2005 Boxster - cyclinder head plug / AOS / high crankcase pressure

[rathayes]

Hi, I am a new user and a bit of a rookie so pardon me if I have missed a similar response in a similar thread but I am in need of some guidance and decision on path forward.

I have an '05 Boxster base with Tiptronic. ~ 126k miles.

Current situation is I lost ~ 2 quarts of oil due to a cylinder head plug 996.104.216.02 behind driver side coming off. Car is at dealer and they think it is the AOS and quoted me ~ $1000. But I don't think they are convinced it's the AOS and say "it is acting like the AOS". Here is where it gets a little complicated:

AOS blew in May 2018 and was replaced at a local shop. Smoke, ran bad, had it trailered to local shop. Had to go back as MAF was fouled. They cleaned the MAF and it has been more or less ok since.

The local shop will warranty it and replace the AOS if they agree it is bad. But their opinion is that a failing AOS would increase vacuum, not increase pressure, so if anything a bad AOS would act to hold the cylinder head plugs in, not blow them out.

So in December 2018 or early January 2019 I resolved a long standing rattle noise that was the oil pressure regulator. I replaced it with the new style tapered piston head (ENU1726 is the bulletin that was released ~ 2006). I also found the piston spring broken so I replaced it along with new metal gasket. I did this myself. Same local shop that did the AOS had diagnosed the noise as tensioner pulleys. The recommened a $1700 repair. Google search with "2200 rpm" and "rattle" and "boxster" led me to the TSB, which solved the noise.

A few weeks later I noticed a small oil leak. I thought well, I did something to the car, so this leak is probably related. So I took it in to dealer (since I felt the local shop had misdiagnosed the rattle and gave me an estimate of $1700 for something that cost $30 in parts). Dealer cleaned the oil up (external mess) and couldn't find the leak.
They sent me away. They confirmed the oil pressure regulator bolt was tight and was not leaking. I shared with them that I had replaced it and asked if they thought it could somehow contribute to issue. They didn't think so.

Next day, bad sound at startup after work. No warning lights on dash and the car was running ok so I drove home. When I got home there was oil all over the underside of the car and on floor. Lost about 2 quarts of oil. This is when i noticed the cylinder head plug - I dropped the plastic under body cover to do a visual inspection.

So:
- pay the dealer $1000 and hope they are right about AOS. Hopefully I have some recourse if they are wrong on the diagnosis.

- risk plug coming out and driving an hour to local shop who may do same for "free". But local shop may come to a different diagnosis and I end up paying for some other repair.

My real question to the group is:
How likely is a bad AOS the cause of high crankcase pressure that is in turn forcing the cylinder head plug to come out.

Thanks! (hope you read this far)

[JFP in PA]

Originally Posted by rathayes

Hi, I am a new user and a bit of a rookie so pardon me if I have missed a similar response in a similar thread but I am in need of some guidance and decision on path forward.

I have an '05 Boxster base with Tiptronic. ~ 126k miles.

Current situation is I lost ~ 2 quarts of oil due to a cylinder head plug 996.104.216.02 behind driver side coming off. Car is at dealer and they think it is the AOS and quoted me ~ $1000. But I don't think they are convinced it's the AOS and say "it is acting like the AOS". Here is where it gets a little complicated:

AOS blew in May 2018 and was replaced at a local shop. Smoke, ran bad, had it trailered to local shop. Had to go back as MAF was fouled. They cleaned the MAF and it has been more or less ok since.

The local shop will warranty it and replace the AOS if they agree it is bad. But their opinion is that a failing AOS would increase vacuum, not increase pressure, so if anything a bad AOS would act to hold the cylinder head plugs in, not blow them out.

So in December 2018 or early January 2019 I resolved a long standing rattle noise that was the oil pressure regulator. I replaced it with the new style tapered piston head (ENU1726 is the bulletin that was released ~ 2006). I also found the piston spring broken so I replaced it along with new metal gasket. I did this myself. Same local shop that did the AOS had diagnosed the noise as tensioner pulleys. The recommened a $1700 repair. Google search with "2200 rpm" and "rattle" and "boxster" led me to the TSB, which solved the noise.

A few weeks later I noticed a small oil leak. I thought well, I did something to the car, so this leak is probably related. So I took it in to dealer (since I felt the local shop had misdiagnosed the rattle and gave me an estimate of $1700 for something that cost $30 in parts). Dealer cleaned the oil up (external mess) and couldn't find the leak.
They sent me away. They confirmed the oil pressure regulator bolt was tight and was not leaking. I shared with them that I had replaced it and asked if they thought it could somehow contribute to issue. They didn't think so.

Next day, bad sound at startup after work. No warning lights on dash and the car was running ok so I drove home. When I got home there was oil all over the underside of the car and on floor. Lost about 2 quarts of oil. This is when i noticed the cylinder head plug - I dropped the plastic under body cover to do a visual inspection.

So:
- pay the dealer $1000 and hope they are right about AOS. Hopefully I have some recourse if they are wrong on the diagnosis.

- risk plug coming out and driving an hour to local shop who may do same for "free". But local shop may come to a different diagnosis and I end up paying for some other repair.

My real question to the group is:
How likely is a bad AOS the cause of high crankcase pressure that is in turn forcing the cylinder head plug to come out.

Thanks! (hope you read this far)

Very likely, have seen it multiple times. But first, some other points:

Your "local shop" replaced the AOS, but with which one? Aftermarket AOS are JUNK, often failing right out of the box. Astute shops do not use them for that reason. If they installed one, you may be looking directly at your problem.

The MAF was fouled? How? It is up stream of where the oil from a bod AOS enters the intake, making it impossible for a failing AOS to foul the MAF.

A dying AOS tends to decrease the amount of available crankcase vacuum due to increased blow by from the low tension piston rings losing seal, not raise it, which probably why you blew out the cam plug due to pressure in the crankcase.

Here's my diagnosis: You need a new shop.

Good luck.

[maytag]

Originally Posted by JFP in PA

Very likely, have seen it multiple times. But first, some other points:

Your "local shop" replaced the AOS, but with which one? Aftermarket AOS are JUNK, often failing right out of the box. Astute shops do not use them for that reason. If they installed one, you may be looking directly at your problem.

The MAF was fouled? How? It is up stream of where the oil from a bod AOS enters the intake, making it impossible for a failing AOS to foul the MAF.

A dying AOS tends to decrease the amount of available crankcase vacuum due to increased blow by from the low tension piston rings losing seal, not raise it, which probably why you blew out the cam plug due to pressure in the crankcase.

Here's my diagnosis: You need a new shop.

Good luck.

I've recently worked-through some odd AOS and Case-Plug related issues myself, so I feel like I have some FHE here.
I agree with everything JFP said, with only one exception: When my case-plug blew-out, the AOS began pushing liquid oil into the intake at such a prodigious rate that I did indeed have oil (liquid oil) on the air-filter element, and dripping out of the bottom of the airbox. remember what happens when you close the tb; any liquid in the upstream at that moment (including any in the AOS tube to the intake) is going to get pushed-back-up. Fouled-MAF is easy, at that point. It happened to me.

I'd put money on a failed AOS, and, like JFP mentioned, it's likely that it was a cheaper aftermarket part, rather than the genuine article. The failure-rate demonstrated on the internet of those cheaper items should make any decent parts-source embarrassed to sell them.

If it were me, I'd replace the AOS and the case-plug and see where you're at. And if you ask me: if you can replace the oil regulator spring and piston, you can replace the AOS. it's not a horrible job.