Car Idles fine, but dies when RPMS lower after accelerating

[Musij]

So I just finished dropping in a working engine on my 318is.
The car Idles beautifully, but after giving it gas and raising it above 3.5k RPM, the car dies as the RPMs lower. I have a check engine and at first it read 1215 so I cleaned the AFM and now I get 1222. Could it really be the 02 sensor? I changed it right before my previous engine died, so it has at most 100 miles on it. I also have a little bit of a oil coming out of my PCV.  Not continuous drippage. Could that be related to the fact that the car isn't running well because something is throwing the AFM off?

[monty23psk]

I would check for an intake leak. If you have unwanted air going in on the suction/release during deceleration, you can have the stumble.

[Nick_318is]

How are your vacuum hoses? Any leaks?

[Musij]

How are your vacuum hoses? Any leaks?

So i haven't chased the hoses yet, my mechanic called while he did the swap and let me know that the old engine he pulled had the mess under intake mod and the new one he was dropping in didn't. So I told him to go ahead and transfer over the new hoses and mod to the new one. I bet he didn't follow a chart and misplaced a hose or two. I'm going to try to to go Sunday and chase the hoses using the chart and see if I find a misplaced hose. Considering the replys I'm 99% sure it's that.

[Musij]

Figured it out! Two hoses were misconnected. Hopefully that corrects the oil coming into the intake. I'll be keeping a close eye on that but at least it now runs beautifully

[monty23psk]

great news. I would remove the intake elbow and clean it if it has any residue of oil. Simple green or any non harsh degreaser will work. Check the throttle place and MAF as well but be careful with the degreaser in those parts.

[Bekverdy30]

This happened to me after I did the mess under the intake. The car sounded cammed so I thought I had damaged the AFM when I took it out. Got a replacement and it fixed the idle but it would die right after I would come to a stop. Turns out the intake manifold gaskets had a crack and once I replaced that, everything was back to normal. Do a smoke test to see if you have air leaks. That's what is probably causing the issue.