imhighlander

Howdy, all. I'm going to directly tap my 996 friend and creator of the Ultimate AOS (which I have on my 996.2) @Porschetech3 and open the thread here also. I'm hoping for some advice as I track down an oil consumption issue in our 2004 986.2 Boxster S.

My wife's 986.2 S is consuming a LOT of oil. I estimate we're now adding a quart every 100-200 miles! The original AOS was replaced a couple of years and maybe 15,000 miles ago by an indy shop "...while they were in there." They replaced the AOS itself, but I don't know that any of the lines or fill tube were replaced at that time. Since then, we fouled a factory cat (at 110,000 miles) and replaced the cats with pretty spendy Fabspeed catted headers. I installed those with the related gaskets, new O2 sensors, etc.

Oil consumption began to increase around 110,000 miles and we're now at around 123,000 miles. We had a compression test done a couple of weeks ago that returned the following:

• Cyl 1: 117 PSI
• Cyl 3: 117 PSI
• Cyl 5: 117 PSI
• Cyl 2: 108 PSI
• Cyl 4: 101 PSI
• Cyl 6: 101 PSI

According to the indy, compression spec is 160 PSI. Leakdown measured 15% across the cylinders, which the indy said is "well within specification." Their report says, "...suspect piston rings are worn out based on reading and causing excessive blow by resulting in low compression and excessive blow by." The shop also diagnosed a small valve cover gasket leak and a leak at the LN Engineering "spin-off" oil filter housing (which I rather regret installing). The car leaves some drips but not enough to explain the volume of oil we're losing.

Today, while the engine was idling, I removed the oil cap (easy enough) but heard audible vacuum pulling from the filler neck. The idle became a little rougher, but not stumbling at all. Put the cap back on, she's fine. Back off, vacuum and change in idle. With the oil cap back on I pulled the dipstick and again had audible vacuum. I understand that "some" vacuum is normal, but I don't really know the difference between "some" and "a wee bit too much."

I believe that a failing AOS can cause excessive suction, but it's usually accompanied by white smoke from the tailpipes. Because our cats are so new, I'm wondering if our AOS has failed again but we aren't seeing smoke because it's getting soaked into the (new, ridiculously expensive) cats.

Final facts: We use 0W-40 Mobil-1 oil, which I change with a filter every 6,000-7,500 miles. The 986 gets regular exercise but sees no track use. (Which is disappointing.)

So a few questions for those who understand the numbers better than I do:

• Do the compression numbers mean it's time for an engine rebuild, or is it reasonable to expect a fair bit more life from the motor?
• How much will a heavier weight oil potentially change the blow-by? What are the tradeoffs/risks of this?
• What else do I need to know to identify the cause of our oil consumption?

I really don't want to wait until we burn through these nice cats. If I need to do an AOS again - which I'll likely use Skip's "Ultimate AOS" since I already did it on the 996 - so be it. But are the compression numbers to the point of justifying a new motor?

The Boxster is probably a "lifetime car" for my wife. She absolutely loves it and we've sunk a lot of work and a fair bit of money into a new top, full Porsche ROW sport suspension, Girodisc brakes, custom interior... so we're "in for a penny, in for a pound." If it's time to ship the car off to get a new motor, so be it. But if the consensus points to just doing a new/upgraded AOS, I'd certainly prefer that answer. But by the time we drop a new motor in it, we'd have a "new" 986 for about $20k.

Thanks in advance for the thoughts and perspectives.

pilot4fn

Start with easiest test first. Porsche recommends now to these older cars a bit thicker oil - 5W-50. See how that changes the oil usage.

ZuffenZeus

At this point I would recommend changing to a 5W40 higher quality oil like Driven DT40/DI40, LiquiMoly 5W40 (with Ceratec), or Motul 5W40 8100 X-Cess

Not necessarily, but based on conventional wisdom, poor compression could mean you either have bore issues that have caused the clearances between piston rings and bore surface to have increased OR the valves are not seated properly.

I prefer to say higher viscosity over "heavier oil". IMHO, you have to be very careful moving to higher viscosities on the average M96 because it can negativity affect the VarioCam VVT system that needs the right viscosity of oil to operate properly. The chain tensioners are also hydraulically actuated. Most people look at the second number in oil viscosity, but it's the first "W" number that is as or more important to remember when choosing a higher viscosity oil. For normal street use, I would not recommend the W number be past 5. Porsche has certified that these engines can run perfectly fine with 5W40 or even newer 5W50 for street use. If your engine has been rebuilt, then listen to the engine builder for oil recommendations because the builder may have different tolerances and more open clearances.

I would test the system vacuum with a Manometer. You need to be about 5 inches of water column. You may want to plan to replace the AOS again. Be thankful, it's not as hard to swap out on the Boxster as the 911. Remove the filter housing and tubing, examine the throttle body for any oil residue or potential pooling. As we know, these engines by design do consume a little oil, but to find the true cause of excessive oil consumption means hiring a trained professional to look deeper into the engine. Scoping the cylinder bores would be a easy first start. You could also look at engine operation using a diagnostic tool like a Durametric and of course a PIWIS. This would give you a better "look" at the vitals.


Then, I would recommend setting aside money for a rebuilding the engine or even full replacement. Remember everything on the engine is 20+ years old and starting fresh would mean many more years of smiles and fun memories, not the ones broken down on the side of the road. Happy wife = happy life

imhighlander

All excellent feedback. Thanks!

I do have Durametric and although the car has a "steady drip" of fresh oil, it's about time for a new filter so I'll move to 5W-40 oil and see how that affects things. I'll also inspect everything along the AOS system as I prepare to do another one of those to see if there's anything else suspicious. Will report back.

imhighlander

This is likely where we're headed. As I mentioned above, virtually everything except the motor and transmission (6-speed) has been replaced in the last four years of ownership. I'm inclined to reach out to Flat6 Innovations for a quote and may use the opportunity to bump to a 3.4 or 3.6.

While I'm thinking on that, any other recommendations for M96 builders who emphasize reliability and longevity?

ZuffenZeus

I'm very biased on that topic, hands down only Jake Raby and his team would touch my Porsche engine.

Porschetech3

Sorry I'm late to the party, I had to gather my thoughts on this one...

Firstly the UAOS will for sure lower the oil consumption by being 4x more efficient at removing the oil droplets from the blow-by.....

This is not usually necessary on a street driven car in good condition..
But it helps tremendously on a Track Driven car by allowing for increased oil level to combat oil starvation..( UAOS also has many other benefits)

The diaphragm is also twice as robust as stock and very to access on the 911 models..

But due to the low compression readings I would say your rings are carboned up...It only takes one long oil service interval to gum up the rings. Todays "low tension rings" are known to carbon up easily and loose compression/increase blow-by, the engines use a bit more oil by design... ...With only 55k miles I wouldn't think the rings to be worn out, so if the bores are not scored, then the rings are probably carboned up... I would suggest running a good cleaner in the fuel tank a couple of times, then test compression again....or opt for the blow-by test that I designed specifically for our M96 engines to confirm the ring seal/blow-by values...

imhighlander

AOS replacement underway and wow! This is so much easier than the 996 (since the motor is rotated 180 degrees). It's clear this unit had failed, considering the oil bath all over the intake and elsewhere. It's taken far more time cleaning up the oil detritus than getting the AOS out.

I've placed an order for new intake boots and a few other bits (and a window microswitch for the 996), so the Box is going to sit a few days until FCP delivers the goods to wrap it up.

Porschetech3

I wouldn't celebrate yet ...

With low compression readings it means there is increased blow-by !! ( if the low compression is due to poor ring seal, which is most likely the case, and not from poor valve seal).

High blow-by values place more burden on the AOS system, or if high enough it is impossible for the AOS to do it's job..

I have specific equipment and tests to test all functions of the AOS, if it is actually bad, you may be lucky...

Send me the old AOS, I can bench test it..

Mikus11

Drivers side - I'd be pulling spark plugs and inspecting bores. With that level of oil consumption and low compression I'd be worrying about bore scoring being near terminal. The proper way is to drop the oil pan and inspect from the rear with pistons at TDC but through the spark plug holes you will see it if its bad enough.

Terminal meaning you blow a chunk off the block and total the motor.

imhighlander

Will do, since I'm already "in there." It seems to me that if the bores are scored, the engine's terminal anyway, right? If there's scoring, would it be possible/worth it to break into the motor to sleeve the cylinders? Seems to me that a 20-year old engine with 125k miles on it - IF the bores are scored - is just a brick waiting to happen. I might as well drop the pan since I have some downtime this weekend and am waiting on a few parts but even if I do see evidence of scoring, I can't imagine it will be worth addressing; I'd probably just wait until she blows and load up on the new motor at that point. It will at least be an education.

8x57IRS

I would give products like https://toralin.com/toralin-compression-repair.html a shot If you do not discover scoring! I know from friends it works. There will be similar products on the market, I have no connection to "Toralin"