How to Diagnose an Engine Miss

This article is limited to diagnosing fuel injected vehicles. Further, this article assumes there is no damaged or altered engine wiring, cracked or worn vacuum hoses, nor are there any engine vacuum leaks around the intake manifold.

Difficulty: Moderate

Instructions

Things You'll Need:

• Mechanics hand tools, a 12 volt test light, a voltmeter, a compression tester, and an old spark plug.

1. Diagnosing an engine miss on a fuel injected vehicle is a lot easier than you may think. Follow the step by step procedures and save yourself a lot of money. On some vehicles all the spark plugs and fuel injectors are not easily accessible. Usually it requires removing the air intake plenum or some other items to get at them. This diagnosis is different and is covered a few steps later. If this is your case, diagnose those that are easy to get to first. If they are all easy to get to, then so much the better. First, take a long handled screw driver and listen to each injector. The "clicking" sound will travel up to the handle and you can hear each one. If one sounds weak, or has no sound at all, chances are you need to service or replace the injector. If you decide to replace the injector, test the voltage first. With the key on there should be 12 volts on one of the two wires that plug into the injector. With the engine running, check to see if the ground side will pulse or flicker a test light when the lead is attached to the positive side of the battery and the light probe is touching the ground wire. If it pulses, the engine management computer is working properly. If not, it's a good chance the computer is at fault.

2. If all the injectors are "clicking" just fine and the engine is missing and shaking, it's time to check the engine compression. To do this you must disable the fuel injection system. Large amounts or raw fuel can find it's way into the engine if this is not done. This fuel will scour the cylinder walls and cause major damage. Pull the fuel pump fuse out and proceed. Remove and "read" all the spark plugs. Label each plug wire so you can replace it in it's proper position. When you "read" the plugs, look for carbon tracks in the plug body. This indicates a crack in the ceramic, which will defiantly cause a miss. Look for excessive wear across the electrodes. When this happens, the plug electrode gap is too wide for the spark to "jump" properly. The manufacturer has a specific gap and wear beyond that can cause problems. After you have all the plugs out and have read them, the fuel system is disabled, and your compression gauge is handy, begin to test each cylinder for proper compression. A good rule to follow is that all the cylinders should have about the same compression ratio, and each should be within ten pounds of each other. Most of today's 4 cylinder engines run around 175 pounds. If they fall as low as 150 pounds, generally the engine won't start at all. If one cylinder is low, that is defiantly your miss and you have mechanical engine problems. You can determine the nature of the mechanical problem by following the next step. However, if your compression is even and good, skip to step 4.

3. A low cylinder indicates a burnt valve or weak piston rings. By adding motor oil to the cylinder, you can find out which it is. Add a couple of tablespoons of engine oil to the cylinder by pouring it into the spark plug hole. "Bump" the engine over a couple of times to distribute the oil. Install the compression gauge and take another reading. If the pressure goes up, the piston rings are at fault and the engine will have to be removed for repair. If the compression stays the same, the oil is passing by a failed valve and the engine head will have to be removed for a valve job.

4. If your compression is good, your injector is pulsing properly, then the next thing to check is for a failed ignition system that supplies spark to the plugs. Using your volt meter, set it to the ohms position. Choose the 100,000 ohms scale and measure the resistance of each spark plug wire. There should be about 7,000 to 10,000 ohms of resistance per foot of plug wire. Check the distributor cap or coil pack for cracks or excessive wear and electrical oxidation. Replace them as needed. Check for a spark at each plug. Using an old spark plug, connect it to the plug wire and lay the plug against the engine. Have a friend crank the car over and notice if there is a bright, blue-yellow spark. If not, then the plug wire, coil pack or distributor is at fault. If the car is equipped with individual coils that mount directly onto the spark plug, then the test is similar to testing an injector. Make sure there is 12 volts to the coil in question. Crank the car over and make sure the test light pulses while the light is attached to the positive side of the battery and the test light probes the ground side of the individual coil. If it fails the test, once again the engine computer is suspect.

Tips & Warnings

• These diagnostic procedures are designed to pinpoint the problem area. If you follow them correctly you should know if you have an engine mechanical problem, a failed fuel injector, or a faulty ignition system. If all of these tests pass, then check these two items.
• 1. Low fuel pressure. Follow your manufacturer's instructions carefully to test the pressure. Low fuel pressure can defiantly cause an engine miss.
• 2. Low air flow metering sensor. Your vehicle with either have a MAP sensor or an Air Flow Meter. If either one of these fail go provide accurate readings, you engine will miss, sputter, and idle poorly. The engine computer is receiving faulty information and is improperly calculating fuel and ignition timing needs.
• These two sensors becoming slightly out of calibration will not always set an error code in the vehicle computer. In some cases they can be tested using an ohm meter and following the manufacturer's guidelines, or they can be diagnosed with a hand held scanner to make sure they are within specified parameters.
• Don't be hasty to replace the plugs and wires thinking a simple tune up will solve your problem. With the advent of platinum plugs, some newer models are rated to go 70,000 miles or more before a plug change is needed.