Why Does an Engine Misfire? Engine Misfire Problem

Why Does an Engine Misfire

The float needle in the carburetor may be stuck due to gasoline gumming, and needs to be cleaned.
The float level may be set too low, and the float setting needs to be redone.
The carburetor’s inlet screen may be clogged and needs to be cleaned.
The fuel pump may be faulty or have insufficient capacity — have it checked.

engine fuel pump — A faulty fuel pump can be the cause of an engine misfire

The fuel pump’s filter may be clogged and needs cleaning or replacement.
The fuel pump’s filter cover gasket may be faulty and needs replacing.
If present, flexible fuel hoses may be kinked, cracked, or pinched.
Fuel lines between the pump and the tank may be bent, clogged, or punctured.
The vent hole in the fuel tank cap may be clogged or restricted and needs cleaning.

Why Does the Engine Misfire at Low RPM and Idle?

A misfire at low RPM and idle can point to a spark plug fault.
The spark plug gap may be too small.
Secondary circuit connections may be dirty or corroded, or the spark plug wires may be faulty.
The distributor cap may be cracked, or the electrode contacts inside it may be burned or worn.
The carburetor’s air filter may be dirty.
There may be a valve leak.
The breaker point gap may be too small.
The carburetor may be out of adjustment.

The float level may be set too high or too low.
The float needle may be leaking.
The carburetor jets may be wrong or loose.
There may be a partial blockage in the idle air passage or fuel jet.
The carburetor or intake manifold flange may be warped or have a damaged surface.
The fuel pump’s intake check valve may be leaking.
There may be an air leak around the valve stems due to worn valve guide clearances.

If your engine misfires at low RPM, review the possible causes described above.

Why Does the Engine Misfire at High RPM?

The spark plugs installed are too hot a heat range; try replacing them with a colder type. But keep in mind that a normal spark plug can also overheat due to poor thermal insulation or a bad plug gasket.
The breaker point gap may be too wide.
The breaker arms may be bent or sticking.
The point spring tension may have weakened.
The valves may be sticking.

The valve springs may be too weak to close the valves properly, or may be broken.
The amount of fuel reaching the carburetor may fluctuate from time to time, occasionally leaning the mixture out too much to burn.
There may be a partial vapor lock in the fuel system (not severe enough to stall the engine completely).
The ignition spark may be weak.

can soot in the exhaust cause an engine misfire

The exhaust manifold may be clogged with soot. (Image alongside)
There may be a blockage in the exhaust manifold, muffler, or exhaust pipe that restricts gas flow.
The ignition timing may be off.
The centrifugal advance in the distributor may not be working properly.
The intake manifold heater may be stuck closed.

If your engine misfires when you rev it up, you should review the possible causes described above.

dirty vs clean carburetor air filter — A dirty carburetor air filter can be one of the causes of an engine misfire

The carburetor air filter may be dirty. (The photo alongside shows a dirty and a clean air filter.)
The choke plate may not be opening all the way.
The throttle plate may have loosened on its shaft.
The fuel pump may not be delivering enough fuel.
Hot spots in the cylinders may be causing pre-ignition.
The valve timing may be off.

Why Does the Engine Misfire When You Press the Gas Pedal?

why the engine misfires, dirty spark plug problem

If the engine idles smoothly but misfires when you press the gas pedal while driving, the following faults — due to the higher compression involved — can interrupt ignition.

1- Weak ignition. On fuel-injected vehicles, the injectors should be checked.
2- The spark plug gap may be too wide.
3- The spark plug may be dirty or wet/fouled.
4- The spark plug’s porcelain insulator may be internally damaged.

If your engine misfires when you press the gas pedal, you should review the possible causes described above.

Checking the Ignition System

Engine misfires are often caused by worn spark plugs or a faulty ignition coil.

Fuel Mixture Imbalance

An incorrect air-fuel ratio can lead to irregular combustion and engine misfiring.

Clogged Injectors

Dirty or clogged fuel injectors can cause uneven fuel delivery to the cylinders and lead to misfiring.

Engine Misfire Problem

Engine Misfire at All Speeds

The spark plugs may be dirty or the porcelain may be cracked.
The spark plug wires may be faulty.
The battery voltage may be low.
The alternator voltage may be low.
The breaker points may be burned, or the contacts may be stuck together.
The breaker point gap may be set incorrectly.
The condenser may be faulty.

Ignition may be weak or absent on one or more cylinders.
The distributor cap or rotor may be faulty.
There may be resistance in the primary circuit, or a connection that intermittently makes and breaks contact.
The primary circuit current may occasionally be shorting out.
There may be resistance or a weak connection in the secondary circuit.
There may be a short circuit in the secondary circuit.
The cylinder head gasket may have blown. If the blown section is between adjacent cylinders, this can be a telltale sign.
The valves may be sticking.

Hydraulic valve lifters may be preventing the valves from closing.
The valve springs may be broken.
There may be an air leak at the intake manifold gasket.
The air-fuel mixture may be too rich or too lean.
The spark plug wires may be shorting out inside their conduit, or that conduit may not be properly grounded to the chassis. All of these can cause the engine to misfire.

As you can see, the causes of an engine misfire can be quite varied. Just as with a cough, where the exact cause can’t be pinned down without an examination, X-ray, or lab tests, diagnosing an engine misfire may require reviewing all the possibilities described above, or even inspecting the engine’s components one by one.