Engine swap troubles

Hey everyone!

Just wanted to share that I completed my first engine swap over the weekend, learned lots, but ran into issues at startup and was hoping someone with expertise can help me out.

I swapped in an engine block from a manual 2011 Mazda 3 (2.0L) into my 2013 (also 2.0, blown engine, auto trans). I was told by the yard and a couple of local mechanics the manual block and auto trans should work no problem.

Everything went well, removed the clutch and plate from the block and replaced with flex plate and torque converter from the auto trans. Note: I did not pre-fill the torque converter prior to install, I was told it is not a must. The engine started and runs super smooth, however, the car will not move. I noticed that the trans fluid level did not get any lower after turning on the car - this leads me to believe that the torque converter is not accepting fluid and is spinning empty - does this make sense?

I've tried:
- removed the trans again and made sure the torque converter is fully seated before reinstalling
- dropped the pan and replaced the trans filter as the fluid was VERY dirty and I thought it could be a clogged filter
- tested the solenoids, all within resistance specs

Questions:
- Does anyone have any experience with auto trans onto a manual block or vice versa?
- Could this be electrical related? the manual had different sensors and it is confusing the ECU?
- Does the car not moving sound like a symptom of an empty torque converter?


Any advice or suggestions are greatly appreciated!

your should check the https://www.mazda3revolution.com/
as well as i have seen many posts on swapping engines

Thank you, will post it on there as well

Not pre-filling the torque converter can cause issues. While some say it’s not necessary, it’s better to fill it beforehand to avoid running it dry. If the converter isn’t taking fluid, that could definitely explain why the car won’t move—basically, the transmission has nothing to engage with.

Since you’ve already confirmed the torque converter is fully seated (which is a common issue), next step would be to double-check the pump in the transmission. If the converter wasn’t seated properly on first install, it’s possible the pump got damaged. That would explain the lack of fluid circulation.

As for the electrical side—yeah, it’s possible the sensors are tripping the ECU, especially since the manual block might have a different setup for things like crankshaft position or input/output speed sensors. The ECU could be getting confused, thinking something’s off and not letting the transmission engage.

It could also be worth manually priming the torque converter by pulling it off and adding fluid directly, then reinstalling. Also, maybe recheck the fluid level while cycling through gears with the engine running—sometimes it just takes a little bit to work air out of the system.

If all else fails, scanning for codes with a solid diagnostic tool might give you more insight into whether the ECU is causing the issue.