In this industry, the difference between success and failure is having the understanding of what is going on. Once you know how things work, you can repair them. If you need to know what is going on under the hood, find the lack of good information frustrating, or simply want to have an edge over your competition you have come to the right place! As an automotive teacher, I can help you. I have worked in the automotive field for almost 30 years, and I understand the frustration you experience when you can't fix a problem and the concern of losing work to others who know more. You need the inside scoop on what's really going on under the hood.

Basic Needs of an Engine:

Let's review the basic needs of an engine. We will be looking at the air/fuel ratio needs of an engine, depending on the condition when it's operating. We'll also throw in a few comments about ignition timing needs during these conditions.

1. Cold Start: When starting cold, the sprayed fuel tends to condense on cold engine metal, so we need a very rich air-fuel ratio to get enough vaporized fuel to run properly. When the engine is cranking there can't be a lot of timing advance or the burning will complete before the piston is at the top of the stroke and it will try to push the piston down backwards.
2. Warm Start: When starting with the engine not as cold, or maybe warmed up, we still need a slightly rich mixture to get started, but not as rich as a cold start. There will still be some condensing of the fuel on engine metal that isn't up to total operating temperature.
3. Cold Idle: We still need a richer mixture than normal running because of the vaporization problem, but not as rich as a cold start. We could also use more timing advance for a smoother idle.
4. Warm Idle: Once we are warmed up, the air fuel ratio can get closer to our middle stoichiometric ratio for clean emissions and we will still have a reasonably smooth running engine. Some manufacturers run at stoichiometry, some may run the engine slightly rich to give a smoother idle.
5. Warm Cruise: Now with higher engine speeds and moderate engine load (higher load means the engine is working harder-it has more air coming in) we can run right at the ideal stoichiometric air fuel ratio for clean emissions or even leaner to get good fuel economy. Our ignition timing needs to be very advanced now. Since the engine is spinning faster, we need to start the spark sooner so that the burn is complete when it's time for the expanding gas to push down on the piston. And leaner mixtures take longer to burn too, since the fuel is more diluted among the oxygen.
6. Heavy Load or Sudden Acceleration: Under these power conditions we need a slightly rich air fuel ratio. It helps to get more power out of the engine. And our timing can't be as advanced because the richer mixtures burn faster, and the heavier load also burns faster. So we don't need to start the spark as soon to have the burn complete by the time we want the expanding gas to push down on the piston.
7. Deceleration or Coasting: This is easy, when decelerating we don't need any fuel at all, so we can turn the injectors off.

Input Sensors:

Now the whole job of the computer is to analyze the inputs to figure out what condition the engine is running under. Then it uses the logic to figure out what fuel, timing etc. is needed for this condition. Then with the actuators, it outputs to the engine what it needs. A problem occurs when some part of this process goes bad.

Since the computer doesn't have eyes and ears like you and I, it has to have sensors to tell it what's going on with the engine. These are the usual types: Engine speed (and position) or RPM, Engine Load, Throttle angle or position, Coolant temperature, Air temperature, Exhaust gas air/fuel ratio, and a Knocking, pinging or detonation sensor. (There may be lots of others, but these tell the major story.)

Let's look at each of these in more detail, and what kind of information you will see on a scan tool. Pay attention to the initials used to symbolize each of the sensors, you will want to know that for later...

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The above info is from pages 24 and 25 of my "Smog Refresher Course" book. Special offer here. Electronic information available worldwide, instantly!

Emission Control Devices

Here we're going to talk about PCV, Fuel Evaporative Systems, Thermostatic Air Cleaners, Air Injection, EGR, and Catalytic Converters. What theory do you need to know about how they work? How do they usually fail and cause problems? How do we test them? And we'll have some sample test questions.

PCV:(Positive Crankcase Ventilation):

These were our first emission control devices. (Unless you want to call a gas cap a smog device, but the first gas caps were vented, right?)

Inside the engine crankcase, where the oil lives and breathes we have gas vapors that got there by sneaking past the piston rings when the piston is compressing the air-fuel ratio or the fuel is burning up. And the more power the engine is developing, the more of these HC vapors get crammed into the crankcase. So, what to do?

If we have a crankcase that is totally sealed, this gas under pressure will eventually blow out the seals. So we have to find a way of relieving this pressure.

In the old days, when I was a child, we had road draft tubes and open oil filler cap covers. That way the pressure could get out, and as the car drove down the road, suction was created at the road draft tube and it helped pull out the gas vapors. Fresh air could come in through the oil cap and help keep the engine oil from being too contaminated by the gas. This helped the oil last longer.

But, then came pictures of L.A. smog, and the smart scientists realized that we could cut down on about 20% of the smog if we didn't let that gas out that we paid for anyway. So now we have what is called a closed crankcase.

The oil filler cap should not let out any fumes. And the oil dip stick should also be sealed, in some newer cars better than some older cars. And we have a fancy PCV valve or orifice that lets the intake manifold vacuum recycle and burn that gas we paid for.

Inside the PCV valve is an orifice regulated by a plug and a spring. When manifold vacuum is high, this plug is sucked in hard against the spring and we just have a small calibrated orifice for blowby to flow through. Then, as engine load increases, the vacuum drops and the plug is pushed back by the spring and we have a bigger opening for more crankcase flow.

But if we have no vacuum, the plug is pushed against the other end by the spring, and we have no flow. This is a flame arrester in case of a backfire. We wouldn't want the flame in the intake to spread to the crankcase. Also in this system, instead of a road draft tube, we have some sort of vent tube usually running between the crankcase or valve cover and the air intake or air filter area.

Here's how the PCV flow goes...

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The above info is from page 45 of my "Smog Refresher Course" book. Special offer here. Electronic information available worldwide, instantly!

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