Lex_GTX

I have a very good understanding of engine, turbo function but I wanted to get some 951 specific experiences and insight as cars have common failure modes. Below is a description of the symptoms and I would like some ideas on what could be faulty or what to diagnose further.

I have a stock block with 130k miles, stock turbo, engine has been resealed and all hoses, gaskets, etc replaced. Engine is not rebuilt. So I have:
- No oil leaks
- No oil in J boot and no oil in hot pipe meaning it's not coming from the compressor or blowby
- Oil/ash deposits on the spark plugs
- Strong compression (145 on all cylinders) and less than 2% leakdown in all cylinders
- Oil consumption of 1qt for around 5-600 miles of Pennzoil 15w40 non synthetic.
- No smoke at startup, no smoke in rear view under vacuum or high RPM alone.
- Boost in the 12-14psi range.
- Smoke under boosted acceleration - blue smoke and strong smell.

One time when I drove the car especially hard I found some oil on top of the oil cap ... really not sure how that got there.

Dougs951S

I would start with checking AOS, you'd be surprised how much oil can get sucked through there. Also running a thinner 40 grade oil will increase consumption. Turbo motors have looser tolerances as compared to say, a 944 N/A. Even more so with a 130k engine. Oil deposits on your plugs would indicate to me worn oil control rings. You might be due for a hone and rering.

Lex_GTX

The AOS - what would I check there? Orings have been replaced and the hoses are brand new silicone. There is no oil in the Jboot or in hose going to the Jboot.

I bought V1 20w-50 oil for the next oil change. If the oil control rings were due for replacement, wouldn't I see the smoke at other times, and not just under boost? I understand if compression rings are worn, I would see it under boost due to blowby but leakdown numbers are great.

Lex_GTX

So in these cars is it common to just have the piston oil control rings fail and not the compression rings? Or are the valve seals more likely?

When started cold the car draws 10 inHg (cold idle) and once it warms up it draws 21 inHg at idle. Every indication I have points towards a strong compression and I am suspecting the valvetrain.

However it it was the guides I would expect smoke on deceleration and if it was the seals I would expect a puff of smoke when I start it cold. None of these happen - instead I get smoke under boost and ashy spark plugs deposits. Also are people reluctant to respond simply because they don't know, or because I am not a paying member on the site?

odb812

Based on this, I wouldn't say it's not the AOS. Is there any oil in the intercooler or the silicon coupler from post-IC pipe to TB?

Dougs951S

a light coating of oil in the IC pipes post compressor is normal, but any pooling is not. Lex, the lack of response has nothing to do with you not being a paying member. This site is here to aid 951 owners, be they members or not. I agree those vacuum numbers sound good, and if your issue was in the valvetrain you would have other issues. Failed oil control rings are not uncommon on this engine or any other engine, and it is not uncommon for them to go while the compression rings are still great. Its possible for them to seal well enough to not smoke when combustion pressures are low, but to allow oil through when you are on boost and chamber pressures soar. Again, you need to be aware that with 130k on the clock and you presumably not the original owner, you have no idea what kind of life that engine has had, or what the bores look like. I would suspect you are in need of a ring job. A lot has to do with how the car is driven. My engine has a tick over 60k miles on it, but it was a race engine for most of its life and it has poorer vacuum numbers than you are posting. Despite the low mileage, my engine is probably also do for a compression re ring. I get about 12 in hg cold, 18 fully warmed up. 1000 rpm idle. Doesn't smoke one bit or use an unusual amount of oil.

Lex_GTX

Thanks for the great responses guys.

I don't have much blowby at all from the turbo side or AOS - actually the hoses are dry. I do see a film on the cold pipe from the IC but nothing in the throttle body. The only place I see signs of oil usage is on the spark plugs. When I pull them out they are ashy and the thread have oil on them - they all look pretty much the same.

If I pull the plugs out and look inside the holes with a cold engine, I see moist piston tops.

Doing a valve seal or valve job is simpler than a re-ring but I don't want to do this if it's not going to help. In most (more modern) engines I work with the compression will drop before oil consumption becomes an issue.

Another question - from what I've read the bores are not machinable. What can be done if they are damaged? And if they are not damaged can the oil control rings simply be replaced without machining?

Dougs951S

The nice thing about our engines is they have some of the nicest blocks around. They are really a metal-composite matrix made up of extremely hard silicone crystals embedded in an aluminum matrix. As such, our coated pistons do not actually touch the aluminum, but ride on this extremely hard silicone. It is very normal for high mileage engines to show literally no wear on the bores after the head is pulled. When I pulled my head recently to do the head gasket, the crosshatching from the factory was still plainly visible. You would of course need to examine it, but it is entirely plausible you could get away with re ringing the engine without honing it. The bores simply do not wear unless something goes wrong. If they are damaged, most machine shops familiar with german imports can lap the bores to restore the hard silicone finish. Bores of this type have been used in BMW's and mercedes engines as well, and a competent shop should be able to do it for ~150 bucks. Also let me reiterate one more time, you most likely do not have a valvetrain issue.

Lex_GTX

Thanks for the great info. Do you also include valve seals as not being a problem when you mention valvetrain?

If rebuilding the motor I assume I should refresh the head as well.

Finally, is a thicker oil going to significantly impact oil consumption? I am on 15w40 and have 20w50 oil on the way.

Lex_GTX

Also I am not a believer in "snake oil" but if the oil rings are seized would something like a seafoam treatment help break down the carbon?

Dougs951S

bad valve stem seals will smoke under decel and high vacuum. A head refresh is not a bad idea at your mileage as a "while you're in there" thing. Thicker oil will definitely decrease consumption. 20/50 is really the proper grade for our cars and with our loose 80's turbo tolerances made looser with age and use, it will certainly help.

Lex_GTX

Excellent thank you. When is oil consumption considered unacceptable? I know what the manual indicates but I am asking more from experience as people seem to consider all kinds of numbers acceptable.

The exhaust smoke under boost (carbon and oil) can be a little embarrassing .

rlm328

The oil seal in your turbo may be giving up the ghost and you are getting oil in the turbine side and blown out that way.

Lex_GTX

Could be the case as well but the ash deposits on the spark plugs make me believe at least some of it is being burned in the combustion chamber.

cruise98

Describe this ash deposit on the plugs better.

I think it is your turbo. With those leak down numbers, I think the rings are fine. It does not take much oil to smoke a lot. If that is the original turbo, it makes me think turbo even more.These kinds of situations are difficult to diagnose.