Mechanics

Misfires

Your reading should be between 11 and 18 ohms. Test all of your injectors; if one injector reading is considerably different from the others replace that injector. Check the fuel pressure regulator, remove the vacuum line feeding the fuel pressure regulator, check for the presence of fuel, if fuel is present the diaphragm inside the regulator has ruptured and is feeding raw fuel into the engine causing it to run rough.

Step 10 - A camshaft is commonly used to operate poppet valves in a piston engine. A cylindrical rod is situated in the cylinder block or cylinder head which has oblong lobes or cams which push the intake and exhaust valves. This force is applied on the valve directly or through an intermediate mechanism such as a rocker arm, lifter (cam follower) and push rods are used to press against the valve for movement. Each valve utilizes a spring which will return the valve to its original position (closed) after the force is removed. If a valve spring has broken or a cam lobe has worn down it will cause the engine to misfire and run rough because it causes the engine to lose compression. You might say" wouldn't either of these conditions show up in a compression test?" and the answer is not always. The problem with a compression test is that it is performed at engine cranking speed. If you have a worn camshaft it will show up only when the engine is running.

In some conditions a broken valve spring can effect different aspects for example: If just one coil of spring breaks off, the spring can still close the valve but only at low RPM's, causing a high RPM engine misfire if the spring breaks somewhere in the middle it will affect both idle and power conditions. To test for this condition, remove ignition coil connector or ignition system or fuel pump fuse to disable power to the ignition or fuel system. Remove the valve covers to gain access to visually inspect the valve train.

Have a helper crank the engine over while you watch the rocker arms or cam lobes, make sure they are all going up and down the same amount, if one or more lobes are traveling less than the others you have a flattened cam lob and the camshaft needs to be replaced. Inspect the condition of the valve springs, use a flashlight and small mirror too aid in the inspection. If a broken valve spring is discovered it will need to be replaced to correct the problem.

Typical Camshaft Configuration
Typical Camshaft Configuration

Step 11 - The head gasket in your engine is used to seal the cylinder head to the engine block. The cylinder head is fastened to the engine block using head bolts that are tightened to a specific torque. When a head gasket wears it can allow coolant or exhaust to enter the combustion chamber causing a cylinder misfire. To test for a blown head gasket, have the engine cold, then remove about a half a gallon of coolant from the radiator. Next attach a chemical tester to the top of the radiator were the radiator cap attaches and fill the tester with testing solution.

Start the engine and watch the chemical tester it will start to bubble as the engine warms up, if the chemical turns blue to yellow the engine has a blown head gasket. If the head gasket is blown between two cylinders it will allow exhaust to enter the cylinder that is not in the power stroke causing a cylinder misfire. To check for this condition perform a compression test of all cylinders. If the head gasket is blown between to cylinders the compression reading will be a little low in two cylinders that are next to each other and compression reading will be close to the same in both cylinders.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-08-16)