Tip & How-To about Mercedes-Benz E-Class
1b) Air : leaks in air intake
Leaks in the air system can occur both before and after the throttle plate. In both cases the additional air entering is not measured by the air mass meter and causes the engine run unevenly. Engines with motor-driven idle control of the throttle plate may be able to accommodate minor air duct leaks. Hot engine restarts can be problematic if the air being drawn in is from the immediate surroundings of a very hot engine.All air ducting should be free of any leaks due to cracks or improperly fitted junctions.
Leaks after the throttle plate and into the inlet manifold severely compromise the ECU's ability to match the amount of injected fuel to the amount of 'measured' air supplied to the engine during 'closed cycle'.Although the ECU does its best to adapt and accommodate the change in air intake the engine remains rough and is less responsive to demand and often the Check engine light will become lit and codes such as P0171 (lean fuel mix) will be set. All vacuum lines should be thoroughly inspected for cracks, splits and loose fitting joints. This entails twisting the pipes to get an all-round look and feeling along the length of every less visible vacuum pipe. Many leaks can be isolated by disconnecting the vacuum lines in turn and temporarily plugging up both the attachment points and the tube ends and then monitoring the effect; a purge valve jammed open or a crack in the tubing on the EVAP system is very hard to detect other than by using this approach. Other points that may avoid easy leak detection, such as the junction between intake manifold and the cylinder head or around the injector body seals, can be tested by methodically spraying the suspect area with a fuel source (cigarette lighter fuel, alcohol, carburetor choke cleaner, propane gas, WD40 etc). A temporary surge in engine revs or perhaps stall, in response to the spray being sucked in and ignited, will confirm a suspected leak.
Engine characteristics in response to an air leak mimic the effects brought about by a faulty throttle position sensor or idle air control valve.All are related to the ECU being unable to maintain a normal ratio between the fuel dispensed and air available.
Possible leaks before the throttle plate:
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