Mechanics

Misfires

The engine in your car is designed to run smoothly with maximum power output and produce as minimal emissions as possible. When your car's engine is not performing properly it can cause, low gas mileage, low power output, increased emissions and possible internal engine damage if left untreated. This troubleshooting guide is designed to isolate the malfunctioning cylinder and troubleshoot to repair the problem. Before we start we need to know one of two things; is the engine running poorly at idle only and seems to be ok under power? 

Or does the engine run fine and it's just the engine idle condition that is the problem. If your engine is vibrating at idling please visit, engine vibrates at idle. If your engine cranks over but won't start visit engine wont start. If your engine won't crank over visit engine wont crank over. If your engine is running rough all of the time or intermittently you are in the right place. Below we have created a guide to aid diagnoses and repair procedure for most common rough engine running problems.

Car Repair Guide

Step 1 - Anytime you have a problem with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, air bag) inspect all fuses in the under hood power distribution center and under dash fuse panels using a test light. If all fuses test okay continue to the next step.

Step 2 - To check for problems with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system) and the fuses test ok a trouble code scan is needed to identify any system trouble. Use a simple scanner tool to retrieve trouble codes and check if they relate to the specific problem, like an ignition coil failure code. If a trouble code is present but does not pertain to the immediate problem like an EVAP code ignore it until a later time, after the engine is running properly. 

The reason we repair non-related codes after the engine is running properly is because sometime false codes can be triggered by a rough running engine. Once the engine is running properly the code present might cycle and turn itself off. You might say "if the engine isn't running right shouldn't it have a check engine light and a trouble code?" 

Sometimes conditions occur that will not be detected by the computer, example: if the intake or exhaust valve operation fails the computer cannot detect the failure because the problem is not sensor related, so the engine doesn't run smooth and the computer thinks everything is okay with no codes. If the trouble code retrieved relates to a cylinder misfire like an injector driver or ignition coil failure first repair these problems then re-test system. If no trouble codes are present proceed to the next step.


How to Check for Trouble Codes with a Scan Tool

Step 3 - Check for broken or dilapidated vacuum hoses on and around the engine, your car's engine is designed to run on a system that can hold vacuum. Vacuum hoses are typically connected to the engine intake manifold and will supply engine vacuum to various accessories like the brake system. Some cars are designed with a larger vacuum transfer hose like Ford that connects the intake manifold to the IAC (idle air control) motor. A broken or dilapidated vacuum line or air intake boot can cause the engine to lose vacuum which will allow the engine to run rough and die. Inspect all engine and accessory vacuum lines to look for missing, torn or dilapidated lines and replace as needed.

Also have a helper rest their foot on the gas pedal just enough to keep the engine running. Check the engine when it is running to listen for any whistling noise coming from the engine that is not usually present. Follow the noise and inspect vacuum lines in that area. Also, when the engine is running it will pull inward a broken or weak piece of the hose to create a larger vacuum leak. Check the integrity of all vacuum hoses at each end of the hose. Typically this is where a vacuum hose fails. If all vacuum hoses check "okay" proceed to the next step.

Failed Air Intake Boot
Failed Air Intake Boot


How to Replace an Air Intake Boot

Step 4 - A pocket IR thermometer laser temperature reader is handy for detecting a cylinder misfire. If you have visual access to all cylinder exhaust manifold ports you can use a pocket IR thermometer temperature detector to detect a cylinder misfire. Start with the engine cold, then start the engine, take temperature reading on all cylinders, if one cylinder is considerably lower temperature then the other cylinders you have found the cylinder that is malfunctioning. Example: on a four cylinder three of the exhaust ports test at 190 degrees and one is at 81 degrees. The cylinder at 81 degrees is misfiring. Once you have found the cylinder that is misfiring use the following tests to find out why it is misfiring, if haven't found the misfiring cylinder continue: (the next step can be combined with step 6 and 7)

Pocket IR Thermometer Laser Temperature Reader
Pocket IR Thermometer Laser Temperature Reader

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)