pressure

If the misfire persists and the engine cylinders are fuel-injected, check fuel pressure and injectors for faults. If they check out OK, run a compression test, poor results suggest head gasket, valve, or piston ring issue. See tests below.

Spark And Misfiring

You already know the spark is the most likely cause of your problem, but here’s a list of the components of the ignition system that can cause a poor spark.

Engine plug wires
  • Faulty plug
  • Fouled plug
  • Badly gaped plug
  • Wrong plug type
  • Damp plug wires
  • Damp distributor cap
  • Cracked plug wires
  • Damaged rotor
  • Cracked distributor cap
  • Faulty ignition points
  • Faulty coil
  • Faulty crankshaft sensor (fitted to later cars)

Fuel

Fuel pump

Bad gas as you know will also cause a misfire. The components and elements that directly affect a fuel system and which may cause misfires, include:

  • Low fuel level
  • Bad gas
  • Wrong type gas
  • Fuel blockage
  • Faulty fuel pump
  • Faulty injectors or wiring (later model cars)
  • Bad MAF sensor (later model cars)
  • Over fueling or under fueling carburetor
  • Manifold vacuum leak

Compression

Compression is the final leg of the stool, it’s the least common cause but when it is compression it’s usually a more serious problem. Running a dry and wet compression test and comparing the results will indicate if the fault is the top end or bottom.

Compression test infographic
Wet test

A leak-down tester is an even better tool, as you’ll likely hear the air escaping from the problem area.

Engine leak down infographic

Here’s a list of compression related components that can cause a misfire:

  • Head gasket failure
  • Stuck open valve
  • Burnt valve
  • Damaged piston
  • Damaged piston rings
Head gasket failure chart


Related Questions

What increases the value of a classic car? Original details like matching engine and chassis numbers will increase the value of a vehicle, but overall condition and scarcity will have the greatest effect on value.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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