Weird throttle behavior: swapped 97 3-series

So we’re building a Lemons car and having some strange behavior after the swap. The car is a 97 318i and we put a 2.8 six (M52) in it from a 98 528. 

Here’s my teammate’s description:

“When driving in gear and I throttle. It sticks at that rpm. So letting off the gas is like cruise control. When I let off the gas the engine doesn't do braking and rpms don't drop. If I come to a red light and stop after about 30 seconds the it will start to idle slower. I can also cheat it by letting the car come to a complete stop and feathering the clutch into first gear. This seams to bring the rpms back in line.”

So... idler valve maybe? Recheck the wiring harness? Break out the hammer and talk sternly to it?

throttle cable clean and not kinked or otherwise bound up?

Is the cruise hooked up? maybe it is getting an erroneous signal to 'set'. I'd unhook the cruise cable from the TB if it is hooked up.

I don’t think the cable is linked, but we’ll double check. I didn’t consider the cruise and that should be easy to test. 

Take a look at the ICV and the throttle body. Make sure the throttle plate is moving freely and that the spring is returning it fully and firmly. Everything should be scrupulously clean; you may need to flush the ICV repeatedly to get it so. Finally, these motors are notorious for vacuum leaks in the intake plumbing, which is not likely the cause of your problem, but which may be making it worse and/or harder to pinpoint, so double-check that everything is sealing up properly and that the elbows aren't ruptured (look at the outside of the accordion sections, both throttle body and ICV).

Second vote for ICV malfunction, if the symptoms are showing up in regular traffic.  If it's happening at 6K RPM, there's no way the ICV passes that much air.

The budget aftermarket ICVs for these cars (and the E30s) are prone to flaking out fast.  I always prefer to clean out the stock unit, or buy a used stock unit and clean that up if mine is totally toasted.

RPM ramp-down on throttle lift is also the subject of certain settings in the Motronic ECU, but I doubt you have been tweaking those, right?

Update:  thanks everyone! I passed your suggestions on to the team.  It was was much appreciated and they looked at a few of the things mentioned.  That said, it ended up being a faulty signal from the TPS, plus some other vacuum leaks (of course).