Unregistrated Intake Air
In general, the Diesel engine and the quite modern petrol engine with mass air flow metre and without any flap in the suction do not have to cope with such problems. But all the other engines, particularly if they are a
little older, may have problems with unwanted air. Typically for this problem is a more frequent change of the idle running, also depending on the engine temperature. One plausible cause may be an ageing gasket
between the grey iron inlet manifold and a cylinder head manufactured from light metal. The
suspicion of unregistered intake air is substantiated if - with
older engines - the idle running cannot be turned back below a certain value any more.
How it works
The pressure between throttle valve and intake valve is an important measure for the petrol engine featuring a carburetor. If no there is no lambda
sensor inserted, the pressure determines almost by itself the amount of fuel that should be added to the air. If part of the sucked in air does not come in through a registered entry, revs and/or mixing proportion
are often not correct. Mistakes appear mostly only in the lower speed range and with idle running. It is very important to find the leaky spot. Information about a recently performed reparation in this area might help,
maybe not quite correctly carried out. The depression in the suction area encloses a wide area of the engine compartment. As the figure on top points out, even the vacuum brake booster might be the troublemaker.
Besides the exhaust gas recirculation even components which are not directly linked to the intake air should be considered. To point out how far depression is spread in the vehicle, door locking systems and pop-up
headlights are mentioned exemplarily. The test can be simplified if you disconnect complete systems for a short while.
In earlier times the suction area - although of course forbidden - was sprayed with a very small amount of petrol from the outside. If the revs of the engine increased, they had found the leaky spot. Today they substitute
water and hope for rev changes - mostly in reverse direction. The water seals for an instant, the engine receives less air. You should be careful not to expose the engine to more than one small splash of water via the
suction air. Because water is at least for the piston of the internal combustion engine not compressible, it can cause similar damages like screws and other metal objects.