The air-cooled 911's have some interesting differences that seem to make checking and changing the oil a bit more challenging for the shade-tree mechanic. I'll try and simplify the process here.
The Big Difference - Dry Sump
The oil in most cars gets pumped up in to the engine as it runs, and then falls back down into a "bucket" bolted to the bottom of the engine block, normally called the oil pan or sump. The process is you warm the car up, turn it off and then open the drain plug in the bottom of the oil pan and let it drain out. Replace the drain plug. Then replace the filter. Finally replace the oil to the "full" line and run the car for a few minute and re-check the oil to make sure you're good to go. Normally there is a bunch of oil in the filter, (maybe a 1/2 qt) so when you screw on a new dry filter, it will lower the overall level once you start the engine up and you'll usually need to top it off. You can check it within minutes any time the engine is off. It's pretty simple.
The older 911s use a system called a dry sump. There is no pan under the engine. You tend to see these systems in race cars and bikes. You can hold more oil in the engine which makes the engine harder to starve of oil in corners. (oil sloshes from side to side in corners which can cavitate your oil pump in extreme circumstances. No Bueno) In fact the oil tank for the 911 is tall and thin, minimizing any side to side shift that would change levels, and it lives inside the passenger side rear quarter panel. The feed to the engine is at the very bottom of the tank, and the car holds about 2X the amount of oil you would see in a standard water-cooled car. A lot of thought went into this stuff.
You can also mount the engine lower in the car with a dry sump since you don't have that "bucket" on the bottom to hold the oil. This is advantageous for race cars as it helps lower the center of gravity.
The process for changing it is nearly the same with a few caveats:
You need to get the car up to operating temp (take a nice 20min or longer! drive) to open the oil thermostat and get oil moving through the oil cooler up front. Those are the big lines running along the passenger side of the car. The oil cooler is in the front PS fender by the horn.
You want to drain the oil from the tank first, and then the engine case. When both of these are empty, then remove the oil filter. If you remove the filter first, you can spill oil all over the place. The mechanic who did my PPI thought the car had a slow leak. In fact, it has not leaked since I bought it, leading me to conclude that it was meatheads changing the oil and spilling it all over that just made it look like a car with an oil leak. <sigh>
Make sure you have a big enough oil catch pan; you need to catch about 10 qts. I used a 16qt I found online.
Materials:
Mechanics gloves. Or latex gloves at a minimum. The oil is very hot and can spill on stuff like skin. Owee. It's also not terribly healthy to be exposed to. So be careful.
2 new crush washers for the drain plugs (don't re-use these!)
15mm wrench for the drain plugs
Oil filter wrench
10 quarts of 10W-60 oil (I used Liqui Moly Synthetic. 10W-60 matches what Porsche recommends for vintage engines over 3 liters)
Oil filter (I used a Mahle OC 54)
3/8" Torque wrench to tighten the drain plugs when you are done. My 1/2" torque wrench wouldn't fit between the heat exchanger and case drain plug, so I'm going with 3/8" here as a recommendation.
Towels, brake cleaner for spills. Etc.
TL;DR
Remove the drain plug from the tank and let it drain. How long? Until you just cannot wait any more. Replace the crush washer and put the drain plug back into the tank. Tighten it to 31 ft/lbs*
Remove the drain plug from the engine case and let it drain. Replace the crush washer and put the drain plug back into the case. Tighten to 51 ft/lbs*
Remove the oil filter. Put a small amount of oil on the new oil filter gasket and screw it on hand tight + 1/2 turn.*
Using a funnel, put about 8 quarts of new oil into the oil tank (about 1/4 way on the dip stick? maybe less). Run the car up to temp and then top it off. I use this funnel for that: https://www.stoddard.com/t52funnel.html.html
Very Important: if you add oil to the full indicator on the dipstick while the engine is cold, the engine will be overfilled and you can do damage to it. If you think it might be over-filled, drain some out. Better to be a little under than a little over. Best case is it will leak, worst case is you can have an engine fire.
The CW I am seeing online is to shoot for the middle of the dipstick or gauge when the engine is really hot, not to fill it. I tried to fill it and caused a leak.
* edited to revise the torque specs to match the Bentley manual, and verified with PCA expert data. Apologies.
Addendum
Switching from 20/50 to 10/60 has made the engine sound a bit quieter and the big puff of blue smoke I used to have on start up is mostly gone. I am hopeful that oil consumption will drop. We'll keep an eye on that.
Pictures
The basics: 10qt of oil, a filter and a pair of crush washers
My preferred oil filter wrench style. Only good for removing old filters, but that's really all you should need a wrench for anyway.
The filler and filter, going into the rear passenger side fender.
My drain pan, I went with 16qt capacity because I really didn't want to get a 10qt pan and have it overflow.
The engine case drain plug
The oil tank drain plug
The oil came out so fast that the pan could not vent air fast enough so a little bit spilled. Not a big deal. Just be ready.
Slightly more than average filings on the case drain plug; I'll be sending oil off for UOA to see what can be learned. The car has about 116,000mi on it. If you don't already use a magnetic drain plug, you really should.
Old filter is off, we are ready to re-fit and re-fill after we clean things up.
This custom funnel for 911's is really great. #BigFan
31 & 51 ft/lbs are the factory setting for the drain plugs. (see above) It seems like a lot, but they are large. It does make them hard to remove (it takes about 2X tightening torque to remove a fastener. You'll need a nice wrench. Remember never use a torque wrench to remove a fastener!
With about 8 qts in we are 1/2 way up the dip stick. Time to run the engine up to temp and then top off while it is running (the normal way to top off) You can use the gauge or the dipstick, either is fine although the dipstick is probably more accurate. What you don't want to do is over-fill. This can cause issues.
Here is our end state later in the day after a nice drive and topping off the system. Always check your level when the engine is hot, running, and on a level surface. This is actually a tiny bit higher than you really need. halfway when the engine is really hot is ideal.
Next oil change, Jan 2023
Mileage: 116,653 (~3600 mi)
Much less debris on the drain plugs, which is great news.
Sending an oil sample out for UOA. Will just be a baseline, but a good start.
Installed new oil line from tank to case.
First off we got a new open-top pan so I would not have the over-fill issue I had last time. File under "D" for duh, but it works.
This is much more like it. The top plug has a thin layer of very fine metal particles, almost like "mud". Much better than the "hair" we had last time. Is it the new oil? (Motul 10W-60 synthetic) Hard to say. A pessimist would say the thing that was getting worn has probably worn away, lol. We'll see what the UOA says as we move forwards.
Once the oil was out of the tank and the crankcase, I dropped one end of the oil hose to let it drain before removing it.
Nice new hose and clamps. The issue is that the old ones get hard and brittle over time, they "plasticize". This new one is much more pliable.
Easy peasey, $25 for piece of mind is a good deal in my book.
Those other lines will be getting refreshed soon. I cleaned them up a bit to help gauge how bad they are weeping.
Jan 29 2023
I noticed a little oil under the car; maybe a tablespoon or two. Not a gusher, but not a drop either. My mind raced, did I mess up somewhere? I immediately checked the return line to the tank I had replaced; it was clean and dry. I looked around and found a drip running down the transmission from a small window in the bell housing - was it my rear main seal? It was definitely motor oil, not transmission oil (hypoid gear oil smells pretty bad. Hard to mix them up!) It was hitting the ground well in front of the engine as you can see below.
I wrote a separate page to track the details and process, but TL;DR it stopped after a day and I am wondering if I over filled the oil just the tiniest bit? I am not sure what else would leak and then stop like that. If we're out of the woods I am very thankful because the rear main seal is a big job to replace; a $40 part that takes a few thousand dollar to get to.
There is a different entry for fixing the oil leak, but TL;DR I put too much oil in and the excess was oozing out the rear main seal. I drained some out and the problem has not reoccured. Lesson learned!
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