Compressor change - oil quantity question

Hi All, I am in the process of changing the compressor on the 2004 996. The compressor worked ok but was a bit noisy so I decided to replace with a new denso unit. It was freshly regassed when I bought the car late 2019, I avoided using the AC as much as possible but it did work fine when called upon through to the cooler weather in March/April, but then did not fire back up after (southern) winter. Gas pressure was low. I noted the instruction to drain both new and old compressor and only add back the amount of new oil into the new compressor to match the quantity removed from the old unit. From my new compressor I only got a few drops of oil from the suction and discharge ports. From the M8 oil port (sealed with a crush washer) I got about 150ml. From my old compressor I got a similar few drops from both suction and discharge ports and only a bit more from the oil port - total quantity approx 5ml at a guess. Seems wrong to only add 5ml of oil back into the new unit, can that be right? Is there a way to determine oil level other than making assumptions about what is in there presently? Only other option appears to be total disassembly of major compoents, flush through then refill with specified quantity of new oil. Shop manual I have suggests 195cc total quantity. Thanks in advance Steve

The air conditioning system capacity is 850 grams (30 oz) of R134. In addition, the compressor needs a synthetic lubricant for proper operation. If you're filling a completely empty system, add 195 ml (6.6 ounces) of ND 8 refrigerant oil. Source: https://www.pelicanparts.com/techar...Air_Conditioning_Maintenance_and_Recharge.htm

This is indeed a bit of a nightmare job .. too much oil and you risk hydraulic lock of the compressor .. too little and over a long period you risk seizing . A new compressor on any make of vehicle should have the total amount of oil for the entire system in it ... 195 as Alex said . As you have mentioned .. you drain out the old compressor and measure the oil amount .. drain out the new and refill the same amount as the old one .. This is what Porsche say to do . The oil in the system .. some will be in tiny pools in items .. condesonsors for example .. some will sit in the base of them .. The majority of oil in an a/c system though is stuck to the pipes .. think of a thin coat of oil over every surface in the system ... surprisingly this does add up ... it moves around in a vapour form . The difficulty is in knowing how much is in a system .. a machine vac will only remove water .. oil doesn't boil in a vacuum so it will stay in the system .. A flush .. the evaporator has pipes on the top .. you try and flush that and you will just fill it with flushing fluid .. can't empty it unless you remove it ... same with condensers and you can't flush a dryer unit . In short .. a flush won't work .. been there and tried it due to a compressor break up .. tried flushing out the debris .. it didn't work and i did each individual component.. We are left with a guess .. 195 in total , unknown amount in a system but if its been open or a leak we can assume 20-30 mls have escaped . I would be comfortable adding that much to a system with a new compressor . if anyone has any better info then i'm all ears to chat about it .. hell i'll even phone you .. but the above is what i would lean towards . I'll add .. give it 3 months and do an evac and record the oil removed .. if 1-2 mls or less then i would add a further 15 mls of oil .

DeMort is bang on as usual, the compressor comes with the total amount of oil required for the system(this is because when they build the cars there is no oil in them and this is how it gets into the cars in the production line). if you overfill the system, your are likely to damage the compressor. If you underfilll the system that isn't good either! the safest bet is to roughly put in what came out of the old compressor. the oil is hygroscopic and would be useless if left open to the elements for a period of time(a wet winters day 24hrs would be enough), moisture in an ac system is the easiest way to kill it or shorten its lifespan! many a technician have little understanding of the importance and maintenance of an a/c system, i regularly hear that a bottle of PAG oil sits on the side of an a/c machine that has been in there for weeks/months goes into cars. Its another common myth that the middle viscosity oil is universal and will go in anything, imagine if we did the same with engine oil!! its all dependant on what compressor is fitted and this details what oil should go in. Most of the time tech's are lucky that its not immediate that the system fails or becomes less efficient at the time of service. The only way we can help to remove moisture is to service the system regularly and vacuum for a good amount of time, the rough general rule is that an hour per kg of refrigerant in the system.

Hi Guys, That's awesome, thanks so much! That all makes it much easier to understand what's going on, and makes perfect sense as the most pragmatic way to approach the conundrum. The history of my AC system is that it had stopped working some time earlier, then the PO got it re-gassed prior to handing over the car. The AC worked well but the compressor was a little noisy so I avoided using it where possible. When I did it worked fine from December through to about April or so when it was cold enough not to call on it. By September/October when I tried it again it didn't come back on, which I presumed to be a leak - hence why the PO needed to re-gas it prior to sale. I'm replacing the compressor first, then will go searching for a leak if I continue to lose gas. It didn't feel right to put only 5ml or so back into the compressor, it seems likely that some oil has gone astray over various evacuation procedures. I've just drained the compressor again and put 30ml back in and am vacuuming it out again. I'll pump it for several hours since I'm doing it at home and there's no rush. Do I have to also replace the receiver/dryer unit? Seems to be Porsche part number 944 573 143 01 (Denso 478-2001). Cheers Steve

I bought a new compressor and struggled to get the back bolt out. I also took out the temp sensor from between the RH inlets to get at it and then couldn't get the sensor back! I gave up and got 9E to do it. They told me that compressors don't fail often and they usually get noisy when there is a leak and the gas level gets low.

The whirring noise that stops when you turn off the a/c is a symptom of low gas .. this is due to a leak . Its best practise to replace the dryer on these whenever significant work is done or if the system has been left open to atmosphere .. a leak and no gas left counts as this . A long vac down ... 24 hours will pull most of the moisture out of the system but i'm sure i was told once a vac down wont regen a Porsche dryer . 944 573 143 01 that looks like the correct part .. you will need new O ring seals .. A/c O rings cannot be reused .. they swell up on contact with Pag oil so once removed they won't reseal .

DeMort is once again correct! The drier cannot be dried out, it will hold about 5g of moisture and after that it will allow the moisture to move around the system. Worthwhile replacing while fitting new condensers and agreed that the O rings are in need for replacing and cheap enough to do so.

Bless you but i have a feeling here your the expert and im just beating you to the answer with what i have figured out over the years ! Thanks for confirming the dryer cant be refurbed .. i didn't think so but had no real proof . 1 KG per hr .. commercial i take it ? with porsche being around 900 grams i assumed a vac down would need to be for far longer .. never really do it for more than 30 mins to be honest as we replace the dryer pretty much as standard but this is useful info for me . The oil type was also interesting .. working on just one make of car i use the Porsche oil but i can easily see a multi car workshop using a more general oil they can buy in bulk .

I work for another German manufacturer as a technical trainer and this is one of the subjects i deliver. The 1KG rule is a oldish one, with modern machines they are more efficient but really its worth a longer vacuum than a shorter one most of our vehicles have approx 500g and a 20mins is nearly about right but i would still recommend a longer vacuum, its free way of helping the lifespan of your system. You have a very good understanding of what's best for the system and the advise you give is spot on, there are many hear says in this subject and lazy techs have got away with issues. For our vehicles we can have different compressors on the same vehicles so we always make sure the techs identify which one is correct(although many use a similar oil nowadays).

Can i just say .... :worship: :worship: Your Compadre's on Porsche taught me and what they new about a subject as you do always impressed me .. no matter what question i came up with they always new the answer . Porsche trainers often have to back engineer things .. Porsche as a company don't tend to give out much info . Your job is a thankless one but the mechanics later on always realise just how much they have learned .. i certainly did . No Longer at an OPC these days so i have to figure things out .. it's always fun though . Porsche use 2 types of compressor which i expect is the same as your Marque .. Fixed and Variable Displacement Compressors . Downside to the Variable ones .. when they go wrong they keep throwing debris throughout the system .. 997 G2 and every pipe and component was covered in metal swarf and paste ... never seen that on a fixed compressor. A flush attempt at each pipe failed .. just can't get it all out. First and last one i've attempted lol . Not sure about your guys but Lazy i would swop for Bonus pay on the Porsche side .. to much rushing and not enough concentrating on doing the job correctly .. The plus side of working for an Independent these days is i get to spend the time i need on faults or issues and as you always need to know how something works before you can fault find then i get to do a lot of reading when anything new arises . Most systems on the German cars are very similar .. if your anywhere near Reading then Porsche is an option to work for !!! I could easily chat for Hours but i think i've wasted enough of your time .. Thank you .... to have an Instructor tell me i'm correct is the icing on the cake i always try my best on here . Total thread hijack but it had been answered anyways Edit ... Just to complete this for the OP .. oil quantities for each component are below . Edit again .. A question if i may ... That would indicate 195 grams of oil in the compressor is ok if it can be started with that much oil in it .. it would never have that much oil in it in normal operation .. As such then surely it would need a massive overfill of oil to damage a compressor ? i've always assumed 40 mls overfilled might be enough but if a compressor can work with max capacity it would indicate a very large over fill is needed to damage it .. as such i can add more oil without fear of compressor damage on a normal refill ? As standard i add 10 mills when i do a regas .

This is fascinating stuff guys, I feel lucky to have such experts contributing. Meandering some way (sometimes a long way) off topic is often the best part of any vehicle forum, so I welcome it. It is interesting to me as while I'm strictly a DIY mechanic for enjoyment, I have a few decades experience working with ultra high vacuum in research applications. Moisture is always an issue when a vacuum system has been 'up to air'. Some of our vacuum systems will take months to pump back to their ultimate vacuum, and require various procedures of baking to release first moisture then actual outgassing elements from the materials. Anyway, great input and thanks again. After reading your input and giving it more thought, I have probably come at this issue the wrong way around in replacing the compressor first. I think I should have had the system regassed and first really traced the source of the leak that caused it to lose pressure, fixed that and only then considered compressor replacement. My info was that the system had been regassed shortly before I got the car, so I *think* it had good gas pressure when the compressor was noisy, but I can't be sure. Oil quantity of only about 5-10ml removed from the old compressor is a concern given the stated residual quantities above. That would suggest there should have been around 50ml in the compressor, give or take. But I'll follow deMort's suggestion and do an extraction after some use and continue to add oil if it appears to need it. Meanwhile it's a work in progress. I visited my dealer and got new compressor connection orings and have the drier unit orings on order likely to arrive early next week. I have an aftermarket drier which was $18 versus dealer at $210, but the orings it seems they are mostly funny sizes to I stuck with factory.

Never changed the dryer on mine and aircon is ice cold. My mechanic reckons unless you live in a seriously hot country there's no point. :dont know:

Alex, if the system has always worked then the amount of moisture is mimimal to a system that has been open to the elements. The drier is a serviceable part, however many manufacturers dont state a service regime, this keeps the cost of ownership down and so benefits insurance rating etc... if you are happy with it, then thats what's most important. The systems on 996's are very good and get nice and cold quickly, yes if you have no reason for concern, why should you spend money? A lot of these cars now have had condensers fail and moisture has got into the system, replacing the drier and a good vacuum will help the lifespan of the system. DeMort another tip for you is to evacuate from the low pressure port, this minimises the amount of pag oil recovered and charge through the high pressure port only. The refrigerant enters the system as a liquid from the service machine and if put in through the low pressure side this is where a gas/vapour should be on a working system and could damage the compressor. On the high side this will be in liquid form and will hopefully minimise any issues.