Intake Manifold

In a car engine, an intake manifold distributes the air flow evenly between the cylinders. In many modern cars, intake manifolds is made of plastic as a compromise between weight and cost. 

Often an intake manifold holds the throttle valve (throttle body) and some other components. An intake manifold consists of the plenum and runners. In some V6, V8, V10, V12 and W12 engines, an intake manifold can be made of several separate sections or parts.

An intake manifold

The intake air flows through the air filter, intake boot (snorkel), then through the throttle body, into the plenum, then through the runners and into the cylinders. 

Path of airflow into an engine

In modern cars, the engine idle speed is also controlled by the throttle body: at idle, it opens at a very small angle. Because the throttle body is almost closed when the engine runs at idle, there is vacuum inside the intake manifold. If there is a vacuum leak somewhere in the manifold, the engine will run rough (unstable) at idle. Many of the problems with intake manifolds are related to vacuum leaks, read more below.

The engine performance can be adjusted by varying the size of the intake plenum and the length or opening size of the runners. For this reason, modern cars have variable intake manifolds where special tuning valves change the air flow through the manifold depending on the engine speed and power demand. Another complexity waiting to break.

Intake manifold problems

Common problems with intake manifolds include vacuum, coolant or oil leaks, reduced flow due to carbon build-up (turbocharged engines)  and issues with the intake tuning valves. In some engines, an intake manifold can corrode or crack (cheap materials) causing either vacuum or coolant leaks. A cracked manifold must be replaced if it cannot be safely repaired.

 

Coolant leaks: In some cars, there are coolant passages inside the intake manifold that can leak, often because of bad gaskets or other damage. If the manifold is not damaged and mating surfaces are in good shape, replacing the gaskets or re-sealing the manifold is usually enough to solve the problem. If the manifold is damaged, it must be replaced.

Vacuum leaks: Worn-out intake manifold gaskets often cause vacuum leaks. This can cause rough idle, stalling, as well as the Check Engine light coming on, although the engine may run fine at higher rpm. If the leaks are caused by bad gaskets, the repair involves removing the intake manifold, checking and cleaning the mounting surfaces and replacing the gaskets. 

Often the source of the vacuum leak could be a cracked vacuum hose or line that connects to the intake manifold. In this case, a broken vacuum hose or line must be replaced. Sometimes an intake manifold can warp causing the gaskets not to seal properly. A warped intake manifold must be replaced. In some cars, a vacuum leak can be identified by a hissing sound coming from under the hood.

Carbon Buildup: In some turbocharged diesel engines, carbon build-up inside the intake manifold, can cause a lack of power, misfiring, smoke and poor fuel economy. Issues with carbon buildup are more common in turbocharged engines. One of the main symptoms is a lack of power. A clogged-up intake manifold might need to be removed and cleaned manually. In some cases, replacing the intake manifold might be a more sensible solution than cleaning it. There are many hidden areas inside the manifold that cannot be cleaned.

Problems with Intake manifold tuning valves: Tuning valves are typically operated by electric or vacuum actuators. Often, a rubber diaphragm inside a vacuum actuator starts leaking and the actuator stops working or is sticking due to carbon build-up.

The vehicle computer (PCM) engages vacuum actuators by turning little vacuum control solenoids on and off. These solenoids also often go bad. This problem is more popular with Audi/VW and BMW engines. 

Whenever an intake manifold is replaced, it's important to clean the mounting surface, replace the gaskets and torque the manifold bolts in the recommended order to the specs. 

Torqueing the bolts means not to overtighten them. Most engine blocks are made from soft aluminium. You do not want to ruin the threads.

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