I have a bit of an updated home-made procedure on what to look for - and where to look for vacuum leaks on the M54 engine... this is based on my experiences with my 2000 E46 330i Manual, but may be of help for other M54 engined cars.
Parts required are:
- 3.3mm Vacuum hose, part # 11657803732 (order a metre of it)
- 7mm Cap for an unused vacuum port, part # 11611727176
- 3.5mm Cap IF you don't have the air pump (see below), part # 11611437560 (you may need a couple of these, see below).
Tools required are:
- Torx sockets (size unknown, sorry), 10mm socket, screwdrivers & a penknife for cutting stubborn old vacuum pipes off.
First thing to do is to remove the cabin air intake tray - twist the three fasteners to remove the pollen filter cover, then remove the pollen filter, then undo the 4 torx headed bolts holding the intake tray in place. At the front of the tray, you will see a cable channel - unclip the cover (it just pops off) and take the 2 electrical cables out of there. Then just lift the air intake up & forwards to remove.
Next go to the passenger side of the engine compartment; the rear compartment houses the DSC unit on my car (the did change later in 2000). the side wall facing the engine is plastic; undo the 2 fasteners, slide the cables out of the top of it, then remove the panel completely. It's a tight fit, but just yank it up.
Next, underneath the air intake tray you just removed, there will be 2 torx screws holding a black panel in place - remove them, and then remove the panel itself (gives you a view of the interior ventilation fan - and more importantly allows you access to the rear of the engine). You need this removed to see / feel the vacuum hoses at the back of the engine.
Then remove the ridged bolt covers to the fuel rail cover on the top of the engine, and remove the 2 10mm bolts underneath. Take the cover off.
Now you have a reasonable view of the top, rear & rear passenger side of the engine - and the engine will still run in this condition.
You could, at this stage, start the engine & listen really closely to listen for hissing, but the engine is a noisy thing anyhow, a little hiss is normal. You could also try the carb cleaner trick, spraying it on the vacuum hoses, but this has never worked for me...
If you have the air pump thing on the drivers side of the engine, follow the output pipe from this. A larger pipe goes to a valve on the drivers side of the engine (part 1 on the diagram below); then it enters a 3.3mm sleeve pipe for about 100mm before joining a harder smaller pipe which leads round the back of the engine before joining to another 3.3mm pipe which joins to the electric valve, No.5 on the diagram below. The 3.3mm pipe sections mentioned above are prone to splitting, so replace them one at a time.
From the electric valve, there is a vacuum pipe that connects the electric valve mentioned above to the inlet manifold, and has a one way valve in it (part 6) as the pump blows air into the engine. Replace the hoses in the same way as above.
Not all M54 engines have this air pump (it pumps air into the inlet mainfold to make for cleaner cold starts); if you don't have it, you will have a small blanking cap at the back of the manifold. Replace the blanking cap (3.5mm one) as they perish / split over time. In theory, this system is one-way (valve is No. 6 on the diagram) but replacing the pipes is no bad thing anyhow.
Beside the above connection / blanking cap, there will be a bigger vacuum cap (7mm) - replace this too, as they're even worse for splitting / perishing.
Next, there's a 3.3mm vacuum line going to the vacuum reservoir located very awkwardly next to the starter motor. This vacuum reservoir provides a constant vacuum (if required) for the exhaust flap IF you have one. Mine is disconnected, so I stripped out this hose & plugged the port on the inlet mainfold. However, if you want to keep the thing in operation, replace this hose too with the 3.3mm vacuum hose. If you don't have the vacuum reservoir / flap, replace the 3.5mm plug like for like (part no. 11611437560).
You should be able to complete the above jobs in under 2 hours including removal of parts to gain access.
The last tip I'll give you may prove a winner....
Under the inlet manifold, tucked away above the throttle body, is the oil separator valve (cyclone, CCV, whatever you want to call it). On older models, this had a vacuum hose attached to it, the other end of which was connected to the fuel pressure regulator on the fuel rail at the top front of the engine. This diagram shows the valve, the vacuum hose to the fuel pressure regulator being part # 6:
The M54 engine saw the fuel pressure regulator change positions, and it was integrated in the fuel filter underneath the car. As BMW didn't want to develop a whole new oil separator valve, they simply capped the redundant vacuum port with a blanking cap. So part 6 on the M54 cars is a blanking cap, which again can split, and should be replaced where possible (difficult access ) by part # 11611437560. Real OEM & dealers are unclear on this, so trust me
Under the fuel rail cover, which you've removed, inspect the pipes which lead from the oil seperator valve into the inlet manifold - the long pipe running parallel to the fuel rail is notorious for splitting - it's lagged to protect it, but they go brittle over time & split. Listen closely in this area with the engine running This pipe should be replaced as part of the oil seperator valve renewal anyhow (part 7 on the oil seperator valve diagram above), but can be replaced on its own easily enough.
If you still fear you have a vacuum leak, the remaining components to test / replace are the main inlet pipes leading from the air flow meter to the throttle body & idle control valve, which may split, particularly on the bellows between the main pipe & the connection to the idle control valve. In addition, the main vacuum hose which connects from the intake manifold to the brake servo should be inspected. If in doubt about the integrity of any vacuum hose, swap it over for a new one.
I hope the above information is of help Sorry it’s a bit picture-light, but I hope it explains the theory behind the vacuum system, and might help you pinpoint that annoying leak, which is generally most prevalent at low revs (revs bouncing under load).