DIAGNOSE – ENGINE MISSES OR IDLES POORLY

by Kyle McFadden

SYMPTOM SUMMARY
The engine misses or idles poorly. The miss usually follows engine speed and may be more pronounced when the engine is under load.

USUAL CAUSE
There are many items that can cause the engine to miss or run roughly. Defective ignition wires, restricted injectors, or engine mechanical malfunctions are all possible.

DIAGNOSIS
Start your diagnosis with an underhood inspection. Look for broken or cracked vacuum hoses. Inspect the spark plug wires for chafing or signs of arching to the engine block. If the vehicle is due for routine service, this should be done prior to spending too much time on a diagnosis. A complete tune-up including spark plugs, spark plug wires, distributor cap and rotor (if so equipped), fuel filter, and air filter will go a long way in correcting the most common problems. If a misfire is still present after a complete tune-up is performed, diagnosis of engine mechanical problems may be necessary. A vacuum test using a vacuum gauge should be performed. A vacuum gauge can be obtained at any auto parts store. An engine should draw at least 15″ of vacuum at idle when connected to a vacuum hose that is connected to the intake manifold. The vacuum should be smooth and not fluctuating. A severely fluctuating vacuum gauge is an indication that there is a valve train malfunction, such as a defective valve or slipped timing belt. Retarded ignition timing can cause abnormally low engine vacuum. A compression check should be performed on the engine to determine if lower than normal compression is present in any cylinder. A compression tester is available at most auto parts stores. Refer to a manufacturers specific service manual for compression specifications. As a general rule, the compression should be above 120 PSI and the lowest cylinder should be at least 80% of the highest cylinder. If the above tests do not indicate a failure, a diagnosis of the fuel and ignition system must be performed.

PRECAUTIONS, TIPS, and NOTES
Often, diagnosing an engine misfire requires the use of special test equipment that is capable of performing ignition and fuel systems test. If the above testing procedures do not reveal the problem, it may be more cost effective to have a repair facility with the above equipment perform a diagnostic test. This usually requires about one hour of labor time. You should ask the repair facility for a diagnostic print-out detailing what was found during the test. Use caution when working around hot or rotating engine parts. When performing a compression test, the ignition system should be disabled to prevent the engine from starting. This is usually done by connecting the spark plug wires or coil wire to a good engine ground away from the area you are working on. Consult a manufacturers specific repair manual for further information.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)