Intake manifold. Problems of air-tightness

One of the more common problems – air suction of the intake manifold. Although the problem is very common, its manifestations are different and sometimes they are “hiding”.

For the introduction – several basic things, which are characteristic of this problem:

  • symptoms: very incorrect idle directly after starting the engine (the engine shivers – misfires; RPM “jumps” etc.) or also suddenly – for the warm engine;
  • sometimes (in the moments, when the idle is incorrect) – loss of power by low torque (starting the movement);
  • error messages regarding failure to maintain fuel mixture (lean or rich), EML symbol in KOMBI lights up;
  • if the cause is damaged CCVV – increased oil consumption*;
  • often (very often, if there are error messages regarding lean mixture) the defect is accompanied with error messages regarding misfires of cylinders.

* increased oil consumption damages Lambda probes and CO catalytic converters (for N43/N53 – also NOx catalytic converter and NOx sensor)!

As we see, the manifestations of the defect are different, so the more detailed description regarding typical scenarios will follow.

Some more addendum – what is typical to this defect:

  • the defect can be sporadic – it means, it can appear and then disappear;
  • usually, the defect is mostly displayed exactly in the moment of the start, but suddenly can appear also for the warmed engine;
  • the defect most of all is manifested in idle and in range of low required torque but doesn’t affect the range of high required torque.

Performing diagnostics – what confirms, that the cause of the problem is directly this problem?

  • idle (offset LTFT) fuel adaptations are 0.00 – they are not confirmed. DME doesn’t confirm offset type adaptations, if the correction of fuel, required in idle, is significant (exceeds planned range) and/or is significantly changing;
  • idle (offset LTFT) fuel adaptations are strongly increased (close to the max value: 1.0/1.5 mg/stk for N43/N53);
  • idle (offset LTFT) fuel adaptations change (are overwritten) or significantly (for banks more than +/-0.2 .. 0.3 mg/stk) changes, depending of the temperature.

In what cases the error messages regarding fuel mixture (lean) will be recorded?

If turning on the engine, additional air is sucked in the intake manifold (which is not measured by air mass meter), but when the engine heats up, the defect gradually decreases/disappears. In this case, directly after turning on (as soon as wide-band Lambda probes will be heated up), DME will try to correct the situation:

  • integrators are increased to +30% (value typical for MSV/MSD systems, for others: max values are 20 .. 30%); Lambda will be increased (above 1.00) for both banks – the lean mixture will be confirmed;
  • if it will not help – will modify (up – with positive pace) values of offset type (LTFT) adaptations;
  • will record the error message regarding lean fuel mixture, if, when reaching the border of 30%, during time X (usually 5 .. 10 seconds) it will not succeed to reach Stoichiometric fuel mixture (Lambda 1.00).

In what cases the error messages regarding fuel mixture (rich) will be recorded?

These error messages are a little rarer, but the mechanism of its registration is the following:

if the defect is manifesting for the warm engine (and the defect appears gradually), for several times DME will modify (overwrites) offset LTFT to significantly positive values (above +1.0 mg/stk), to inject additional fuel (to compensate the air, sucked in the intake manifold and not measured by air mass meter) and to maintain the correct fuel mixture;
if the defect doesn’t manifest for cold engine or for a warm engine, when starting, in this case, DME detects, that the fuel mixture is very rich (thanks to modified LTFT):

  • DME will swiftly reduce integrators to -30% and modified offset type LTFT “down” (with negative pace);
  • Lambda will be lowered (below 1.00) for both banks – will confirm rich mixture;
  • will record the error message regarding rich fuel mixture, if, when reaching the border of 30%, during time X (usually 5 .. 10 seconds) it will not succeed to reach Stoichiometric fuel mixture (Lambda 1.00).

Why the defect manifests exactly in idle and in range of low required torque?

In the idle, the intake manifold (for N43/N53, working in Homogeneous mode, for others N series engines – working in “non-Valvetronic” mode – it means, directly after starting the engine, for M series – without any additional requirement) has very strong air shortage. The throttle is practically closed, the engine tries to suck the fuel in the cylinders “with all power”. In such conditions even tiny (in diameter 1 .. 2 mm) defect in the intake manifold will destroy completely all fuel mixture stabilizing system! Exactly – for DME to become totally unusable (misfires, shivering, loss of torque, error messages regarding fuel trim), it’s enough with even a tiny crack or hole!

On the contrary, in conditions of larger torques, when the throttle is opened, the air shortage ins the intake manifold decreases, and at the same time – the opening range (area) of throttle increases. The impact of the defect decreases very swiftly and becomes close to 0 already by average torque inquiry.

What are the most typical causes of this defect?

  • the damaged membrane of CCVV. The air is sucked via membrane’s ventilation holes (on the top of its body);
  • the defect of the CCVV pipe (usually – close to the engine block – in the most hardly visible place);
  • defects of the EGR valve, DISA valve O rings.

The defect, which has a similar impact, but is located before throttle – damage of the pipe to the intake manifold. This defect will cause problems both in idle and in range of partial load. The error messages regarding lean fuel mixture, plausibility/data conformity of air mass meter.The good news – this defect can be located easily when visually checking the pipe.

How to detect the location of the defect?

How to check the damage of the CCVV membrane, read here;

to check airtightness of other places, usually, the smoke test is used. It will identify places, not airtight. How to perform a smoke test, read here;

Still, we have to keep in mind some nuances:

while performing the smote test, the engine is not working – it doesn’t vibrate. If the cause of the defect is a connection of any pipe, the defect can manifest exactly for working (vibrating) engine. So, when performing the smoke test, try to move all connections of the intake manifold.
If the cause of the defect is the damaged membrane of CCVV, it can happen, the increased pressure (in time of smoke test) presses the damaged place closed, and the defect “hides”. So the damage of the CCVV membrane has to be checked separately – checking the air suction via ventilation holes of the membrane, while the engine is working.
Performing the smoke test, the pressure difference between “surrounding/intake manifold” usually is smaller than in real idle conditions and “opposite”, it means, the pressure in the intake manifold is increased, not lowered. Sometimes these different conditions are the cause, why the smoke test cannot the pressure detect some tricky defect.

While performing the smoke test, we have to understand – this test helps to identify places, where the additional air is sucked, but, if it does not identify any damages, it’s not a guarantee, that the additional air is not sucked in! If the engine has symptoms, mentioned before – check the CCVV very carefully, if it doesn’t close – the hub has to be replaced.

If it’s complicated to perform the smoke test, you can go to the closest workshop – they have to have the equipment to perform the smoke test.

In the image – typical condition of CCVV membrane after 5+ years of use.

Example No.1:

As we see, MSD has not confirmed offset adaptations (they are 0.00), the fuel mixture is very lean (even by +30% additional fuel supply MSD measured Lambda 1.32). The defect observed, when starting the engine (also warm).

Example No.2:

MSD has confirmed offset adaptations, but for the warm engine, they grow and reach +1.5 .. 2.0 mg/stk. Instead, for the cold engine, the defect (air suction) doesn’t manifest, and the error message regarding the rich mixture is recorded.

Example No.3:

The defect of additional air suction doesn’t manifest directly after starting the engine, but (see the example No.2) before the engine has created very positive offset type adaptations. When the defect is not observed, the fuel mixture becomes very rich (Lambda drops significantly below 1.00 for both banks).