Second thoughts on changing AOS
 

I am getting ready to take my first road trip in the box. This will be through the Mojave desert where authorized Porsche service centers will be non existent. Bubba's service station is about as good as it gets! My car is a 98 w/92k on the clock. Since it had the original water pump that I recently changed out and with no indication the AOS has been changed in 18 years, I ordered a new Genuine Porsche unit from Pelican to replace this week. However reading Ben006 thread with JFPinPA and Jake Raby stating new units failing all the time, I am reconsidering. Sometimes TMI is TMI. I am certainly not afraid of the work but I do not want to change working fine old for possibly failing new. I also know the "if working fine why mess with it" theory. I would like to change but now not sure. This is a Genuine Porsche AOS, any suggestions to reduce my anxiety??? :barf:Thanks

If the unit is bad, it will be bad right out of the box. I would not be afraid to install it, but test it afterwards (we use a digital manometer, but you can rig up one from a yardstick and some plastic tubing). On a cold start, you should see 4-7 inches of water vacuum, which should level out to 5 inches or water as the car warms up.

We test every car that comes through the shop, regardless of why it is here, and we automatically replace the AOS on any car that we pull the trans out of for whatever reason, particularly 996's.

There isn't a whole lot of rhyme or reason why these things are what they are, but I would bet on a new unit over one with nearly 100K on it.

Thanks JFP for the quick response. Your last sentence is exactly why I am considering changing it! Should have everything here by Thursday to change out. Thanks again.

Monty

Not to derail, but do these typically fail due to mileage or age?

Neither. From everything I've read, they can be bad right out of the box or they can last forever. They fail when they fail and there's really no rhyme or reason why.

The usual item that fails is the diaphragm . It splits and is not accessible for diy replacement.
The cause is probably heat cycles/duration. But overfilling with oil or some other way of 'swamping 'the diaphragm is alleged to harm it.
I read of some in no-frost areas,disconnect and bypass the coolant connections on the AOS.

Atmospheric pressure has a lot to do with the AOS failures. If you live near the coasts these units will fail more often than Boxsters living at higher altitudes. Jake Raby said he has never seen a failed unit from Colorado. I have replaced mine four times and the Jӓgermobile is driven from 20 feet to 120 feet above sea level every day.

911monty - I had exact same thoughts 2 summers ago. I was planning a 2000 mile road trip. I bought a new AOS to carry as a spare. I thought the whole drive - how would I change this thing on the side of the road with an engine that has been running for hours.
My Original AOS had 75k+ miles on it at the time
I took 2 more 2000 mile road trips last year with the original and the new as a spare.
This year I bit the bullet and put in the new one. I haven't done much driving yet so the jury is out.
If I do take any Road trips, I'll take my old one as the spare. At least I know how to change it

Here's an example off what usually fails with age. Notice the diaphragm split. http://986forum.com/forums/uploads02...1462373422.jpg

HAHA! I can relate to those thoughts. I get them even close to home at my mileage. I don't know if you've ever driven through the Mojave desert, but it's a bit like the surface of the moon in appearance and the temperature of Hades. I'm pretty sure the lizards would jump on the hot muffler to cool their feet off!:D No place to be broke down for sure, so I don't want to even be thinking of the AOS out there.

+ 1 about the Mojave Desert

911monty,
I had been at Willow Springs with the POC in early September, and your biggest concern is to keep yourself hydrated, something like survival mode, (literally)... LOL

After the first weekend there, I went ahead and installed the third (center) radiator

@ 911monty

There is a saying: never change a running system. When it fails you can fix it. Why will you fix it before it fails. I wouldn't do that, because it's not an easy fix and if it fails there won't be any big harm to the engine. Just relax and enjoy your car.

Regards, Markus

Gilles; Yeah it really is no joke out there.

Funny (or not) you mention the radiator. I have had both radiators off and cleaned. Fans working well. New WP and LN low temp stat and still my temps swing from just a tick below the 180 mark to the right edge of the 0, 103 deg C or 217 f. indicated on the climate control at 70 to 80 deg f ambient when working the engine in the hills and twisties rowing 2nd to 3rd. I am considering upgrading stock rads to the CSF rads. This way I replace the 18 YO rads without all the mods for the center rad which should end up about cost neutral. This trip is kinda the litmus test.

Thanks Marcus; This may be TMI but there was never a saying more true when it came to changing a perfect condenser while changing points in your distributor! Ahh the memories :)

However in this case I can only imagine my old brittle AOS diaphragm is being held together by the baked on oil fumes coating it! I would much rather change out in the comfort of home with plenty of micro brews than on the moon. ;)

I overfilled my 2000S. It was fine until I tracked the car. AOS failed. Common failure when driving on a track with too much oil.

+ 1

I am getting too old to perform heavy, unplanned emergency repairs on the side of the road… :p

Update; Changed AOS today. It was the previous version ending 03, so it was most likely changed at some point. This probably explains the crooked leaky clamp on the oil fill pipe connection. Also found the lower tube quick release connection broken on 1 side, I did change that o ring but may change the lower tube if it gives me a problem.

So this is definitely not a job I'd want to do on the side of the road. Originally tried on ramps, but removing rear tire made it doable albeit with a lot of up and down.

Idle was low and died 3 times on startup, revved on the 4th start and settled into normal idle. No Smoke! :) Drove about 40 mins and all seems well. :dance: