<p>I have a Tevo Tornado that I've outfitted with an official BL-Touch auto level sensor. I can see the bed probing run, and I can see the Z axis slowly adjust during x/y moves, so it's doing <em>something</em>. However, you can see that there appears to be a systematic tilt:</p>

<p><a href="https://i.stack.imgur.com/sacY6.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/sacY6.jpg" alt="BL-Touch tilted bed plane"></a></p>

<p>Any ideas what could be causing this? The bed, gantry and print head is tight, no wobble. Here's my start code:</p>

<pre><code>G28 ; home all axes
G29
G1 Z5 F5000 ; lift nozzle
</code></pre>

<p>I have mesh leveling enabled with a 5x5 grid and correct probe offsets. The bed itself is on PETG printed standoffs instead of springs to eliminate any jitter.</p>

The problem with the Tevo Tornado design is that the design is as such that the Z-axis is powered by a single stepper (under the assumption that you have not added a second Z stepper).
This means that the level relies on the rigidity of the X-axis assembly, more specific the play on the guide wheels. Apparently the BL-Touch level determination suffers (the up and down movement while probing) from this design as can be seen from the tilt around the Y-axis direction. This effect causing the tilted level plane is called
hysteresis
. Now that the Z-axis moves up and down you experience much more problems than using a mechanical switch. The effect is more pronounced when the mass of the hot end carriage is furthest from the Z-axis lead screw.
Note that an extra stepper can also cause tilted level when the second stepper does not move in sync (e.g. missing steps). In such designs, a single stepper (geared) belt driven 2 lead screw has better performance in that respect.