Bypass Coolant to AOS/OVS Mod? Imre's Idea.

Has anyone tried Imre's idea of eliminating the coolant flow through the AOS/OVS? Is the purpose of having coolant flow through the AOS only for frigid climates or does the hot coolant serve another purpose in making the system work better? If the only purpose is for freezing weather, I would consider this because it would certainly eliminate the risk of intermix and coolant leak due to AOS problems. It would also eliminate the silly plastic coolant fitting that breaks so easy on the front of the engine.

I have the same question and haven't seen an answer anywhere yet. If this feature is strictly for cold climates, I would be inclined to bypass the AOS.

Coolant is for cold climates. In the case of BMW AOS for example, they use insulating foam around the tubes and such since they don't use coolant.

Sooooo, fill the AOS coolant lines with expanding foam and bypass the water line? LOL

A local indy shop that only works on 911s (of all years) told me that the AOS bypass is fine. But I didn't do it, I replaced my AOS.

I bought a new one as well but it seems this could prevent a future problem or at least eliminate that particular failure.

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Not sure it is for freezing weather only. The AOS wants to be kept hot so water vapor doesn't condense out to liquid when it comes in contact with the relatively cool AOS. Otherwise, the water vapor returns to liquid form and returns to the crankcase and the build up of water in the oil just accelerates. This could also apply to any unburned fuel, too. I note the 911 has coolant routed to the AOS while the Boxster (at least my 02) doesn't. But I also note the Boxster AOS is located on top of the engine which is itself located inside an engine compartment that isn't directly exposed to air flow from outside the car thus the Boxster AOS runs hotter. I suspect removing the coolant from the AOS would be a case of jumping from the frying pan into the fire. Or trading off the risk of an intermix from a failed AOS to that of a engine with a spun bearing or some other lubrication related issue arising from heavily contaminated and diluted oil.

All good points Macster. The 02 - 04 is mounted different and I wondered if the engine heat would keep it warm enough. Maybe just assume it has a valid purpose and risk the next leak. The original lasted 10 years.

Maybe in slow traffic on a warm day the engine and engine compartment heat is sufficient. Maybe. At least until the engine compartment fan comes on. But what about the cooler times? Or when the car is moving down the road. There's a lot of air flowing through the engine compartment to remove the heat. The air flows downward so the AOS is going to get cooled right down and stay cool.

Macster, I disagree. The engine will get plenty hot for the AOS to not cause condensation issue. I have no idea where you get these theories but coming up with them has to be time consuming as they are very convincing to those that don't have much experience with engines. I'm fairly certain that line is for extreme colds and short distance drives. I think its worth a shot bypassing it.

So you think Porsche put the coolant heated AOS there for looks? Geesh. You want to bypass the thing, run a non-heated AOS, have at it. Your car. Your engine. Your theory.

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You disagree with Macster on this point and in doing so disagree with the Porsche Engine designers. So you have more experience with engines than the Porsche Engineers? Then you go on to say that Macster is convincing to people or THOSE that don't have much experience with engines. YET you are only half convinced that you are correct with your reckoning when you say things like :- I am FAIRLY certain and I THINK it's worth a shot bypassing it. You my friend are all bologna. I don't THINK so I know so from your posting. Not that Macster would have told you so, as he is more a gentleman than I am. If you did this MOD, would you return it to the original as you pass on the car to the next owner. I know not. And some poor unaware buyer buys what could be an exploding time bomb especially if it moves from one part of the country to another with a different climate. If I ever had to buy another 996, I would not buy it from an owner who frequents this or any other Porsche Board. You scare me half ****less with your mods.

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You literally appear dumber every...single...time you post. stick with applying decals to your vehicle, because mechanics are simply not your calling.

Holy s#$%! QC, that was rather harsh. Hurdis starting to perform lots of DIY's which is awesome at any age and he makes interesting posts. I welcome and respect anyones opinion and can certainly see arguments for keeping the coolant through it or not. Is it common practice for the track guys to bypass the AOS? Seems like it would be less hoses/potential leaks to worry about while tracking. I have not decided which way to go with this. I was hoping others had tried it and could give opinions.

Different Pcar engine but I use a catch can between the AOS and the intake. This is a 99% track car and you have to manually drain the catch can. After short runs in cold weather, all that comes out of the catch can is water with a little oil mixed in. Hot weather, very little water and a little oil. My AOS is NOT water heated. From what I find the my catch can, I would agree that the water jacket is there to help speed up the process of vaporizing any water in the AOS in cold weather. This is a better unit (Motorsports Cup Air -AOS- Oil Separator) that was designed for racing to avoid hydro-locking an expensive race engine...a bit pricey $800. http://www.suncoastparts.com/product/9XXSEP.html

Awesome! Thank you. That answers the question for us street guys. The Macster strikes again (though I do see Imre's point for track use).

I hate it when the ' You think you know better than Porsche Engineers? ' card is played... Porsche Engineers design to many objectives, including a significant amount of regulatory requirements (noise, safety, emissions, etc). Not necessarily the 'best' way to do something, rather the best solution that meets all requirements including cost sensitivity. Engines would likely still be vented to the atmosphere if performance/simplicity/longevity were the primary goals. Heck, Porsche probably would have never gone water cooled in the first place if it wasn't for increasingly strict emission regs...

If I ever convert mine to track use (unlikely), many things under the hood will be gone.

There should be separate forums for 'Trackers' and 'street drivers'. Obviously two different breeds of Porsche drivers who should not be mixing.

Being a DIY doesn't mean you understand how and why things are done by the factory. You work on enough cars, not all Porsches, and you'll start to see certain patterns and then MAYBE you'll start to also understand how things work. Yes, as I wrote before, I beleive that coolant lime is there for extreme cold conditions.

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I realize this thread is quite old now, but I'll add my two cents- I'm in the process of replacing the AOS in my '04 Cab, and indeed the coolant hoses where there, but not connected to anything! The car was originally delivered to Florida first owner, and according to the carfax had not been replaced.....so was it delivered that way?

CarFax is not reliable for parts change, especially small parts ones. Do a search on this forum for recent comments on CarFax accuracy. I doubt it was "delivered that way..."

The odds of everyone with a 996 bothering to Carfax an AOS - especially this far along in the game - are long or nonexistent. Plus, Carfax is a scam. I have the thread to prove it.

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To answer as simply as possible, No. It is after all a 13 year old car that probably had dozens of non-dealer mechanics poking their grubby mitts into it. I always cringe when KK's old threads are bumped. Should the guy who bought his car also check for expanding foam in the AOS?

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This one seemed pretty complete, first owner mostly warranty and maintenance stuff at Porsche of Orlando, which included headlight switch and front crankcase oil seal replaced. Second owner used The Collection of Coral Gables, and listed such service items as brake fluid, tires and maintenance inspections. Granted, the AOS could have gone out and the owner decided to have it service somewhere else - which probably is the case. The bottom line is that they didn't hook these coolant lines back up, but I wonder where they are suppose to attach to , ie, what was done to plug them up at that end?

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I have no idea if the bypass would work but this cracks me up as well. Because they did such a fine job with the design of the M96 engine!

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Yep. Agree. Of course it gets plenty hot. In fact, I heard that the engineers at Porsche were just going to leave the AOS off, but Macster got in touch with them and started complaining. The engineers were �fairly certain� too. ;-) I ran my 996 for a while with the AOS lines cut and plugged. Did it bc of a leak in the lines or AOS. Here in the Kansai area of Japan the weather is @ lot lije mid-SOuth Carolina. Snow is generally only a dusting. Temps get low enough sonetimes to put ice skim on the dog�s water. But I replaced the libes and AOS. WHY? Because Porsche engineers seem to think it is a good idea. If I lived in Okinawa maybe I would have left it off, but here? Not chancing it.

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Well said

Another zombie thread has arisen!

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sorry to bring up an old thread! But I've been having a issue with a coolant leak, which from pics so far shared on FB people seem to think its AOS coolant supply link. I probably need to have my engine removed to have that replaced plus a bunch of other hoses given the car is a UK 2002 ( C4S manual) but need a temp fix. i have just very disconnected the supply No1 line to AOS and as stated the upper connector broke off. I've ordered a 5mm brass connector to pop in here. now as a temp solution to stop the leak can I cap this connector off ( as one side is threaded) and will blocking the supply and leaving the return feed as is , cause any problems? i dont want to break anything else and start messing around with the return line too. The car would potentially be driven in cold weather, slow speed short trips till fixed properly.

I have a coolant line to AOS by-pass kit that is for UAOS "only", but can be used as a temporary fix for AOS coolant leak ( or you can install the complete UAOS System). Can be found here for $25 www.ultimateaos.com scroll down to bottom of page.... It eliminates both AOS coolant lines entirely and caps the system at the origin of the coolant lines...( would need to be removed before cold weather usage or the UAOS System installed)

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thanks ill check that out. In super short term emergency, ie next few days, would be there any harm in just capping off the aos coolant supply line only? Leaving return as is ?

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If you have a leak at the AOS, capping just the supply line will not suffice ( it will still leak due to pressure from the return line)...

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thanks, I'm still getting my head around how this works. but if the feed line is blocked off how could the return line allow the aos to leak coolant?

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Answer :: " the path of least resistance"... Coolant/liquid will flow on the path of least resistance, if you cap,block,pinch the supply line, you have just created a total resistance to flow in that direction, but the pressure/flow is still on the return line, so the coolant will flow in the opposite direction to the AOS and leak because there is no resistance to flow in that direction... You MUST cap both lines, .....or by looping the lines around the AOS.... ie... connecting the supply line to the return line, thereby by-passing the AOS...

Many MR2 Turbo guys are running catch cans without any coolant flowing through them. They experience zero problems. Maybe one of our aftermarket vendors should sell a bypass hose, I would buy one.

Ooops deleted post.

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I tried to bypass the AOS coolant supply by making a loop of lines 12+13 in the pdf of the AOS supply system. i used a non return valve in the T connection to where number 12 would've plugged back in. So basically the line going up by the pulley (13) was connected to the number 12 that would've gone down into the Tee by the tank. however this join leaked abit due to size of the connector I had available despite the screw flange being very tight, and then seemingly another pipe , perhaps the end of number 20 going into the block, started leaking quite badly. This would be a drip with the coolant tank open or a full on leak when engine running. I'm kind of gutted as at least before I tried to disconnect the number 13 and broke the very weak connector, it wasn't leaking that bad. i think I may have to get the car recovered to the specialist at this point. My only hope of saving that is to connect everything back the original way and remove the loop, with a repaired connection for the coolant feed up into the AOS from 13 ^ where it snapped.

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Are you talking about dumping blow-by gasses to atmosphere, or just eliminating coolant to the AOS? There are several benefits to recirculating blow-by gases back through the engine for street cars.. It cools the combustion temperatures allowing for more ignition timing , it helps keep moisture out of the engine, it adds a little power and efficiency to the part throttle and cruise modes.....and it keeps the Feds from getting pissed off, they can get very ugly when pissed off.. If you are just talking about eliminating coolant to the AOS, on the 996/997, the factory flow regulator is located in a very cool place, and the moisture/water vapor in the blow-by gasses will form ice in the high velocity/low pressure area in the flow regulator and will block the port in cold weather operation... So the 996/997 factory AOS MUST have warm coolant flowing through to keep from Iceing, but with the UAOS System, the flow regulator is moved to a warm location near the throttle body and does not need the warm coolant, so it can be eliminated in the UAOS System and still used in cold weather operation...

I tried a temp fix today by making a loop of 12+13 coolant lines _ the AOS supply and return lines. However because the small connector by the central pulley was broken, i think it began to leak on the other side of that , quite badly. I blanked off the T with a non return valve and that was ok. i guess I'm gonna have to wait for the replacement small connector in the middle for the aos coolant pipe, then just connect it back the original way so i can atleast drive it to the garage. Getting fed up of this car ! Was just running ok, after a garage messed up my front wheel hub after not tightening it up after doing the front shocks.