Diagnose Engine Misfire (Part 3):Ignition And Compression Misfires

Engine Related Issues

By Rodney (Mechanics)

Table of Contents

Read Diagnose Engine Misfire (Part 2)


Once you remove the spark plug, in case you also have isolated it to a single cylinder and have a misfire, the cause will be obvious. If the insulator of the plug is broken or cracked, then you have found the issue. Check for plug boots and wire damage, if the plug is wet but appears to be OK. End to end, using ohmmeter to measure the resistance of plug wire. As the rule, the resistance per foot shouldn’t exceed 8000 ohms, if the resistance exceeded the specifications of the vehicle manufacturer, the wire should be replaced.

If you have fouled plug, it mean the source of a misfire problem is found, however the cause of the plug to foul still have to be determined. The oil will be burned by the machine if you found heavy black oily carbon deposits. The valve guide seals that worn is the general cause it occur, another possibility of the entry of oil into the combustion chamber caused by worn cylinders and rings. The temporary fix for this misfire is replace the spark plug, however the foul plugs will continue occurs until you fix the problem about oil consumption.

To know the oil getting past the rings or the valves guides, a compression test or leak-down test should be done. When the oil is less squirted into the cylinder (wet compression test), it telling you if the cylinder holds good compression or there the cylinder has a little leak-down, it mean replace guide work or/and valve guide seals to solve the problem. Positive valve guide seals available on most late model engines. In most case, seals are cracked or worn but the guides is OK. Replacing the seals on some engine do not need removing the head.

The way to install new seals can be done by removing the springs using external spring compressor, also the valve-train hardware need to be removed after the valve cover has been pull off. In order for the valve not to drop into the cylinder, the spark plug hole must be connected to a regulated air hose. However to get to the valve springs there will be a lot of disassembly involved and may have to remove the head if it’s on OHC engine.

Indication of a coolant leak either through a crack in the combustion chamber or past the head gasket shows heavy whitish to the brown deposits on the spark plug. If this type of problem isn’t get fixed ASAP, it could bring more greater problems or even worse problem. Coolant can cause ring and makes a lousy lubricant, and if get into the crankcase or cylinder, it can make bearing and cylinder damage.

Overheating also can caused by coolant loss, it can get the aluminum cylinder heads warping or cracking. If you have a problem like this, check the internal coolant leakage by do pressure test the cooling system.

Detonation damage or preignition of spark lugs telling you to check the timing, the cooling system operation and conditions causing the lean air/fuel mixture. Also colder heat range plug need to be switched. Short trips to stop and drive can cause the accumulation of normal deposits quickly on the spark plugs, especially if the machine has many miles and there has been an oil leak through the guide valve and the ring.

In the plugs there can be a rapid buildup of normal deposits caused by short trips stop-and-go driving, especially if the rings and the valve guide seals leaks, so the oil can pass through and the engine has a lot of miles on it. Switch to a one-step hotter spark plug, if you want it fix.

Bad Spark Plug
Bad Spark Plug

If the cylinder is weak and there no problem with plug wire and spark plug, and if the problem is compression related then a compression or leak-down test can be run. The most likely leakage of compression and seal loss is the exhaust valves, so if there low compression is found, a wet compression test should be done to get the assurance of the problem, whether it be a rings or a valves.

When run wet test and there no change in compression, it telling you a blown head gasket or bad exhaust valve (valve related problem) is the problem you may have. However if the wet compression test result reads the compression is significantly higher, it means the cylinder walls or/and the piston rings are worn.

Valve job the way to fix a leaky valve, and replacing the gasket is the way to fix a leaky head gasket. Also, replacing or overhaul the engine is only way to get cylinders and rings worn problem fixed.

Rounded cam lobe also can cause low compression. If the valve won’t open, the compression will be low because the cylinder can not breathe normally. IF you suspect this kind of problem, you need a visual check the cam and the valve-train.


If the compression and ignition components are fine in misfiring cylinders, the possibility remaining problem is fuel-related issue. To start, check the voltage at the injectors. When the engine running and the injector should also buzz, it mean the injector is good. Injector is dead if there no buzzing, and if the reads results would be no-voltage, it means the injectors is fine, however the problem could be a computer driver or the wiring issue.

Check the injector, if the injectors clogged or dirty, it still spraying fuel and buzzing, but won’t deliver enough fuel to the cylinders. Minor repair can be done by on-car cleaning, it can removing the varnish deposits, and no restricting fuel delivery and no restricting the injector anymore, however you need to replace or do off-car cleaning if the injector is clogged and caused a steady misfire.

Check the injector performance response to changes in air/fuel mixture, by observing the injector performance on a scope. If the injector is not receiving voltage or dead, it will show flat line. When you turns on the injector and the line is drop and when the current switched off, the line is peak, it tell you the injector is working.

How long the injector is on can be shown by the injector scope pattern. By momentarily pulling off a vacuum hose, you can make artificially lean of the air/fuel mixture, and/or into the manifold, feeding some propane make artificially rich, there a corresponding change that you can see in injector as the responds the computer input from sensor of oxygen. There is a problem in computer or the Oxygen (O2) sensor is dead.

Always check the spray pattern when you testing or cleaning the injector. If injector produce a cone-shaped mist of fuel vapor, it mean the injector is good. The injectors need attention when you see on spray pattern a solid stream of fuel or solid streamers. The injector needs to be replaced if off-car cleaning doesn’t work to restore the spray pattern to the normal condition

Possibility of all injectors is dirty, in case the random misfire can not be isolated to certain cylinders. Make sure the pressure regulator is not defective or pump is not weak by checking the fuel pressure. Fuel pressure can be reduced by plugged fuel filter. Air leak can upsetting the overall air/fuel mixture, check the intake vacuum if fuel pressure is within a specifications. A leaky power brake booster, or a leaky EGR valve may be another cause that missed.