TOPIC: Air Oil Separator for PCV/Breather hose re-route

On several Groove/PCV Re-Route jobs I've seen oil/crankcase condensate and water condensate moving thru the breather hose into air intake duct/TB, making a mess. I/We dont need that crap going into engines, plus this phenomena becomes visible due to the PCV hose re-route. Pre-Mod it was there also just not visible! But some might blame us/our mod for it. This crap is always being sucked from crankcase thru PCV valve into intake--BAD news Batman!

Here's a way to stop this. Install an AOS into the breather hose, after the PCV tee but before the air duct connection. A small collection drain bottle is added to bottom outlet of AOS to collect the liquids and can be emptied at oil changes easily.

The Euro OEM's put a hose from drain on bottom of AOS connected to dipstick tube to return separated oil to oil supply. I dont want to do this, as this mix contains oil, crankcase vapor condensate and water condensate duing cold/damp weather forming mildly corrosive mix.

Todays install used a small Jaguar AOS from Ebay, a homemade drain bottle and 5/8" heater hose. This engine in a '89 Nissan Maxima (Caveman Grooves posted this past July I think) has PCV mounted direct to intake/manifold vac. So PCV is capped, breather hoses from valve covers loop and join at tee. Tee then connects to intake port of AOS, the middle one. Top outlet AOS port then connects via more 5/8" heater hose to breather hose nipple on air intake duct.

AOS works by interrupting gas flow, flow hits a 45 degree angle wall before going up and out top port of AOS. this interruption causes entrained vapors/mist to separate out and run down into bottom drain port out to catch bottle I made. Catch bottle is 3"long 1" dia. pvcGray sched. 80 nipple w/ pvc female end caps screwed on w/ Teflon tape. A Nylon 3/8" hose barb w/ 1/4"NPT pipe tap threads is installed in one end thru drilled/tapped hole w/Teflon tape on threads. This cap I tightened firmly, the blank cap is tightened snug but still able to unscrew to empty catch bottle when needed. Simple--only cost like 6 bucks to make this catch bottle--I resorted to this after looking ALL over for suitable small bottle/container. The AOS itself-I found an Ebay auction for 2 of these for like 32 bucks incl. shipping. These AOS and similar may be available elsewhere (foreign or Euro auto parts suppliers/stores?) Other likely AOS part candidates are for BMW engines in '90's I think, they are black tapered tubes w/ 2 larger hose nipples up top and small drain nipple at bottom. Look for these on ebay (around 30 bucks) I just won one of these today.

The black tapered BMW ones use cyclonic action plus tangental gas flow entry to do the job. AOS becoming common on Euro. engines due to increasing use of direct injection. W/ direct Injection there is No fuel going thru intake anywhere so crud buildup is serious issue. But later model applications are co$tlier more complex and exact application designed so not suitable to our purpose IMHO. Far as I know w/ the Jaguar part on today's install and the BMW one I won today entry port is below top outlet port and drain is bottom small port.

So here's the pics, note the happy driver/Owner her Nissan runs Great now, passed smog inspection today w/ this installed no prob!

TracyG Gadgetman Reno

OK to add to this topic here is a guy on Youtube with a VW Jetta who installed a BMW cyclonic Air Oil Separator in his PCV line. This will give an idea of what I'm talking about. You can easily see how he installed the AOS, how the hoses were run, etc. This is the style of AOS I think could be good as a universal part for this mod. It looks to be pretty much available on Ebay, I have not tried to locate it through regular auto parts stores.

TracyG Gadgetman Reno

To further illustrate the BMW type Air Oil Separator, here are pics of mine that arrived today from an ebay seller. Got it for just under 30 bucks shipping was free.

This should help anyone interested in using an AOS to divert oil water and crankcase condensate from the re-directed PCV/Breather hose mod.

This type separator looks more available on Ebay right now there are at least 2 sellers w/ price within a buck of each other. Ought to be available through some auto parts stores, don't know which ones or price.

The other style gray Jaguar one I used on my freind's 89 Maxima might be less common; I don't see any more on Ebay now. Be aware there are several other styles of these out there for newer BMW and others but many are more complex and expensive, this looks to me to be a good universal style one, it's about 8.5" inches tall. Needs installed w/ the 2 larger 5/8"inch hose nipples UP small drain nipple DOWN, use whatever for a catch can/bottle.

It's important that the drain be connected to a closed airtight container to maintain system integrity.

TracyG Gadgetman Reno

OK today I emptied the drain bottles I made for my twin Air Oil Separators, on my '85 Subaru GL Wagon (w/ double Grooves).

I took a coke bottle and filled it 1 ounce at a time, marking 1 OZ. graduations. I unscrewed the bottom cap on each drain bottle, poured it into a clean soup can then thru a funnel into the coke bottle. I got a bit over 1 OZ., nearly all water condensate.

On the Subaru flat-4 engine there is a nipple on both valve covers-both are now breathers (driver side was PCV hose, PCV capped). Air cleaner has nipples for both hoses, so each breather hose goes to an AOS, then on to air cleaner. Driver side uses BMW type AOS, pass. side a Land-Rover type. Both work well, though the driver side BMW one got more water.

I'm now theorizing that using long hoses to hook up the AOS's is beneficial--in that the longer than stock hose allows more time and area for crud to collect/condense inside the hose. The driver side uses almost 8 ft. 5/8" ID heater hose, looping behind spare tire from AOS to air cleaner. NOt by choice, getting a good shot at the air cleaner nipple for this hose dictated this. I figure so what, not hard to spray out the hose w/ carb. clean once in awhile!

And now there is no more water/oil milkshake goo coming thru into the air cleaner! Engine seems a little happier, idle is a bit smoother.


I got this much slightly oily water condensate after about 150 miles of intown/hiway mix miles. Glad I thought of this...

Tracyg Gadgetman Reno

OK I was just at a gas station, and while checking my oil I opened both drain bottles and dumped 'em out. Didn't measure anything, but both drain bottles had a good amount of water in 'em. Bet I dumped out 2-3 ounces of water total.

This after emptying them both last time, with around 300 miles total since then.

Tell ya what--these AOS's Work!

Tracy G

A further update on my dual air/oil separator setup, and the PCV hose re-route on my Subaru.

I was seeing a lot of oil smoke in my exhaust and rearview mirror, from time to time this spring. So about 2 weeks ago I checked the metal water bottle homemade one, and the drain bottle on the BMW AOS. The metal water bottle one was totally full of water with oily stuff, the BMW drain bottle was filled with milkshake goo. So I dumped both out, and the exhaust smoke ceased shortly after. I keep the crankcase hoses hooked up in stock type config, with an restrictor in the PCV hose, in winter. This uses manifold vacuum to pull crankcase fumes thru both separators, to keep water condensate from building up in crankcase.

Then yesterday, I reconnected the PCV and breather hoses into the PCV re-route config. This makes a noticeable increase in throttle response and power.

We had a very wet winter and spring here, I wound up emptying both separators twice this season. I probably kept 20 OZ. of water out of the crankcase!

Tracy G

This is just a thread bump. But, about a month ago, I emptied out both AOS'ses on the Subaru, they were full. I re-did the PCV/Breather hoses back to the PCV Re-route config. w/ PCv valve vacuum source capped. And did an oil change to 15-40 oil. Since then, the oil smoke in the exhaust stopped, oil consumptoin has gone way down, and throttle response has improved.

I'd prefer to keep it like this, I'll have to see when fall rolls around, if colder wet weather results in water in the crankcase. Having the PCV system connected to the man. vac. source, with a hose restrictor, does pull water vapor out of the crankcase, w/ the 2 AOS rigs. But I found the drain bottles tend to fill up then allow water vapor and oil crankcase crud to be pulled into the intake manifold, resulting in exhaust smoke. But, this was result of such a wet winter, and the fact that the all aluminum Subaru engine acts as a water condenser, if driven in short commuting trips in winter.

In hot summer weather, the PCv re-route config. works great.

Tracy G