BMW Repair: Off Idle Response, intake hoses, vacuum lines

Question
QUESTION: Hi Evan   I recently bought a 2001 325ic (convertible) with less than 30k miles. It is a manual, and the last VIN is JY41736 . It has an "off idle" problem. It starts easily, idles smoothly, but when I take off in first (or reverse) it hesitates and surges unless I keep the rpm over 1500.  As a test, I put the trans in neutral, slightly increased and held the throttle and watched the tach.  It went from 600 to 1100 quickly, then slowly back down to 700 then back to about 1100 then back down etc.  It was not running roughly during the hunting exercise..seemed like I was easing off the throttle and then accelerating again (but I wasn't).  It would take about 4 seconds to go from 1100 to 700 and back to 1100.  So far, I have changed out the air filter, and cleaned the MAF and there might be a small improvement, but not much. The problem starts to show within 2minutes of starting, and seems unaffected by A/C loading.  No engine codes show on the OBD2.  Above 1500 rpm it runs like a champ.  What do you think?  Thanks Les

ANSWER: Les, first idea is unmetered air leak. Use a smoke generating machine to slightly pressurize the intake track with smoke with the mass airflow removed and duct plugged off. use the vacuum port that has the two vacuum lines in the main duct that the mass air clamped to to pressurize. Look for smoke leaks. Especially at the duct off shute from the main duct that branches off and goes to the idle valve located just above the throttle body. Faulty MAF is possible, but don't generally see off idle throttle response suffer when they fail. More likely a vacuum/intake leak. Idle valve sticking. Throttle valve sticking or faulty potentiometers in the throttle (usually accociated with check engine light). Tell you what I would do, smoke test it, remove throttle and idle valve. Clean both with solvent or spray throttle body cleaner/brake cleaner. Replace any suspect gaskets or cracked intake hoses. Reinstall and recheck for proper operation.
-Evan

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QUESTION: Hey Evan  Thank you.  I understand, but don't have access to a smoke generator. (I can pressurize and spray soap bubbles..think that will work?).  Alternatively, I could run the engine and spray WD40 around the suspected joints (while the engine is off idle..to see if it smooths out).  Hate to be a PITA but.. where are the potentiometers located, and how difficult is it to get access and check them out?  (I have some basic test equipment.DVM, Scope etc.. and Im supposed to be an EE). Can you recomend a shop manual or online source which clearly shows how to get access to and clean the IAC and throttle valve?  It looks like a nightmare to me to remove the ducting.  Thanks Les

Answer
Leslie, anything combustible will work for an engine running leak test like you suggested. We use this test also, but a smoke machine is far superior for system leaks. There are two potentiometer in the Throttle body. One goes from 0-2.5Volts, The Other Goes from 0-4.5volts. The potentiometers don't usually fail, but it is something you could check. The Air box, Mass Airflow meter, and ducting need to come out to remove Throttle and Idle valve. The ducting is held together with 6mm hose clamps, they are hard to get to if they are positioned face down.
You also may need a T-40 torx bit to remove the Anchor over the idle valve. Bentley manuals are probably the best,
Chilton and Haynes are okay aswell.
-Evan