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Parking Brake The parking brake holds one or more brakes continuously in the applied position. The parking brake employs the regular drum brakes on the rear wheel. Instead of hydraulic pressure, however, a simple mechanical linkage is used to engage the brake shoes. When the parking-brake pedal is depressed (or, in some cars, a hand lever is raised), a steel cable pulls taut a tension lever; other cables draw the brake shoes firmly against the drums. The release knob slackens the cables and disengages the brake shoes. The parking brake is self adjusting. An automatic adjuster in the piston moves on the thrust screw to compensate for lining wear.

PCM Power Control Module

PCV valve   See positive crankcase vent valve.

PFI  Port Fuel Injection

Pinging  The sound an engine makes when it is suffering from detonation or preignition. This is often a warning that the gasoline being burned has insufficient octane, and it can be a very hazardous condition for an engine.

Pinion  A small diameter gear with a small number of teeth designed to mesh with a much larger gear wheel or a toothed rod (rack). Used in rack-and-pinion steering and for speed reduction with an increase in power.

Piston, Rings, and Wrist Pin The piston converts the potential energy of the fuel into the kinetic energy that turns the crankshaft. The piston is a cylindrical shaped hollow part that moves up and down inside the engine's cylinder. It has grooves around its perimeter near the top where rings are placed. The piston fits snugly in the cylinder. The piston rings are used to ensure a snug "air tight" fit. The wrist pin connects the piston to the connecting rod. Pistons are made of aluminum, because it is light and a good heat conductor. The piston head or "crown" is the top surface against which the explosive force is exerted. It may be flat, concave, convex or any one of a great variety of shapes to promote turbulence or help control combustion. In some, a narrow groove is cut into the piston above the top ring to serve as a "heat dam" to reduce the amount of heat reaching the top ring.

Piston  A partly hollow, cylindrical metal engine part that is closed at one end and fits into the engine cylinder. Connected to the crankshaft via the connecting rod and usually fitted with rings to seal it in the cylinder.

Discover How The Piston works in The Engine

Piston Rod A shock absorber rod which operates the piston in a telescopic damper

Plastigage  A thin strip of plastic thread, available in different sizes, used for measuring clearances.
For example, a strip of Plastigage is laid across a bearing journal. The parts are assembled and dismantled; the width of the crushed strip indicates the clearance between journal and bearing.

Ply Rating A measure of the strength of tires based upon the strength of a single ply of designated construction. An eight-ply rating does not necessarily mean the tire has eight plies, but rather that the tires has the strength of eight standard plies.

Positraction  General Motors' trade name for a limited-slip differential.

Pneumatic Tire  A circular tube of rubber or synthetic rubber and fabric, and sometimes also steel, attached to the rim of the car's wheel, having resilience due to its containing air under pressure.

Poppet Valve A valve structure consisting of a circular head with an elongated stem attached in the center, very much like the shape of a mushroom with a flat top. It is designed to open and close a circular hole or port. Its name comes from the fact that it pops up and down. In engines, it is the valve used to open and close the valve port entrances to the engine cylinders.

Porsche    A vehicle brand of which the Series 356 for 1949-64 are milestone cars. The 356C for 1965 is a milestone car.

The official "date of birth" for the first Porsche is considered being June 8, 1948. Number one, a two-seat 356 with a 1.1 litre centre engine, 40 HP weighing 650 kg. The following 50 speciemens of the so-called 356/2 were hand built in Gmünd/Kärnten (Austria). These cars almost resembled the classic 356 and are today, if at all to be found, almost priceless.

Port Fuel Injection (PFI) A type of fuel injection with at least one injector mounted in the intake port(s) of each cylinder. Usually the injector is mounted on the air intake manifold close to the port. Port fuel injection improves fuel distribution and allows greater flexibility in intake-manifold design, which can contribute to improved engine breathing. Also called multi-point injection

Positive Crankcase Vent Valve  Abbreviated PCV valve. An emissions-reduction device that has been used on most cars from the late '60s to the present. Blowby and other factors can allow combustion gases, as well as some unburned air and fuel, into an engine's crankcase. The PCV valve uses engine vacuum to remove these gases and reroute them back into the intake system, so they can be burned, rather than dumped into the atmosphere.

Power Brakes Power brake are of four general types: vacuum suspended; air suspended; hydraulic booster, and electro-hydraulic booster. Most power brakes use vacuum suspended units, which contain a large vacuum-powered booster device to provide the added thrust to the typical power-brake. Pressure on the brake pedal pushes forward a rod connected to the pistons of the two master cylinders. The pistons begin forcing fluid into the front and rear brake lines. At the same time, the brake-pedal pushrod positions the vacuum-control valve so that it closes the vacuum port and seals off the forward half of the booster unit. The engine vacuum line then draws off the air, creating a low-pressure vacuum chamber. Atmospheric pressure in the control chamber then pushes against the diaphragm, dividing the two chambers. The pressure on the diaphragm, which is locked to the pushrod, forces it forward, supplying even more pressure on the pistons. The safe driver is always ready to apply the total force needed to stop their vehicle, even if the engine quits (removing the power assist).

Predetonation The undesirable "knock" or "ping" that occurs when the ignition of the air-fuel mixture occurs before the ignition spark. Also known as "pre-ignition".

Primary Circuit  The low voltage (6 or 12 volts) path of the ignition system which goes from the positive pole of the battery to the primary windings of the coil, through the breaker points, and into the ground which leads back to the negative pole of the battery.

Primary Winding
Primary Windings    The low voltage (6 or 12 volt) windings in ignition coil, the primary winding is heavy wire; secondary winding uses fine wire.

Primary Wire
Primary Wires

The wiring which serves the low voltage part of the ignition system. Wiring from battery to switch, resistor, ignition coil, distributor points.

Propeller Shaft  The long hollow tube with universal joints at both ends that carries power from the transmission to the differential on front-engined rear wheel drive vehicles.

Proportioning Valve  A hydraulic control valve which limits the amount of pressure to the rear brakes during panic stops to prevent wheel lock-up.

Push Rod A push rod is a connecting link in an operating mechanism. Two examples are the rod between the valve lifter and rocker arm on an overhead valve engine, and push rods at the piston ends which apply pressure to the brake shoes.


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The objective of this Web Page is to familiarize you with basic auto maintenance
-  in some common emergencies -
not to make you an expert in auto mechanics

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