BMW Repair: E30 2.5L stalling at idle, spark plug gap, vacuum leaks

Question
I've just made a swap on a 1991 318is.  Complete interior wiring has been replaced by a 1991 325is wiring.  Engine is a 1987 2.5L. The engine starts well, run well over 2000rpm+ but stall at idle. I've replaced every vacuum hose (in fact the engine has been completely rebuilded) checked spark plug gap, tested the MAF, replaced the IAC and nothing seems to work. I'm out of idea now...

*1991 318IS
*1991 325IS interior wiring
*1987 engine/tranny/MCU/engine wiring

Answer
I think you might want to recheck your engine for vacuum leaks.Check the intake making sure you clean the old gaskets off. I have a 525i that I swap an engine over,I thought all was well until I tried to start it.Unfortnately it did the same as you stated.I searched and searched I relooked it over and that is what I found the gaskets were not completely cleaned off the intake manifold.When I put the new gaskets on,the head,I hurried and didn't look at the manifold to make sure it was clean.