This post is only to pass along what I learned about the (complicated!) 2003 T6 PCV system, because I had difficulty assembling this information from various forums and the Volvo parts system available on some dealer websites, as well as inspection and experience on my own vehicle. Much of this must applies to other model years, and probably also to the XC90. Probably most (but not all) of this info can be found somewhere else, but it seems worthwhile to collect it in one place. If others care to correct my errors and add to it, that would be great.

Crankcase vacuum
As the turbo bearings are lubricated by circulating engine oil (and cooled by circulating coolant), many people have noted that it's especially important to avoid PCV clogging severe enough to raise the crankcase pressure (during boost) to the point that those bearing seals fail, leading to an unholy mess in the air system and expensive turbo repairs. This must be the reason why Volvo went to such lengths to keep the PCV hoses and banjo bolts warm to avoid condensation of oil sludge. In particular, both supply and return coolant lines nestle on either side of the longish PCV hose in a harness (Volvo 30731004 aka 30622348) running from a banjo bolt (Volvo 31325709 including check valve) in the (RH) underside of the intake manifold, past a Tee on the top of the nearby oil trap (Volvo 9497454 in my case, but 8670306 for other serial numbers), then all the way left and back to a nipple on the intake pipe of the LH turbo, near the firewall.

My reason for reviewing these parts is to point out that they imply that it allows only a hardly measurable crankcase vacuum at idle, contrary to what you may read elsewhere in various places. (Non-turbo crankcases probably do have traditional vacuum at idle.) The reason is that the only pressure drop from outside air to the turbo intake, and hence through the hose harness to the top of the PCV oil trap is due to (slow) idle air flow through the air filter. The limited suction through that banjo bolt orifice can't produce much pressure drop through the PCV hose harness, unless that harness is clogged. Hence, significant crankcase vacuum at idle (measured at the dip-stick) may be evidence for clogging in that part of the PCV system, opposite to non-turbo engines! On the other hand, excessive positive pressure in the crankcase is always cause for concern. On the T6, boost pressure in the intake manifold closes that Banjo bolt valve (if you have one), so positive crankcase pressure is limited only by venting into the LH turbo intake. (I suppose during boost, this may be assisted by increased pressure drop across the air filter impedance at high flow.) So that coolant warmth in the PCV harness defers condensation of sludge until it gets dumped into the left-side boost system, which is too big to clog.

There is an amusing consequence of this arrangement. It can be convenient to be able to run the engine while the boost pipes are removed from the top of the engine, during maintenance work. (You may have to unplug the MAF cable at the sensor.) When you do this, you may hear an alarming rattling noise coming from the open pipes on the left turbo. Don't panic — this is coming through that PCV hose from the oil trap, presumably the sound of PCV air flow bubbling through oil in the trap. It seems to be amplified by the big air pipe geometry, because it stops coming from there when you disconnect the PCV hose from that nipple, and it's not nearly so loud coming directly from the small PCV hose.

Question 1: Is that PCV trap cleverly designed to bubble PCV flow through a bit of oil in there to better remove clogging vapour? Or does that gurgling sound suggest that I have a blocked oil drain from the trap?

Question 2: If someone could explain why the PCV trap has 3 ports connecting to the engine block, that would be interesting. One is for sucking, one is for draining, and…? Maybe separate sucking on the valve cover and crankcase? Or an emergency drain for when the drain to the sump gets clogged? Or controlling the level of oil in that trap bubbler?

Air leaks in boost air pipes
If large enough air leaks develop in the boost air pipes, a trouble code is generated by the discrepancy between the MAF and the oxygen level measured in the exhaust stream. But even smaller leaks are presumably detrimental to performance, and this forum may be swarming with guys who might lose some sleep over that. My limited experience suggests that a common cause of such air leaks may be cracks from stress on the quite rigid pipe system due to faulty engine mounts or repair procedures that disturb the pipes or move the engine on the mounts. I found several cracks at the big hose clamps. The biggest was at the clamp on the throttle assembly. That rigid pipe (Volvo 30645294 in my case) running from the intercooler has a bit of flexibility from a couple of single-leaf bellows, but any engine movement will stress it at the throttle. When I got such a trouble code, I found a big crack there, probably due to failure of the upper engine mount. Smaller cracks at the clamps on upper pipes I "fixed" for a while by putting a couple layers of electrical tape under the clamps.

My main purpose here is to mention that I found it much easier to find even small boost-air leaks by pressuring the boost system with the engine stopped, using an air source of high flow at low pressure that is far enough away to reduce its noise, and allow hearing leaks. Positive pressure allows you to also localize big leaks with your fingers. I ran a (dry!!) ¾" garden hose from the exhaust of a (residential built-in) vacuum cleaner to that PCV nipple on the left turbo inlet. (By removing the PCV hose from that nipple, I avoided pressurizing the crankcase.) Of course, I also had to disconnect the rubber bellows from the air cleaner outlet and plug it (with the lid from a jam jar clamped inside the bellows end). If your earlier model year doesn't have a check valve in that banjo bolt in the underside of the intake manifold, then you need to plug that too. (By the way, the thread of that hole is M12×1.5, so you could use such a bolt + O-ring, or any BMW oil drain plug, it turns out.) If you do have that check valve, then using positive test pressure means you don't have to worry about it, but with suction, you would. Anyway, hearing leaks this way was a breeze(!), especially using a length of hose to my ear as a stethoscope. (Results may vary.)