Myth 1: ‘Resistance Drives Current.’ High resistance causes high current. After all, you need a lot of current to overcome that resistance. What’s more, there’s often high heat at and around the resistance as well, so obviously it must draw a very large current.

Contrary Reality: High resistance limits current flow.

Explanation: First, think of the extreme cases: of an open circuit and a short circuit. Increase the resistance, and what happens to the heat? An open circuit has infinitely high resistance, but no current flow and no heat at all. Now drop the resistance. How about a short circuit? It has no resistance, gets plenty hot, and certainly flows enough current to melt a metal fuse (after which it becomes an open circuit and drops to room temperature). I teach Ohm’s Law as V = C x R, where V is volts, C is cur-rent and R is resistance. Usually we see the formula, E = I x R, but I prefer the more familiar characters. Ohm’s Law really is a law: Electricity really does behave that way, not the mythical.

Myth 2: Check connections with ohmmeters. An ohmmeter is your most effective tool for locating poor cables or connections.

Contrary Reality: Voltage-drop testing with the circuit under load is faster, easier and more reliable.

Explanation: An ohmmeter tests for resistance using a very low current. For example, a battery cable will usually ohm-test good if just one copper strand remains intact, even though all its other strands are broken. Testing for voltage drop from one end of the cable to the other with the circuit under cranking load provides a truer measure of the cable’s capacity to carry a working current. Look for no more than 0.1 volt-drop per connection for most heavy power circuits and 0.05 volt-drop per connection for low-load PCM electronics circuits like those for engine coolant temperature or mass airflow sensors.

Myth 1: High hydrocarbon (HC) readings indicate a rich mixture: More gas in the exhaust must have meant more gas in the intake. Where else could it have come from?

Contrary Reality: HC readings simply indicate residual fuel in the exhaust, whether that came from an ignition fault, a mechanical loss of com-pression, a lean misfire or an overly rich intake mixture. Any of these factors could have caused high sniffer numbers. Most frequently unusual HC readings flag an ignition or compression problem.

Explanations:

Carbon Monoxide (CO) indicates a rich mixture (too much fuel for the air).

Oxygen (O2) indicates a lean mixture (too little fuel for the air).

Oxides of Nitrogen (NOx) show hot combustion with combustion chamber temperatures over 2500 degrees F, often from a lean mixture during high engine loads.

Hydrocarbons (HC) indicate residual fuel in the exhaust, from whatever source.

Carbon Dioxide (CO2) reflects combustion efficiency. When everything works right and the fuel/air mixture burns cleanly, CO2 should be high, while HC, CO, O2, and NOx should all be low.

Myth 1: Eventual compression is good compression. As long as the compression readings eventually come up, the rings and valves are sealing okay.

Contrary Reality: The first puff of a compression test tells you the most about cylinder wall condition. It should be at least 40 percent of the final (fourth-puff) reading. Under operating conditions, each engine power stroke is preceded by just one compression stroke.

Explanation: Scored cylinder walls, excessive cylinder wall taper or damaged rings will all cause lower than normal ‘first-puff’ readings. Check engine compression with the battery fully charged, engine hot, all spark plugs removed, ignition disabled and throttle wide open. Four compression strokes (eight crankshaft revolutions) are sufficient to assess a cylinder’s condition. For consistency, crank the engine the same number of revolutions for each cylinder.

Myth 2: The higher the compression reading, the better. If some’s good, more’s better, right?

Contrary Reality: There is most certainly such a thing as too much compression. Excessively high compression readings indicate problems resulting from a worn exhaust cam lobe, from excessive milling of the cylinder head or from carbon buildup in the combustion chambers (often from overrich running).

Explanation: An engine is designed for a specific compression ratio. As the compression exceeds that, the engine’s tendency to knock, to destructive detonation, increases rapidly. Ordinarily, engine wear will reduce compression over a long period of time, but if it increases for one reason or another, the damage can be even greater.

Myth 1: The safe and effective test for spark is to pull a cable from one plug, roll back the boot, hold the end a quarter of an inch from the block and crank the starter.

Contrary Reality: This procedure can usually tell you whether spark is present, but not whether the spark can ignite the mixture. It’s neither a safe nor a reliable test.

Explanation: It doesn’t take more than a few kV for a spark to jump a quarter-inch gap in open air; it takes much more to jump across the electrodes of a spark plug under the combination of pressure and fuel mixture in the combustion chamber. You want to determine whether the spark is sufficient, and not merely whether it’s present. Using a spark-tester with an adjustable air gap allows you to view the spark more critically. Better still, hook up your ignition scope to view the secondary ignition patterns with the spark plug wires connected.

This was brought home during our class when a great deal of time was spent verifying fuel delivery and flow on a no-start Honda. We had previously verified that spark was present, but it turned out to be insufficient due to a weak coil.

As to the safety of the pulled-cable technique, remember any secondary ignition spark will find its way to ground somehow. Somehow may turn out to be through your hand, up your arm and down your leg to the ground. Then you may feel disposed to sit quietly on the concrete floor for some time while meditating on the role of intense pain in human consciousness.

Myth 2: The best way to identify a weak cylinder is to pull plug wires one by one on a running engine.

Contrary Reality: This worked fine on the Hudson Hornet, but it’s unlikely to solve any problems on a modern car and can cause some new problems of its own.

Explanation: Again, any spark will find its way to ground somehow. Besides the safety considerations above, that fact may involve using the ignition module as a ground path, with some expensive damage. Then you’ll have two independent problems: the one that brought the car in and the one you caused with your ‘balance test.’ Besides all that, many current engine management systems control idle speed so well that you may not be able to detect any difference in idle speed or quality when temporarily disabling the secondary ignition to individual cylinders.

Myth: If the fuel pressure meets the manufacturer’s specifications, you have some other problem.

Contrary Reality: It is entirely possible to have good fuel pressure coupled with inadequate fuel flow (volume). We have even seen this on cars that have run out of gas.

Explanation: As the fuel pump flow simulator photo proves, you can have pressure without enough volume. Most commonly, this comes from running out of gas or from excessive resistance in the fuel pump electrical circuit. When coupled with an inaccurate fuel gauge or sending unit, a modern automobile that has run out of gas (without the driver’s knowledge) paradoxically can be one of the most difficult no-starts to diagnose.