Intake Manifold: How It Works, Symptoms, and Fixes

What an Intake Manifold Does

The inlet manifold (or intake manifold) is designed to disperse air evenly to each cylinder of the inline engine, so the engine can mix air and fuel and burn it properly. On many contemporary engines, the manifold actually contains the port fuel injectors; older carbureted or throttle-body-injected systems instead fed a premixed air-fuel charge through the head(s) with long runners that stretched down from a central carb/throttle body location on updraft and side-draft manifolds.

Plenum vs. runners explained

The plenum is the main room, and it stores air. From it, a separate runner supplies each intake port. Their size and length are optimized to balance the flow velocity and the back pressure of air, allowing cylinders to be filled more completely with air on each intake stroke, further enhancing torque, power output and drivability.

Materials: aluminum, cast iron and composites

Manifolds are typically constructed of aluminum or cast iron to provide heavy duty strength and durability. Plastic or composite is used in many modern designs to save weight and reduce the propagation of heat that cools incoming air somewhat, increasing efficiency.

How filtered air gets to and through the manifold

The purpose of air filter and maintenance

The air cleaner assembly into which the intake air flows is provided with a filter, and the filter filters the intake air. That’s because this filter catches the dust and chunks that would otherwise mar cylinders or destroy sensors. When it’s time to replace, then Engine Air Filter must be replaced along with its engine companion – the Cabin Air Filter.

Intake design types: EFI vs. carb, variable length

Carbureted vs. fuel-injected setups

On carbureted and early throttle-body-injected engines, a fuel/air mixture travels through the manifold. Many newer engines rely on multi-port fuel injection, with injectors mounted in the manifold at or near the runner entrance and making runners all or mostly air only for more consistent mixture and emissions.

Variable-length and resonance tuning

Many variable intake manifolds can vary runner length; long runners enhance low-end torque, short runners high-rpm power. Resonance (Helmholtz tuning) is used by many systems to coordinate pressure waves in order to cram more air into cylinders over an even wider rpm range.

Diesel swirl flaps: advantages and dangers

Some diesel manifolds employ swirl flaps or discs as small butterfly valves near the intake ports to promote a better air/fuel mixture in the low RPM band. They can become gummy, sticking or even damaging them over time by EGR soot buildup. In the worst case, a tearing flap could be swallowed by the engine and cause catastrophic damage. Inspection and the need to clean or replace is very Important.

Common intake manifold problems, symptoms and risks

Vacuum leaks: whistling, rough idle and stalling

Either a cracked plastic manifold, split vacuum ports or gasket failure allows unmetered air into the engine. Symptoms of failure are a whistling noise at the idle, shaky and irregular idle or poor driving behavior with lean codes or even stalling. Small gaps may be temporarily filled, but replacement is the reliable repair.

• Whistling/hissing noise at idle
• Rough idle, hesitation, or stalling
• Check engine light with lean codes
• Fall off in fuel economy or lack of power gains

EGR soot and swirl flap failure (diesel)

Too much soot builds up and airflow is restricted, the swirl flaps can become stuck. Complaints can range from limp mode, smoke and intake limitations. If left unattended, they can work their way into the cylinders and cause extensive damage to the pistons and valves.

How to replace an intake manifold

Modern vs. older vehicle considerations

Direct-fit replacement manifolds and gasket sets are now available on a very large number of the most popular applications. Classics, as well as certain rare models, may require salvage yard parts, optional online marketplace goods — or advantageous custom-fabricated manifolds can be expensive and involve expert fitting.

Parts, gasket sets and fitment (what to buy)

If you are contemplating the addition of an MAP sensor, fuel injectors and your authority recommends it, don’t forget a JBR-specific manifold for use with internal-boundary cutting manifolds.

Quality Parts List

• Vehicle specific lower intake manifold (LIM) to cylinder head gasket set
• Upper intake maninfold gasket
• Throttle body gasket
• Injector O-rings
• Nuts/Washers
• Hose-Clamps

As always use factory tightening sequence and specs. Post install you’ll have to smoke or spray test for vacuum leaks and remove any fault codes.

Performance upgrades: plenum, runners and powerband

Runner length, plenum volume & powerband

Aftermarket intake manifolds optimize the length and cross section of runners, as well as plenum volume, to move the torque curve. Longer, narrower runners are optimised for low to midrange torque; shorter, larger runners and bigger plenums help make high-rpm horsepower. Choose a manifold properly based on your engine family and rpm range.

Composite/carbon options and heat management

Composite or carbon manifolds tend to weigh less and suffer less from heat soak, as well as often being modular for simple runner swaps. Results vary depending on application; gains without supporting mods and tuning may be nominal on a stock engine.

FAQs about intake manifold leaks, costs and reliability

What does an intake manifold serve?

It sends clean air (and, on some systems, the fuel-air mix) into each cylinder for burning.

Intake Manifold Gasket Symptoms

Hissing/whistling; rough idle; stalling; lean codes; misfires, poor fuel economy or visible cracks and oil/coolant owns when applicable.

How Much Does It Cost to Replace an Intake Manifold?

Costs vary widely by vehicle. Estimate manifold and gasket set with a fitment tool for you exact year make and engine then add local labor hours.

What is a plenum and runners?

The plenum is the air chamber; runners are each of the passages that lead from this chamber to one intake port.

Is plastic intake manifold a reliable thing?

Yes, they are widely available and lightweight, but plastic becomes brittle with age and heat. Check for cracks and replace gaskets after an aging period.

What are swirl flaps and should I get rid of them?

They’re diesel manifold valves that help generate the swirl at low rpm. They help drive better but can become fouled; many people clean or replace them, or convert over to a safer design — vs just pulling the plug.

Summary

The plenum and runners of the intake manifold mete air to each cylinder, dictating the torque curve and horsepower. Keep the air filter new, be aware of symptoms of a vacuum leak, and tackle diesel swirl flap issues early. Use the correct gaskets and torque sequence for repairs. For performance, opt for a man­ifold spec’d out for your engine family and rpm goals.