Intake Manifold, how it works, common problems, diagnostic, repair

February 15, 2016
Intake Manifold
Intake manifold

An intake manifold distributes the intake air flow evenly between cylinders. In many cars, an intake manifold also hosts the throttle valve (throttle body). An intake manifold consists of the plenum and the runners, see the photo.
The air flows through the air filter, intake boot, then through the throttle body, into the plenum, then through the runners into the cylinders.
The engine performance can be adjusted by varying the size of the intake plenum and the length and shape of the runners. For this reason, modern cars have variable intake manifolds where special tuning valves alter the air flow through the manifold depending on the engine speed and power demand.

Intake manifold problems

Intake manifold problems
Intake manifold problems Click for larger photo

Common problems with intake manifolds include leaking gaskets, carbon build-up and issues with intake tuning valves.
In some cars worn-out intake manifold gaskets (in the photo) can cause vacuum leaks. This can cause rough idle, stalling as well as the Check Engine light coming on with the codes P0171 and P0174. The repair includes removing the intake manifold, checking the mounting surface 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 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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Intake manifold performance tuning

Ferrari F12tdf - Focus on powertrain

The power of the engine is greatly depends on the amount of air that can be delivered through the intake. Changing the size of the intake plenum as well as the length of the intake runners can greatly influence the engine torque curve as well as overall performance. For example, in this video, Ferrari engineers were able to get more power out of their V12 naturally aspirated engine by changing the intake manifold shape and size and adding adjustable trumpets. Adjustable trumpets effectively change the length of the intake runners according to the engine speed. Longer intake runners add torque at lower RPMs, while shorter intake runners increase torque at higher RPMs.

While switching to a custom-made intake manifold can be expensive, some performance enthusiasts opt for porting and polishing of the intake runners and the cylinder head. The idea is to smooth out the imperfection and match the size and shape of the intake runners to the openings in the cylinder head. If done properly, porting and polishing can improve the air flow into the engine.

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