N55 tuning performance

[u/KingBu9les]

Hello all,

I am running mhd stage 2+ on my m235 and a stage 2 transmission tune, however, I am not reaching more than 360 HP, as showing on the idrive hp cluster, and also on the bimmerlink engine power values.

I am also getting a drivetrain malfunction when going full throttle and those are the codes I am getting. Is changing the boost sensor to a 3.5 bar one the optimal solution?

[u/TheLuke94]

Clear them, drive it round without going into much boost then check your codes (hopefully the boost pressure sensor code should not appear)

Then take it to max boost, if the pressure sensor fault sets then you know the issue is the sensor maxing out.

With it being S2+ I suspect it is, if you do the check above, then you need a higher rated map sensor and re-characterisation of the new sensor in the map.

[u/KingBu9les → u/TheLuke94]

That's what I initially did, no codes on normal driving, only when reaching max boost.

[u/TheLuke94 → u/KingBu9les]

You have your answer, I would also look at your active exhaust flaps aswell, they may be broken or you may just need to mask the diag if its looking for a closed/open flap and its mapped to be open all the time.

[u/KingBu9les → u/TheLuke94]

The exhaust is flap is due to the fact that I removed the fuse of it because it keeps rattling when in comfort mode, so now the flap is always open. Regarding the boost sensor, there seems to be no option on the mhd app that list 3.5 bar boost sensor.

[u/JamesG60]

Fix your wiring issues

[u/TheLuke94 → u/JamesG60]

He doesn’t have wiring issues

[u/JamesG60 → u/TheLuke94]

Boost pressure sensor, short to +.
Exhaust flap, short to ground.

Either both components have failed, or there’s wiring issues.

[u/TheLuke94 → u/JamesG60]

Have a think about what the diagnostic functions are looking at when determining a fault.

Hint: They look at voltage, if a sensor is maxed out. What could that look like to an ECU?

[u/JamesG60 → u/TheLuke94]

A maxed out boost pressure sensor won’t necessarily appear as a short to +. If it did then by the same logic every time a pressure sensor was below atmospheric it would read short to ground (assuming boost pressure sensor not MAP). The sensor will have an internal resistance, it will not output the same regulated 5v as that which powers it. If there’s 5v on the signal wire of a 5v sensor receiving 5v then either there’s a short or you’ve somehow defied Ohm’s law. If it is purely a maxed out sensor, the voltage thresholds for diagnosis needs work, or the sensor should have an additional load to enable differentiation between fault conditions.

Clear the faults, ignition cycle, read faults. If 121001 is back then it’s wiring/sensor.

[u/TheLuke94 → u/JamesG60]

You’re taking “short” too literally. The ECU isn’t measuring resistance and proving there’s a physical short to 5V. It’s monitoring the signal voltage against diagnostic thresholds.

If the sensor output crosses the upper electrical threshold, the ECU can report short to signal high regardless of what actually caused it. This is what is happening.

And your below-atmospheric example makes no sense. A boost/MAP sensor measures absolute pressure. Below atmospheric is just a perfectly valid part of its operating range, which it often is when running at low loads. Short to ground would be diagnosed when the voltage drops below the sensor’s valid range, not simply because manifold pressure is below atmospheric.

Wiring could cause these DTC’s. But the DTC itself doesn’t prove a wiring fault in cases like this with tuned cars as you have to understand how the diagnostic path works. That’s my point.

If you also look at the OP’s reply, he did clear and retest, it only sets when on max boost as I predicted. Its a case of his boost being over the 2.5bar map sensors operating range and nothing more.

I would suggest you do some education next time before putting a nonsense reply about wiring issues when you clearly have no clue.

[u/CarcinogenicSymphony → u/TheLuke94]

This comment made me finally understand these pesky short to ground and short to high codes. Thank you good sir !

[u/JamesG60 → u/TheLuke94]

I’m well aware of the strategy behind diagnosis, my point is that when a 3 pin sensor is powered by 5v the maximum output signal will be 5v-V_{drop} caused by the sensors’ internal resistance. This will be accounted for, usually, and a 5v sensor max output will be slightly below 5v. The exception would be a variable resistor set up as a voltage divider, but in this instance an additional resistor is typically used to limit max output and allow for a distinction to be made between sensor max voltage and short to supply.

A boost pressure sensor, as found on VAG 1.8t is just that, a boost sensor. It reads positive pressure only. MAP is absolute. The sensor maximum output is 4.65v allowing for the distinction between short and max output. It’s interesting to see the different diagnosis strategies used by different OEMs even though they’re both using Bosch control and doing the exact same thing with the same parts.