rhkwon

AOS DIY and Top End Refresh

1999 Porsche 996 Cabriolet C4 Manual Transmission

Before I get into the nitty gritty of the actual AOS replacement, let me give you some background on my car. I bought this car about a year ago from California with about 127,000 miles on it and it now has 133,000 miles on it. Car drove perfectly all the way to Texas but we all know that perfection never lasts. After a couple of days, the code for a faulty spark plug coil pack came on and the engine ran rough. So, I changed all 6 coils with the updated 997 Beru coils and new Bosch spark plugs. I also changed the motor mounts. Ran great for a few thousand miles. I noticed every once in a while, that the temperature gauge would fluctuate up just a tad but nothing to worry about. Then last September my daughter stole my car while I was away on vacation in Cancun with the wife (Risky Business) and drove it to Tyler TX and the lower 3-way splitter hose failed. So, got the car towed to a pretty good European mechanic and he replaced the splitter hose and the coolant hard line pipe and flushed the coolant. After a couple of months of enjoyable and care free driving I noticed the smell of coolant and find out that the top of the coolant expansion tank is leaking. I figured it was the bleed valve and so I tightened the bleed valve bolts (after much cursing to get to the 2 bolts covered by the side of the engine). This did not work so decided to go cheap and replace with a Uro brand bleeder valve. The bleeder valve would not seal correctly so I decided what the hell, rather than chance it and try to buy an OEM bleeder valve and then the real cause being a cracked expansion tank, why not replace the entire cooling system? I bought every single coolant hose from front to back, a new water pump, new thermostat/housing, gaskets and new bolts. After cleaning the front radiators of debris (which I highly recommend every few years), I decided that I would leave the front end of the cooling system alone for another time. This will be Phase II when I decide to replace the radiators. Although the hoses are 20 years old, I honestly think that the main front and the 2 heater hoses under the middle of the car could last quite a few more years.

Since I already had everything out and apart, I decided I might as well change out the AOS. Hearing horror stories about this procedure did not inspire any confidence in me. I have never had the white smoke symptom but an occasional faint lumpy idle (which could mean a million things) caused me to think the AOS could be the culprit. The clincher for me was not having any documentation that the AOS was ever replaced (which I later found out that it had indeed been replaced) along with not wanting to have to remove everything again in order to replace it if it did actually fail. I don’t know about you guys but I am lazy as hell and I hate doing things twice. Kill 2 birds with one stone. Measure twice cut once, etc. I think I went overboard on this project but I can never leave well enough alone. Most of the parts I replaced could easily have lasted much longer but again I am very lazy and I’d rather just do it and not have to worry about should I have replaced that part while I had the chance or when it was much easier to do? Besides, the labor I saved doing this by myself basically made all the new parts and beer free!

The DIY guide on Renntech https://www.renntech.org/topic/37442...placement-aos/ proved invaluable and if you have a 1999 – 2001 manual transmission this should be your guide and you should be fine. This is the ultimate guide and my details are just ancillary with a little bit more detail. I did notice that there are some minor vacuum hose differences between 1999 and 2000-2001. Pelican also has a good DIY. I did not have to lower the engine. There were no special tricks or unusual surprises where I got stuck. Take lots of pictures and bag each set of bolts for each particular part that is removed. Human nature is to forget things and you will forget how a part went and a picture is invaluable.

After having completed this job, I have come to the conclusion that the AOS replacement is not difficult at all. Keep in mind the later model year 996 may be slightly more difficult but for the 1999 it is very easy. What makes this job appear to be so difficult is the totality of having to methodically remove so many different parts to get to the AOS makes it look so daunting. Removal of each separate part is not difficult at all. Don’t be intimidated. Be confident in your abilities. I’m an idiot. If I can do it, you surely can. Take your time. Go slow. Take lots of pictures and/or videos. Write down notes. Bag your bolts and parts and label them. Buy your parts before hand. Even if you buy all the parts you think you will need, you will probably order more parts later on because something broke or looked old.

I have basically removed everything and am now in the process of putting everything back together and hopefully everything will go smoothly.

Here is a summary of the steps in the order I used to get to the AOS. Note that these steps are after having jacked up the car, draining the coolant and replacing the water pump/thermostat:

1. Remove air box. Disconnect the electrical connector.
2. Remove engine coolant hoses. You do not have to do this but I had every single coolant hose removed as these were all being replaced anyway. And guess what? Removing all these hoses gives you tons more space to work! I would rather replace a hose on any car when given the opportunity as opposed to being stranded out in the middle of nowhere. Gives you a ton more room to work and prevents snagging. I even replaced the 2 heater hoses that are located below in the middle of the car fore of the engine. In order to keep track of which hose went were, I took a lot of photos and even left a few dangling so that I would know how to put them back together.
3. Remove air pump, associated hoses, and disconnect/loosen the air change over valves of which there are a total of 2. The first is just to the left top of the alternator which I left unbolted from its bracket and later replaced. The part number is the same for both. I replaced mine with Pierburg brand 7PP906270.
4. Remove fuel filter along with the rectangular shaped fuel line. Use a 19mm crow foot and a 17mm wrench. Although my fuel filter had only 12,000 miles on it, I decided to replace it along with a new fuel line as some of the fuel line rubber had cracks in it maybe from me pulling on it when I removed it.
5. Remove coolant expansion tank – easily removed once the fuel filter and fuel line are removed. Slides right out from the holding bracket to the right towards the engine. Just be careful to disconnect the coolant level sensor electrical connector first. Now is also a great time to buy a brand new expansion tank with a new sensor, cap, bleed valve and overflow hose. These stupid tanks are so badly designed that any time is probably a good time to replace these. My Toyota Land Cruiser has 300K on it and the coolant tank has never been replaced. Why Germans continue to engineer a split tank somehow glued together and capable of splitting at these very seams is beyond me. Well, to make money on replacing of course.
6. Disconnect all the electrical connectors, vacuum hoses, throttle body valve and hose connectors that go to the AOS.
7. Remove the alternator. Removing the alternator gives you a ton more room to work under the intake manifold when accessing that last intake manifold bolt in the back. I also noticed that the pulley was making a slight grinding sound and with no documentation that the alternator was ever replaced, I decided to replace the alternator anyway. Secret tip: rather than pounding on that god damn bushing for hours and putting hammer marks and dents all over your car and weakening your tensioner bolt and the lower bolt, just remove the engine block cap (part number 997-101-021-00) that is held on with 6 10mm bolts. This cap attaches to the coolant hose that goes to the joint support connector that cools the AOS and attaches to the 3-way splitter hose. Removing this made alternator removal a breeze. And make sure to replace the gasket with a new one 996-101-331-50. Gasket only costs $5 which is well worth it as opposed to hours of pounding. Another reason to go this method is that it looks like significant pounding of the long tensioner bolt could cause strain on the hole of the cap where the lower bolt goes through which could then cause a coolant leak. Torque back the bolts to 7.5ft/lbs. Now would also be a great time to replace the coolant hose and the joint support bracket. Once the alternator is removed, you can see the “sunflower face” of the AOS in the back! Be patient, you are getting really close now. I replaced mine with a brand new Bosch unit and rebuilt the old one for $90 as a spare.
8. Remove the 6 upper intake manifold bolts. The first 5 are easy as hell. The 6th one is also fairly easy. I used a ¼” ratchet with a 4-inch wobbly extension and 10mm socket. There is a large dome shaped plastic 993-110-140-03 vacuum reservoir that can get in the way if you have X-large hands like me and can make getting your right hand into the right side of the manifold for ratchet guidance difficult. Secret tip: remove the Cut Off Valve for Air Injection (part# 993-113-245-01-M100) and the 2 associated air hoses connected to it (the longer hose attaches to the air pump and the shorter elbow hose attaches to another valve that is attached to the engine block). This will give you tons more room to get your right hand in there. Now is also the perfect time to replace both of these valves, the 2 air hoses and the dome shaped vacuum reservoir. Once the alternator is out, removing the last bolt is really easy. For me, removing just the longer air hose and the cut off valve gave me enough room. Don’t forget to undo the thin vacuum hose that is connected to the dome shaped vacuum reservoir. Removing the cut off valve gives you much more space to get your right hand to hold the bolt and guide the socket onto the bolt. You can also use a magnetic bolt pickup tool/stick as extra insurance so that you don’t lose or drop the bolts. For bolt number 6, I used a 4-inch extension and a ¼ socket wrench and went straight down from the top. On the 1999, there are 2 fuel lines that appear to run across and over the bolt but this is not the case. This just appears that way due to the angle that you are looking at it from. You will have a perfect straight shot down to the bolt. You can use your right hand to guide the socket on. Whatever feels comfortable for you. When I had the bolt loosened just enough, I used my left and right fingers to turn the bolt undone and carefully cradled the bolt out. You can also use a magnetic pickup tool/stick attached to the bolt as extra insurance so that it does not drop and get lost. I did notice that all the bolts had blue thread Loctite on them.
9. Remove the throttle body. Remember to disconnect the electrical connector and the AOS hose. Remove the 4 large hose clamps (use either a screwdriver or 5mm socket) of which there are 2 on each side to remove/slide off the rubber sleeve. Then remove 10mm hexagon nut from the throttle body housing mount (basically a rubber grommet that slides over a screw that bolts to a bracket that attaches to the engine block) located on the bottom left corner (right above the power steering pump pulley) that gives the throttle body extra bracing via a bracket. After 20 years, the rubber on mine broke apart. Just replace with a new one 999-703-265-00. Be careful not to drop the nut or the grommet! After the 2 left side clamps are undone slide the boot to the left and go on to the right side and undo the 2 right side clamps. At this point you can remove the throttle body. With the throttle body out, you will have lots of room to loosen/undo the 2 rear left sleeve clamps. Sliding the left sleeve to the left onto the intake manifold will allow for extra room to slide the throttle body to the left and out. Now is a good time to clean your throttle body and the rubber sleeves. The front left rubber sleeve has a thin plastic vacuum line that connects to it as well as the rear right rubber sleeve. I noticed some obvious heavy oil residue in the lower intakes as well as a nice oily sheen in the intake manifold and the rubber sleeves. Wipe clean and/or replace the rubber sleeves making sure which vacuum line goes to which rubber sleeve.
10. Remove the upper intake manifold. Just lift up the top hoses/electrical wires, etc. that are at the rear of the engine and the ones connected to the throttle body regeneration valve out of the way so that the manifold can slide right out towards you. Be careful not to snag on any wires or hoses and it should come out easily and cleanly. Wipe clean the inside of the intake manifold as best you can. Once the manifold is out, you will have a straight and easy shot/access to the AOS. That beautiful sunflower face stares right back at you. With the manifold on the engine, the AOS looks so far away and hidden. With the manifold off, the AOS is actually very close and access is easy! At this point, I contemplated removing the passenger side manifold but this looks like a far more involved process including removing the AC compressor and the power steering pump so I left it alone. I wiped clean the inside of the passenger manifold as much as I could.
11. Remove the AOS. Jack up the car on all 4 sides so that you can have plenty of room to work. Removal of the driver side rear tire is not necessary at all but doing so can give you some extra space. To give you a sense of direction, I slid underneath the car head first with my head directed towards the front of the car. Once your head is slightly past the AOS, you can shine a light onto the AOS. You should have decent access to the AOS with your left and right hand. First remove the AOS hard plastic hose connector on the right side with your right hand. I used my right hand to pinch down the 2 ribbed areas and was able to pull the connector off. The trick here is to slide yourself to the left a little bit and with your head facing towards the passenger side of the car you can manipulate your right hand by feel and disconnect the connector. I tried using my left hand at first but the angle and available torque was not good. There are a bunch of miscellaneous unrelated electrical wires right below the connector that look like they will get in the way but don’t worry about these. No need to undo or move them out of the way. Now you can either cut the bellow in half or just undo the bottom clamp. Whoever previously replaced the AOS on my car used a black zip tie! If you have an Oetiker clamp on the bottom of the bellow, use a plier cutter and then remove the final part of the bellow. If your bellow is attached via a spring clamp, I highly recommend using a coolant hose spring plier. Then unbolt the 2 10mm bolts that hold the AOS to the engine block. I used my right hand and a ¼” ratchet with a long 10mm socket. Take your time and the bolts will eventually come out. It got a little tiring because the bolts are fairly long. They also had blue Loctite on them. Use a big long flat tipped screwdriver to carefully pry the AOS to the drivers side out away from the engine block and then you will be able to pull the AOS out from above the engine. There was only a bit of oil that dripped out of the AOS on mine. Most of the inner passages of the various hoses and intake manifold/boots had a thin coat of oil on them. Not sure if this is normal but I would think that a thin mist will eventually turn into a heavy mist which in turn will become a heavy coating of oil. Some of the hoses had drops of oil running out/down so I think it was a good time to replace the AOS.

The following are optional items that are readily and very easily replaced now that you are in this deep:

12. Replace oil filler tube and cap. Driver side bolt easily removed. Easier to tackle the passenger side bolt from the right using your right hand. Use a 4” extension to get the two 10mm bolts out.
13. Replace driver side knock sensor. Using a 13mm socket or wrench, undo the bolt and the connector and replace with Bosch 996 606 125 00.
14. Replace starter. First undo the ground and electrical connector. Then, using an extra-long extension or a combination of several long extensions, undo the 2 15mm bolts that attach the starter. I replaced mine with a new starter and had the old Bosch unit rebuilt for $90 as a spare. The unit I replaced looked like it still had lots of life left even after 6 years of use. The contacts still had about 2/3 of wear still left.
15. Replace Secondary Air Injection Check valve 964-110-950-02.
16. Replace all vacuum lines/hoses. Although you could leave well enough alone, I highly recommend replacing as many if not all the rubber vacuum lines as possible. This is the best chance with the easiest access to do this. Some of the lines are very thin plastic lines, standard 3.5mm rubber lines and several semi u (L) shaped rubber connectors known as L fittings 928-574-717-07 which help connect several of the valves’ hard nipples to the thin plastic lines.
17. Replace the regeneration valve from throttle body for fuel vapor system and the associated rubber vacuum hose. OEM Porsche costs $220 and the exact replacement from Bosch is $40. The only difference is the 5” hard plastic hose fitting attached to the valve. You can try to reuse or buy a similar type hose and heat it with a heat gun so that it seals. I went all out and just bought the OEM Porsche part. I believe this part was on its way out as I had begun to hear whistling sounds from the back when in 6th gear cruising on the highway I would tap the accelerator to gain speed.
18. Replace the engine mounts. Very easy and simple to do. Just jack up/support the engine and replacement is easy and cheap. Makes your muffler pipes look better too.
19. Replace both idler pulleys and the tensioner pulley. All 3 pulleys were making a grinding sound when I spun them freely so I had the idler pulleys replaced with Dayco brand and the tensioner OEM. Good time to replace those bolts also as there is a lot of stress on these.
20. Replace plastic water line support (2) for large driver and passenger side coolant pipes 996-106-421-51. Easy to take off and replace, especially with all the hoses removed. And although not really necessary, I am very **** so I replaced mine just so that my engine bay looks cleaner.
21. Replace serpentine belt. Self-explanatory.
22. Replace air temperature sensor and rubber grommet. Again, not totally necessary and not made any easier as this is located on the passenger side intake manifold. But with the driver side manifold removed, replacement is a breeze. First disconnect the electrical connector and then remove the sensor and the grommet. (My grommet was a bit hardened from age and heat and the male part of the sensor was just loosely inserted into the hole and not seated fully into the grommet). Then remove the sensor and the grommet. Install the grommet first. The foundation for the grommet is a circlip shape and so if you first put the sensor into the grommet and then try to install these two together as one piece onto the manifold, it will not work. It took me forever to figure this out but you need to install the grommet first and then using some soapy water onto the grommet, push the sensor in to the grommet with your right hand while at the same time using your left hand to support the grommet from underneath (and made so much easier now that the driver side manifold has been removed). I had tried to install this prior to removing the driver side intake manifold and it took me forever to figure this out. I even lost the sensor and grommet down into the engine bay but was able to retrieve it after the driver side manifold was removed.
23. Replace the intake manifold gaskets. You could reuse the old ones but why?
24. Replace oil and filter.
25. Replace rear differential fluid.
26. Replace elbow vent valve and O-ring that connects to AOS air hose on the passenger side. See https://rennlist.com/forums/996-foru...nger-side.html for details. And note, once the power steering lines are removed, this is an extremely easy job compared to what I had to fight with in the beginning as explained in the post.
27. Replace dipstick and dipstick tube O-ring. My dipstick was bent, old and dirty so replaced with a new one. Also changed the o-ring on the dipstick tube. Original o-ring was still in good shape. To remove, use a 10mm socket with an short extension and undo the bolt. Then start pulling the dipstick tube straight up. There will not be enough room for the tube to go directly up all the way as the tube is fairly long and will hit the top of the engine compartment. Gently pull the tube towards you little by little making sure not to bend the tube. If the remainder of the bottom of the tube that goes into the engine seems to get stuck or caught, just gently wiggle it back and forth upwards. Of course, with the engine out or lowered when replacing the engine mounts, there is plenty of clearance for the tube to be pulled straight up. Replace the o-ring and lube it with a little bit of grease so that it seals correctly. Torque specs for the bolt are not necessary as the bolt will stop when it is fully threaded.
28. Replace Check Valve. Using either a very slim machined down 27mm crows foot or a 27mm Williams 3527M 30 degree service wrench, remove the check valve with the elbow breather hose still attached (much easier to remove with the check valve removed). And remember to remove the long breather hose that goes from the cut off valve first. Be careful when unscrewing the valve as the elbow breather hose will hit the dip stick housing. The new Check Valve Porsche OEM part number is 993 113 250 03 and you will also have to order a new sealing ring 900 123 011 30 (or use the existing ring). I would also recommend replacing both breather hoses at this time. My check valve was in serious need of replacement with black soot, rust, and gunk all over the place on the outer housing tubing and the inside. The moisture really gets into this valve and rust was the result. Secret tip: the check valve is not on the block really tight so I think one could use a big wrench or pliers to get this undone and installed. Just enough torque to get the sealing ring sealed is all that is required. No need to manhandle the thing back on.

rhkwon

OK curiosity got the best of me again and so I decided to replace some additional items.

1. Remove passenger side intake manifold. There is no need to remove any of the A/C compressor bolts/lines or the power steering items. Once the rear long resonance tube and the 4 rubber sleeves (be careful of the vacuum lines) are removed, access to the rear 6th intake manifold bolt is easy. The resonance tube also has a plastic vacuum unit 993-110-462-02 attached to it. I replaced mine. The bolt is located just to the right of the heat exchanger. Using my left hand, I used a 1/4“ ratchet with a long 10mm socket. The remaining 5 bolts are easy to remove. I first broke the bolts with my right hand and after that used my left hand to ratchet the bolts undone. You have a lot more room, torque and turning angle this way. The front most bolt removal is made easier by removing the nut that holds the alternate positive battery lead and then moving it out of the way. It’s not a straight direct shot down so I used a wobble extension to undo the bolt. The bolts were blue Loctited from the factory. There is a plastic vacuum hose that connects to the front passenger side rubber sleeve that will need to be disconnected as well as the air temperature sensor electrical connector. Then just remove the intake manifold. Now you can go to town and replace the remaining vacuum lines, gaskets and clean up the area in general. The air temperature sensor and grommet replacement is now a breeze with the manifold out.
2. You will now also have easy access to replace the fuel pressure regulator. First slide the retaining clip off directly to the right with a flat tipped screw driver. Remove the vacuum hose that connects to the FPR. Then pry and wriggle the FPR directly up using pliers. Note the direction and travel of the vacuum hose that goes from the FPR via a rubber vacuum hose connector to hard plastic vacuum line to the rear most sleeve on the manifold. Both green O-rings were in decent shape but compared to the new ones were definitely in need of replacement. The new FPR will come with new O-rings.
3. Replace the power steering upper tank and/or O-ring/pump/return line and high pressure line. My tank was weeping slightly so I replaced all 4 O-rings and tightened the screw connector down. Siphon out the fluid first and wrap a towel around the connector to avoid spills. After removing the upper and lower tank and the PS pump, I noticed that the upper and lower tank had been replaced recently but the pump and high pressure line were still original. The return line had been replaced recently. There is a good write up on this on the forum on PS pump/hose replacement. Very simple and easy to do. The AC compressor is also easily removed at this stage. Don't forget to install a new green o-ring that goes on the high pressure line connection to the pump. The new replacement hose will have the o-ring already installed. This o-ring on mine was deteriorated badly to the point of disintegration when rubbed. If you are replacing the entire hose, the end connecting to the hard lines will just click in place.
4. Remove/replace heat exchanger. With the passenger intake manifold off, you will now have easy access to the heat exchanger. I replaced mine with a brand new unit and also replaced all 4 O-rings and the long breather/coolant hose that goes to the coolant expansion tank. First disconnect the bracket at the center rear of the engine compartment that holds the breather hose up. Then remove the 4 5mm hex bolts. Now lift straight up on the heat exchanger and let the respective coolant and oil to flow out a bit back into the engine and make sure you don't cross contaminate the respective passages. Lifting straight up will allow oil and coolant to drain back cleanly. Or if you don't want to take a chance then have plenty of rags ready to soak up any additional spillage. When removing the heat exchanger from the engine compartment turn it upside down to avoid spills. Both oil O-rings were still in excellent condition. However, both coolant O-rings were definitely in need of replacement and were very near failure as they had become soft and mushy and were disintegrating so it was good that this was replaced. I also noticed coolant residue on the outside of the heat exchanger where it looks like one of the O-rings had been weeping for a long period of time and on the verge of failure. Clean the mating surface and the o-ring recesses of oil and dirt using brake cleaner. The O-rings will drop right in on top of the engine and then place the heat exchanger back on. As the heat exchanger is made of aluminum, torque back to approximately 7.5 ft/lbs.
5. Replace oil level sensor. Now easily removed using a 17mm socket or wrench. The new sensor will come with a new copper gasket.
6. Replace oil pressure sender unit. You have lots of room now and will not need to work from underneath the car. Disconnect the 2 electrical connectors. Use a 19mm crows foot and a 4 to 6" extension with a 3/4" socket wrench and you can now do this not on your back but sitting down in a chair and in your sleep. New unit will come with a new metal gasket.
6. Replace passenger side knock sensor.
7. Replace long plastic AOS breather hose that goes from AOS to the elbow connection on the passenger side. Now very easy to do, especially with the PS lines out of the way and both intake manifolds off. The new replacement hose is a different design and the accordion tubing near the passenger side is now solid plastic probably for more durability. The only negative is that it takes a little bit more finagling to get the hose through the manifold.
8. Replace tensioner, pulley and lever. I would recommend replacing the old pulley with a Porsche OEM tensioner pulley ($~90) as the kit comes with all the necessary hardware including new pulley, bolt, 24mm nut, aluminum washer and pulley cover/washer. With the PS pump and the AC compressor out of the way, removal is now an absolute breeze. I also replaced the tensioner, lever, bolts and both lever seals as the lever arm was a bit loose. First remove the pulley itself. I kept getting confused on which way to turn the socket but finally figured it out. The open ended wrench on the 15mm bolt behind the pulley should be pointing northeast and once you have the open ended part onto the bolt, try to push the wrench forward towards the front of the car (towards the PS bracket) where there will also be a small protrusion so that the wrench can be held steady. Use both the protrusion and your right hand to hold the wrench as steady as possible. Then using a big breaker bar and a 24mm socket and pushing left with your left hand, undo the 24mm nut by turning it left. Keep everything steady and even and it will come undone easily. No heat torching or partner necessary. Very easy to do and not a lot of force was necessary to break it loose. Then to remove the tensioner, you can either remove just the tensioner by itself or remove the PS support bracket and then remove the tensioner while it is out of the engine. There are 2 hex head bolts that attach the tensioner to the PS support bracket. There is 1 hex head bolt that attaches the lever to the tensioner. 4 bolts hold the PS support bracket to the engine block. Removing the tensioner allows you to replace the 15mm pulley bolt with a brand new one. Make sure to place the new washer back on and apply red Loctite. Torque to 44ft/lbs.