Leakage at the intake manifold

Auto Motor Klassiek » Practice and technology » Leakage at the intake manifold
Purchasing classics there

Because air is also needed for the combustion of the mixture in the engine, there is not only the carburetor or injection in the inhalation path of an engine, but also the intake manifold through which the air that enters through the air filter is fair across the cylinders is being devided.

That intake manifold is mounted on the cylinder head at the level of the intake valves and has the air filter as a 'hat'. The spacing and shape of the intake manifold tubes should vary as little as possible from cylinder to cylinder so that each cylinder can receive the same amount of air / mixture. The air must also encounter as little resistance as possible during the flow in. That idea used to be of secondary importance to 'normal' cars. The intake manifolds were often simple ornaments of cast iron (or aluminum) and little value was attached to the 'tuned lengths' of the intake manifolds.

Easy score

And that was easy to score again in the early days of engine tuning, because with better coordinated intake and exhaust routes, mixture / air and exhaust gasses had a much more unimpeded course. The engine got more and more evenly distributed mixture over the cylinders, and with a 'spaghetti' exhaust the breathing improved even further. Okay, the current chip tuning is even easier. But did you know that it makes the leasing companies very nervous? And the insurers also respond quite perkily.

Formerly made of cast iron

It also happens with modern classics that the fuel injectors are mounted in the manifold instead of the engine getting its mixture through a carburetor. In the past, intake manifolds were made of cast iron or aluminum. On new cars, by the way, we increasingly see plastic intake manifolds. But in all cases such a manifold can leak. In proportion, leakage occurs more often with an exhaust manifold than with an intake manifold.

This is because the exhaust manifold is much heavier thermally loaded due to the constant flow of hot (approx. 900 degrees) exhaust gases than the intake manifold. But here we focus on the intake side. If a leak has occurred in the intake manifold, for example, a crack, the fuel-air mixture will not work properly and the engine will run considerably worse because the mixture is too poor. If an engine runs better at operating temperature with a (partially) engaged choke, then that is a nice indication. Often this is most noticeable with no load. Air that enters the engine in this way is called false air. Apart from a crack in the manifold, there may also have been a leak at the gasket / seal on the cylinder head.

Check for leaks

If a large leak is present, the suction of the false air will be clearly audible. That kind of false air suction can often be located by spraying some brake cleaner on the suspicious spot. Brake cleaner is highly flammable. So if the stuff enters the combustion chambers through a leak in the intake path, the engine will go down faster.

When replacing possibly present manifold gaskets, make sure that no old gasket remains. The best tools for this are: good lighting, a mirror and a gasket scraper. And patience.

For carburetor engines, the false air can also be sucked in because carburetor components are worn out.

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