Intake manifold, how it works, symptoms, problems, testing

Updated: October 07, 2019
Intake Manifold
Intake manifold
Intake air flow
Intake air flow

In a car engine, an intake manifold distributes the intake air flow evenly between the engine cylinders. In many cars, an intake manifold also holds the throttle valve (throttle body). An intake manifold consists of the plenum and runners, see the photo.

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

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.

Intake manifold problems

Common problems with intake manifolds include leaking gaskets, carbon build-up and issues with the intake tuning valves. In some engines, an intake manifold can crack causing either vacuum or coolant leaks.

Intake manifold problems
Intake manifold problems

A cracked manifold must be replaced.

Worn-out intake manifold gaskets (in the photo) often cause vacuum leaks. This can cause rough idle, stalling as well as the Check Engine light coming on. For example, the OBD-II trouble codes P0171 and P0174 are often caused by intake manifold vacuum leaks. The repair includes removing the intake manifold, checking and cleaning the mounting surfaces and replacing the gaskets. Check this forum to have an idea. Often the source of the vacuum leak could be a broken vacuum hose or line that connects to the intake manifold. In this case, a faulty vacuum hose or line must be replaced. In some cars, a vacuum leak can be a identified by a hissing sound coming from under the hood. Read more: Vacuum leaks: common sources, symptoms, repairs.
In some older GM vehicles, an entire intake manifold had to be replaced due to a coolant leak.

In some engines, for example, Volkswagen TDI Diesel, carbon build-up inside the intake manifold can cause lack of power, misfiring, smoke and poor fuel economy. Check this post to see the photos. Issues with carbon buildup are more common with turbocharged engines. A clogged-up intake manifold might need to be removed and cleaned manually, or replaced all together.

Problems with Intake manifold tuning valves

In many cars, tuning valves are operated by vacuum actuators. Often, a rubber diaphragm inside a vacuum actuator starts leaking and the actuator stops working.

How to test vacuum actuators for tuning valves.

It's easy to test the vacuum actuators with a handheld vacuum tester. If a vacuum actuator leaks, it must be replaced.

The vehicle computer (PCM) engages vacuum actuators by turning little vacuum control solenoids on and off. These solenoids are also often go bad.

Problems with intake manifold tuning valves are also common. For example, in some GM vehicles, a failed intake manifold tuning valve can cause the Check Engine light code P2070. The repair involves replacing the tuning valve. In many BMW vehicles a failed DISA valve installed in the intake manifold is a common issue. In some cars, especially the ones with a turbocharger, intake tuning valve problems could be also caused by carbon build-up.

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