Which Engine Parts Are Plastic Today

Many modern engines use plastic for non-structural components such as intake manifolds and ducts, valve covers, thermostat housings and coolant pipes, oil filter and some oil pans, timing chain guides, PCV and vacuum parts, sensor housings and connectors, airboxes and resonators, as well as certain fuel rails (on port-injected engines), pulleys, fan shrouds, and water-pump impellers. By contrast, high-heat and load-bearing parts—like the block, head, pistons, crankshaft, rods, and exhaust components—remain metal.

Where Plastics Are Commonly Used on Engines

Air intake and induction

Automakers widely use engineering plastics in the air path for weight savings, acoustic tuning, and corrosion resistance, especially on gasoline engines and many light-duty diesels.

• Intake manifolds (glass-fiber reinforced nylon/polyamides, often with integrated runners and PCV passages)
• Air filter boxes, snorkels, and intake ducts (polypropylene, ABS, PA)
• Acoustic resonators and Helmholtz chambers to reduce intake noise
• Mass-airflow sensor bodies and housings (with metal sensing elements)
• Throttle-body housings on some models; commonly, the internal gears are polymer even when the housing is aluminum
• Charge-air cooler (intercooler) end tanks on many turbocharged applications (often glass-filled nylon), though the cores remain metal
• Intake swirl/tumble flaps and actuation linkages inside manifolds

Together, these parts illustrate how plastics enable complex shapes and airflow tuning while trimming weight, though components near turbos may still default to metal due to heat.

Fuel and emissions subsystems

Plastics are prevalent where fuels and vapors must be contained without adding mass, with material choices driven by pressure and chemical resistance.

• Fuel rails on many port-injected engines (multilayer plastic rails); direct-injection fuel rails are typically metal due to very high pressures
• Fuel lines, quick-connect fittings, and evaporative emissions plumbing (nylon, fluoropolymers, multilayer constructions)
• PCV valves and separators, often integrated into plastic valve covers
• EVAP canister purge valves and housings
• EGR control solenoid housings and connectors (the EGR valve body and cooler are usually metal due to exhaust heat)

In these systems, plastics balance permeation control and weight, but extreme pressure or exhaust heat still necessitate metal in key locations.

Cooling system components

The cooling circuit uses robust thermoplastics for corrosion resistance and integration of complex passages, though heat proximity varies by engine layout.

• Thermostat housings and coolant outlets/inlets
• Coolant crossover pipes and tees on some engines
• Water-pump impellers on select applications (others use metal or composite)
• Expansion tanks and caps (system-level, not on the engine block)
• Radiator and charge-air cooler end tanks (system-level, commonly plastic crimped to metal cores)

These parts demonstrate plastics’ assembly and corrosion advantages; however, repeated heat cycles can age some housings, making proactive inspection important.

Lubrication system and covers

Polymers help integrate oil passages, reduce noise, and simplify assembly for upper-engine closures and lubrication hardware.

• Valve covers (widespread use of glass-fiber reinforced nylon with molded baffles)
• Oil filter housings and caps (common in European and many modern engines)
• Oil pans on some vehicles (composite/plastic pans with metal inserts and baffles)
• Oil fill caps and dipstick handles/tubes
• Decorative/insulating engine covers and undertrays

These components show how plastics can integrate functions (like baffles and PCV passages) while reducing mass and noise versus traditional stamped metal.

Timing and drive accessories

In timing and accessory drives, plastics are used where sliding interfaces and noise control matter, or where cost and weight advantages are significant.

• Timing chain guides and tensioner shoes (polymer wear surfaces on metal backings)
• Front-end accessory drive idler pulleys and covers (plastic on some models, metal on others)
• Fan shrouds and many engine-cooling fan blades (under-hood, not always engine-mounted)
• Belt tensioner housings or pulleys (application-dependent)

These uses leverage low friction and damping characteristics, though heat and load dictate when metal remains the better choice.

Sensors, connectors, and small hardware

Electronics and small fittings rely extensively on plastics for packaging, sealing, and weight, with careful polymer selection for heat and chemical exposure.

• Sensor bodies and connectors (MAP, MAF, cam/crank position, coolant temp, oxygen sensor connectors, etc.)
• Harness connectors, clips, and conduit
• Vacuum reservoirs, check valves, and nylon vacuum lines
• Ignition coil housings and coil-on-plug boots (elastomers and high-temp plastics)
• Various caps, plugs, and service fittings

While small, these components are numerous and illustrate how plastics underpin modern engine electronics and serviceability.

What Is Rarely Plastic (High-Heat/Structural)

Certain components face extreme temperatures, loads, or wear that currently demand metal or ceramic materials.

• Engine block, cylinder head, head gasket layers, and head bolts
• Pistons, piston rings, cylinder liners, crankshaft, connecting rods, camshafts
• Exhaust manifold, turbocharger housings and wheels, catalytic converters, and EGR coolers
• Valves, valve seats, valve springs, lifters, and most timing chains/gears
• High-pressure direct-injection fuel rails and injectors’ critical metallic bodies

These areas endure sustained high heat and mechanical stress that exceed the capabilities of even advanced thermoplastics in mainstream production.

Materials You’ll Find

Engine plastics aren’t consumer-grade; they’re engineering thermoplastics and composites tailored to heat, chemicals, and mechanical demands.

• Polyamides (nylon PA6, PA66, PA6T, PA612), often glass-fiber reinforced for manifolds, valve covers, and housings
• PPS and PPA for higher-heat, chemically aggressive environments
• PBT and PET for connectors and under-hood electronics
• Polypropylene and ABS for ducts, airboxes, and covers where temps are moderate
• Fluoropolymers and multilayer constructions for fuel and vapor barriers
• Elastomers (EPDM, FKM/Viton, silicone) for seals, boots, and hoses
• High-performance materials like PEEK appear in select, demanding niches

The choice depends on temperature exposure, fluids, pressure, and stiffness needs, with fillers like glass fiber boosting strength and heat resistance.

Why Automakers Use Plastics

Several practical advantages explain the shift from metal to plastics in many engine-adjacent parts.

• Weight reduction for better fuel economy and emissions
• Design freedom to mold complex shapes, integrate baffles/passages, and reduce part count
• Corrosion resistance against coolant, oil, and fuel
• Noise, vibration, and harshness improvements (damping and acoustic tuning)
• Cost and manufacturability gains at scale

These benefits accumulate across multiple components, contributing to efficiency and refinement targets without compromising function when materials are well-chosen.

Limitations and Failure Modes

Plastics come with trade-offs that inform maintenance and design choices, especially as vehicles age.

• Heat aging and embrittlement leading to cracks (e.g., thermostat housings, valve covers)
• Creep or warping under sustained load/temperature
• Chemical incompatibility if the wrong polymer meets the wrong fluid
• UV and ozone exposure on under-hood parts near the cowl if not stabilized
• Mechanical damage from over-torqued fasteners or improper service
• Known historical issues: plastic water-pump impellers and coolant tees on some makes, swirl-flap failures inside manifolds

Understanding these risks helps prioritize inspections and choose upgraded materials where available during repairs.

Variations by Vehicle Type

The extent of plastic use varies with engine technology, duty cycle, and packaging constraints.

• Gasoline passenger cars: broadest plastic use in intake, cooling, and covers
• Turbocharged engines: more heat shielding and metal near turbos; plastic still common in upstream intake and charge pipes with proper materials
• Diesels: higher combustion temps can shift more components to metal, but plastic manifolds and covers are still common in light-duty applications
• Heavy-duty trucks/off-highway: greater bias toward metal for durability and service life
• Motorcycles and small engines: simpler systems; plastic airboxes, covers, and some cooling/fuel parts are typical
• Hybrids: same engine trends; additional plastic covers and cooling manifolds for hybrid ancillaries

In short, duty cycle and under-hood temperatures dictate where plastics can safely replace metal without compromising reliability.

Maintenance and Repair Tips

A few habits can extend the life of plastic engine components and prevent avoidable failures.

• Use manufacturer-specified coolants, oils, and service intervals to minimize chemical and thermal stress
• Inspect plastic coolant housings, crossover pipes, and oil filter caps for seepage or hairline cracks during routine service
• Replace aged quick-connect fittings and vacuum lines proactively; brittle nylon can split
• Follow torque specs, especially on valve covers and housings with molded-in seals
• Consider upgraded materials (e.g., aluminum thermostat housings or metal impellers) when reputable and compatible
• Protect exposed under-hood plastics from rodent damage and excessive heat soak where possible

These steps can mitigate common aging issues and keep plastic components reliable well beyond the warranty period.

Summary

Plastic is ubiquitous around modern engines—especially in air-intake parts, valve covers, coolant housings and pipes, oil filter housings, some oil pans, timing guides, and the myriad sensors and connectors—because it cuts weight, cost, and noise while enabling complex designs. Core hot and load-bearing parts remain metal. With proper materials and maintenance, plastic components are durable, but heat and age can make certain housings and fittings service items over time.

How to identify automotive plastics?

Plastic parts can be identified by an SAE code that is typically stamped on the rear of the part. Match the code(s) with the one found in the Plastic Identification and Refinishing Systems section of Service Information (SI).

What plastic is used for engine parts?

Polypropylene. Polypropylene is used the most frequently of any plastic in automotive manufacturing. Being a thermoplastic polymer, it can easily be formed into almost any shape. It has excellent chemical and heat resistance and is generally resistant to impact.

Are engines made of plastic?

There have been exceptions. Notably, a US firm called Polimotor developed an engine that used a reinforced plastic for the engine block and various other components, including piston skirts, connecting rods and the oil pan. Some surfaces, the piston crowns and the combustion chambers were reinforced with with metal.

What car parts are made of plastic?

4 Most Common Plastics in Car Manufacturing

• Vehicle carpeting.
• Seat upholstery.
• Cable and wire insulation.
• Bumpers.