95twinturbo

I have a 1995 993 with Protomotive Stage III Twin Turbo mod. Engine was overhauled less than 2,000 miles ago with used pistons and cylinders. Turbos were also overhauled with check valves added.

The car produces a large blue smoke puff on startups (2 times out of 3, no rhyme or reason for when) and uses about 1 quart oil per 400 miles, but zero exhaust smoke while running. No visible oil leaks. No sign of oil making its way from turbos to intercooler.

Cylinder leakdown shows all cylinders <= 5% except #3 and #5 = 12%.

Before starting a top end, I borescoped each cylinder and several showed oil puddling at the bottom of each cylinder wall. Pistons and valves showed normal carbon deposits, heavier on #3 and #5.

So I have engine out and disassembled for inspection. Here are the measurements (all in millimeters):

Cylinder diams (measured 56mm from top per WSM):
Measured 90 deg from pin alignment:
100.05625
100.05625
100.05625
100.05625
100.07500
100.05625

Same as above but turned 90 deg (same direction as pin):
100.03125
100.03125
100.03125
100.04375
100.03750
100.03750

Piston diams measured at bottom of pin bore:
99.99500
99.99500
99.97000
99.99200
99.99500
99.99500

So based on average piston diam measurement, the largest piston-to-cylinder clearance is 0.07375mm (cylinder #3) and next largest is 0.06125 (cylinder #5), with others all at <= 0.05mm. According to Wayne book, maximum allowed is 0.1mm so I;m about half way there, but still good.

Valve guide play as measured per WSM is 0.1mm for all intake valves and 0.2mm for all exhaust valves, so I'm well within the maximum of 0.8mm. The valve seals look fine but will be replaced.

The cylinder walls are still showing crosshatch with no evidence of piston or ring damage. I didn;t get a chance to measure ring land gaps but will with new rings. No metal shavings found in oil drain plug magnet.

Am I correct to think all the above is within specs and should not be cause for excessive oil consumption, so the only logical explanation is rings did not seat properly during break-in after last rebuild? I drove at less than 4,000 RPM's for the first 500 miles on mineral oil and tried to frequently change RPM's but didn't drive it too hard.

Thanks for your input!
Peter

NineMeister

Driving at low rpm and part throttle will not do enough as you need cylinder pressure forcing the rings into the bores in to bed in the piston rings. Ideal ring break-in is to use maximum torque for very short periods, keeping the engine oil cool and allowing the combustion chambers to cool down between full throttle squirts. Start at lower rpm on boost, then after 30 minutes or so be up at peak power rpm to do same. We usually do this on a dyno before the car hits the road, you may not have that choice so be very careful where you run.

95twinturbo

Thanks Colin for the advice, I don't think I stepped on it enough when I broke it in.

Churchill

Your rings didn't seat. I would be careful how you proceed now. The p/c's were used when you did the build, and now they have 2K miles on them of non-seated driving. They're probably glazed. I would at a minimum bubble-hone the cylinders, or better yet send them to a machine shop to be freshened up. And reassemble with new rings.

pp000830

Could you be overfilling the oil and the excess oil is being fed into the intake?

95twinturbo

Could be, cylinders still have some cross-hatch but minimal, I re-honed them and will be putting in new rings before reassembly.

95twinturbo

I've kept the oil level between the dipstick marks and my valve guides and seals all checked out. If there is excess oil, I don't know how it could make its way to the combustion chamber other than through the rings.

pp000830

Overfilled oil is sucked directly into the air intake through the intake manifold. 993 engines are notoriously easy to over fill. Some find that the dip stick reads too low even when checking the engine oil fill correctly with a hot engine. Pick up the intake manifold, do you feel anything beyond very light oil residue inside of it? Slight oiliness may be normal as part of the emissions recirculation function. If it is wet overfill could be your issue.

95twinturbo

Very interesting. One of the first things I checked when taking engine apart was the MAF sensor to see if oil had made it way to it. No oil found there, so I assumed that was the end of it. I didn't think to check the intake manifold downstream of the MAF sensor. After reading your post I went and checked the inside of each of the six plastic intake risers and they all have a film of oil, I would say closer to wet than a residue so you may be on to something here.

Do you have any suggestion for troubleshooting once the engine is back together? In the aviation world oil overfill gets vented out directly into the atmosphere via an open hose and usually leaves a greasy belly that's easy to spot, here the attached WSM intake diagram shows rubber hoses labeled D and E both "to the oil tank", is that what you're talking about and would it make sense to insert a clear hose and see if there is oil passing through when I have the engine back together and running? Since both hoses are connected to the intake manifold, are they be under a relative vacuum on acceleration, which would help suck in oil? Also do you know how to make sure the oil dipstick reads correctly with engine warm?

Thanks for your insights!

pp000830

Not sure how to adjust for this. Some owners fill the oil a quart short and then top off a hot engine just until the oil touches the tip of the dip stick. Some fill the oil when hot until the gauge on the dash registers half way and they note that as their fill level.

These cars hold a lot of oil so filling the oil to the absolute maximum level is not really that important. You will know if over-fill is your issue when you fill it a little short and your oil consumption seems to fall off dramatically.

I recall one owner who's consumption went from a quart every 500 miles to a quart every 2500 miles. There has also been some discussion that overfill contributes to air injector clogging or is misdiagnosed as needing new valve guides.

Having the gauge on the dash float around a little vs being pinned on full also gives a little additional feedback as you use the car. In my case on my 96' Carrera I change out both filters, I drain the tank and the crankcase, I do not unhook any lines to drain them and it takes less than 10 quarts to fill the engine measuring it when hot. If you are getting near 10 quarts or even more you are probably overfilling. There are other threads on Rennlist discussing this topic.

Is is possible to adjust the dash gauge by removing and slightly bending the float arm on the oil level sender in the oil tank.

Not sure what path the blow by from the crank takes or the path of the overfill, others may give you some insight into this.

Lats but not least I am not a professional engine guy. I have never done any internal engine work so take my comments as a shade tree mechanics opinion in context to other's views.

Best of luck with this,
Andy

95twinturbo

Andy we need more shade tree mechanics like you!
Thank you again for your help!
Peter.

nine9six

Peter,
Pretty sure the over filled oil gets sucked up and ingested through the Secondary Air Injection (SAI) system.

A faulty SAI check valve is part of the problem.

95twinturbo

My SAI system was removed as part of the Protomotive conversion.

95twinturbo

UPDATE: as part of the Protomotive mod, the two oil tank breather hose connections were plugged at the intake side. At the oil tank side, one hose was fitted with a small oil filter-looking breather so vents to atmosphere. The other hose was just plugged. It also looks like the oil film I reported earlier is mostly in the intake risers --the further away from the heads, the less oil there is. So I think that eliminates the possibility that oil was overfilled, I'm back to the improperly seated rings scenario.

Churchill

You don't burn a quart every 400 miles because the tank is overfilled. This is a nonsense theory.