TPS and/or ICV problem

[KanistaZ@xxxxxxx]

I need some help diagnosing an idle quality issue I'm experiencing.  

Last summer I transplanted an engine from a '89 635i into my '87 535is.  I've 
been driving the car with the throttle cable adjusted super tight in order to 
bypass an idle issue that came up originally, and that I couldn't get worked 
out.  While the throttle position switch was being bypassed at that point, the 
extra tightness of the throttle cable kept the idle up at an acceptable level.

The symptoms I'm experiencing are that at idle, the RPMs fall very low 
(around 400-500 RPMs) in a cyclical manner, about every 7-8 seconds, then they leap 
up to 1000, then settle in at a normal ~700 for a minute or two, only to go 
through the cycle again.  This consistently kills the engine with the AC on, so 
it's become a real pain this summer.  

In an effort to get to the root of the problem, I have now adjusted the 
throttle cable correctly, and want to figure out what's keeping the system from 
working correctly.
So, I have adjusted the throttle cable to allow the TPS to give it's 'click', 
and sent the idle signal along.  I've also checked it for continuity, and it 
sends the correct signals at idle, and at wide open throttle, so it appears to 
be operating correctly.  My only doubt regarding the TPS is that it is the 
one that was on the car originally, which was set up for an automatic 
transmission, and my car is a 5-speed.  (Is there any need to switch to the unit 
designed for a 5-speed?).

I've also checked the functioning of the Idle Control Valve by installing a 
brand new unit.  However, there was no change in idle quality (or lack of idle 
quality!) with the new unit.  So my assumption is that the ICV is not getting 
inputs somehow from the TPS.  This may be a wiring issue.  It depends if there 
are connections that go through the fuse box, which is where I had to splice 
in the newer (round) connector into the older (rectangular) connection.  If 
the signals go directly to the ECU and back out to the ICV, then I'm not sure 
what's up, unless it's the earlier issue of 'there being a different TPS for 
automatic' vs. '5-speed'.

There is also another possibility, and that is that I have the AFM adjusted 
incorrectly.  I've got a replacement on the way, but don't expect it to be 
affecting the idle as described above.

Could it be either of the above, or some other issue?  Any help would be 
greatly appreciated.

Ken Mitchell
Durham, NC

BTW, other than this idle issue, I'm very pleased with the performance of the 
M30-b35 engine in the older body!
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