Auto Repair Help


by Kyle McFadden

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

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.

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.

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.)

10 responses to “Diagnose Engine Misses Or Idles Poorly”

  1. Tonya gill says:

    Hi I have a 93 ford explorer I got a fuel pump put in my truck then every thing that’s connected with the fuel went out and I had to replace, like the mass air flow cencer , and a few more things but now i may need to replace my ignition control module. My car is hard to get to start and when I sit with it running and not moving it wants to cut off. Please help me I don’t no what I should do now!! Thanks

  2. 2002chrysler town&country cly#2 misfire

  3. josh says:

    I have a 1996 nissan sentra 5 speed and this hose is bothering me like crazy. I have recently replaced a clutch, cv axel,thermostat,fuel filter, distributor, coils and spark plugs and my car still looses power going uphill. I notice that when im in neutral, the car kinda woops, when i rev it up its slow and then catches itself. umm, i checked the fuel pump( even though i dont know what im looking for but it still squirts??? I want to check the fuel injectors but I dont know how to check if they are squirting or not, oh the hose lol. heres a question i posted on yahoo answers with the pictures regarding the hose. Please if someone has any ideas thank you

  4. Tonya says:

    Hi I have 1993 toyota corolla and it has been missing , got the distributor repaired . After 3 week the car is missing again .I have check all the plug , please help dont know what else to do

  5. suleiman says:

    My motorcycle has problem of miss I try to do all process causes miss but problem still have

  6. Brian Ingham says:

    1993 mustang 2.3 dual plug. Automatic 90000 miles intermittent check engine light tach reads at 50 % of true rpm

  7. Jaren says:

    Hi i have a golf one..2006 carb. Just did a service and one spark plug was very black with oil on it. I believe the car is missing. I done the engine about two years ago and i always felt that there was a miss on the engine and the mechanic stated that nothing is wrong. What can i do to know for sure that it is a miss. And how much will it cost me to sort it out. Thank you

  8. Someone put a serpentine belt on my 2000 Buick century and ever since it has been missing. It never missed before

  9. lawrence says:

    hi got a jeep wrangler 1998 model 4 l. it starts well cruse well till hot. it starts jerking and stalls. at this point its usually hot.after cooling or after sometimes it cruses ok till hot again. what can i do

Leave a Reply

Your email address will not be published. Required fields are marked *