Hello new guy here i work at a independent shop and im not as custom with newer mazdas as I am with chevrolets "4 year dealership experience" installed a new cylinder head gasket on a 2018 mazda 6 2.5l non turbo engine with cylinder deactivation head was replaced due to being cracked and squirting oil on the exhaust after replacement started vehicle and the vehicle has a extremely rough idle just like it has a missfire hooked scan tool up to it and vehicle has no dtcs and under missfire data there is none figured maybe timing was off by a tooth and after pulling crank pulley and valve cover verified timing is where it should be checked all my engine bay grounds and all is where it should be under load vehicle does great its just at idle I am really stumped now due to there being no dtcs and no real starting point to solve the issue ill take all the advice yall can give

 

Welcome to the forum..

 

When Ypu scanned the ECU hopefully using a Diagnostic scanner directly connected to the OBDII Diagnostic connector did you look in the permanent DTC as well the I/M monitors. Based on your info you should find more information. See what you find and post back .
I changed my head on my Mazda 3 , 2018 2.5 with less then 15k miles from its new life from Mazda when I purchased with only 3 miles on the odometer and was very careful removing all the electrical connectors , sensors and any parts needing be moved to RnR the head. To add to the slightly different service I installed 2 prototype performance cams and some other upgrades in parts and casting configuration. The engine fired like nothing was done and ran fine on the first try. So my thinking is something in your project was over looked or damaged in work. The ECU should have some clues .
I did have to do some ECU Calabration changes to take advantage of my Head RnR but your service would not and should not have needed or required that.

 

Have a good look for possible air leaks with the intake ducting/manifold area.

 

That's a great idea but the only thing he should have had his check engine light activated and found 1 or more DTC in the ECU caused from a lean condition from the intake leak. The ECU is very sensitive to those.

 

Hopefully that would be the case. Different makes different priority with logging a code. Some will be very fast and others take some time to register. A competent person that knows what data should be on the specific car should be able to read the info from a scan tool. Our beloved Holden could take up to 6 months to register a code with a lazy O2 sensor and Dumbo's would replace multiple parts because they couldn't read scan tool data.

 

Checked scan tool data and no dtcs at all even pending or memory performed a compression check and cylinder 1 has no compression checked scan tool data with a snap on scan tool and also a think car and there was no missfire at all on live data on either scan tool performed a cylinder pressure leak down test and intake valve is opening but exhaust is not (can't hear air leave exhaust) customer came in and decided to have insurance total vehicle due to not having a issue until his wife was driving and hit a pothole we think when she hit the pothole exhaust hit and caused the head to Crack leading to a head replacement so either cylinder 1 cylinder deactivation lifter had collapsed or it was a faulty cylinder head from mazda still find it crazy that it's only running on 3 cylinders and there was no missfire or cel at throttle engine runs smooth but unfortunately due to customer taking the vehicle I can't give a fix

 

Interesting, if you disconnect a single coil while rung you get min. 1 DTC. If you disconnect 3 coils the engine will run on one cyld and you get several DTC. lol.
I only know because I did this out of curiosity to my 2018 Mazda 3. Back in the day ….a long long time ago pre dating even electronic ignitions we would have bets who had the best adjusted carburetor with our cars aka Hot Rod or muscle car depending how far pre 1985 you want to go… lol , how many plug wires you could pull on a V8 and it would still idle ROLMAO
The trick is leave 1 and 8 on a Chevy connected and it will stay running on just 2 cylinders ,at least long enough to win the challenge. 😂

Anyway it is very strange the ECU on your Mazda showed nothing???????
Snap-on scanners don’t lie… lol I own a Solus with $$$$ invested in it and all the upgrades.