What are you going to tell the customer when you can't fix their vehicle right the first time?

Get the answers to these questions and more right here. Quickly discover what you need to know so you too can perform successful auto repairs. As an automotive teacher I explain things in plain English so they can be easily understood. I cover electrical, computers, emissions, engine performance, as well as how to diagnose effectively and efficiently. Let's get you some auto repair information now. (CA smog techs please click here.)

Let's start with some basics...

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. more information...

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