Can someone explain the general idea? I understand the intake sucks in air.
I understand there are several vacuum hoses that somehow divert this air suction for various purposes.
What are these purposes? Why does a vacuum line go to the fuel filter, for example?
Why does a broken vacuum line cause idle problems when these lines are not part of the air intake chain?
I understand the path of the air intake (air filter, intake boots, MAF, throttle body, intake manifold,…valves?)
But, what exactly causes the air to get sucked into the intake? Is there some sort of fan/blower?
Is it entirely created by the pistons moving down and sucking air via the open intake valve?
The pistons suck air into the cylinders thru the intake system (air cleaner, intake manifold, intake ports, and finally, the intake valves). At 2,500 RPM in a 2 liter 4 cylinder engine, the pistons are attempting to pull about 90 CFM in. The bigger the engine and the higher the RPM, the more air it attempts to pull in. That is a lot of sucking, and given that the throttle valve is rarely open wide, it creates a considerable vacuum in the intake manifold. The most common use for it is powering the brake booster, but it does other things as well. I could be wrong, but I’m not aware of a vacuum line to the fuel filter. When you “floor” the throttle pedal, the vacuum all but disappears. The reason vacuum line problems cause idle/mixture problems is that the vacuum lines are connected right into the intake manifold. If something goes wrong with them it will admit more air in than is supposed to go in there. Air should come in only thru the throttle valve.
The vacuum is created by the action of the piston in the cylinder. When piston travels down in the cylinder and the intake valve opens, a vacuum is created that sucks the fuel/air mixture into the cylinder. The intake valve closes, te piston moves up and compresses this mixture. A spark occurs and the piston is forced back down with the intake valve closed. Finally, the piston move back up as an exhaust valve opens to expell the burned fuel.
The fuel/air ratio is supposed to be precisely controlled. If there is a vacuum leak, this fuel to air ratio is upset and the engine doesn’t function as it should.
The vacuum is used to operate the power brake booster. In many cars, it operates the various doors that direct the flow of air through the heating and cooling system to the floor, the dashboard vents, or the defroster slots. On many cars of the 1940s through the 1960s, the windshield wipers were operated by vacuum. The school buses that I rode as an elementary student had vacuum operated windshield wipers. When the driver accelerated the bus, the wipers would come to a standstill on the windshield. When he released the accelerator the wipes would really fly back and forth across the windshield. What caused this? Think about the hose on a vacuum cleaner. If you put your hand over the hose, you can feel the pull increase. The same thing happened on the school bus when the driver released the accelerator. The plate in the carburetor restricted the airflow and increased the vacuum. Thus, the wipers sped up. On the other hand, when he depressed the accelerator, the plate in the carburetor opened and the airflow through the air intake increased. Therefore the vacuum was reduced and the wipers slowed down. When it was raining, the driver would have to release the accelerator to increase the vacuum to get the wipers to move. Rainy weather always meant a jerky ride to school.
Yes, modern cars have a vacuum line going to the fuel tank. It is part of the emissions system, used to test the tank for proper air tightness and/or leaks. That is why a loose gas cap can light the CEL.
Back in the days when there was no electronics, vacuum adjusted ignition timing by moving the distributor. Broken vacuum line = wrong ignition timing = rough idle.
Today’s cars have sensors and wires instead. My last car had a distributor above the glove box. Many call it a distributorless ignition