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Mysterious 993 oil consumption

Just when you thought you were safe from yet another post asking for advice on excessive oil consumption, here we go. This is my 1995 993 with Protomotive Stage III Twin Turbo mod. Engine was overhauled less than 2,000 miles ago with used pistons and cylinders. Turbos are overhauled and have oil check valves with no evidence of oil making its way into intercooler.

The problem is 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.

Cylinder leakdown showed all cylinders <= 5% except #3 and #5 = 12% (pic #1 and #2)

I borescoped each cylinder and most showed oil puddling at the bottom of each cylinder wall. The pistons and valves showed carbon deposits, heaviest on #3 and #5.

So I finally pulled the engine out and disassembled it 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

This means that based on the average piston diam, 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 Dempsey book the maximum allowed is 0.1mm so I'm about half way there but still good?

Valve wobble is 0.1mm (all intake valves) and 0.2mm (all exhaust valves) so well within maximum of 0.8mm, rules out leaky valve guides. Valve seals look fine.

Upon disassembly, cylinder walls are still show honing crosshatch pattern, with no evidence of any piston or ring damage. No metal shavings found in oil drain plug magnet.

So... my 2 questions to Forum members are:

1) am I correct to think all the above is within specs and should not be cause for excessive oil consumption?
2) is the only logical explanation that the rings did not seat properly on break-in after the 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 since read that driving hard from the beginning is better. I would be interested to hear if improper break-in can cause such oil consumption.

Thanks for your input!
Peter



Old 04-28-2017, 01:49 PM
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It isn't so much the term "drive it hard", but more correctly, you do need to load the rings in order to get them to seat in properly.

Below is the basic procedure for an American single cam, push V8, so not totally applicable for our overhead cam engines, but still basically sound advise non-the less.

1.} First the obligatory 20 minute run in at a set 2000 rpm {no driving} to break in the cam and settle all the bearings.

2.} Then start the ring seating procedure with a 1/2 dozen 15-30mph 1/2 throttle pulls to start putting load on the rings.

3.} Then let the engine cool down a bit by continuing with a 5 minute easy drive.

4.} Then finish up the procedure with another 1/2 dozen 30-60 mph 3/4 throttle pulls.

5.} Cool her down again, and drive the next 500 +/- miles never exceeding 3/4 max rpm.

This was more or less the standard I was taught in school, and has worked for me for many years and many engines.

Mark

Last edited by full quack; 04-28-2017 at 04:28 PM..
Old 04-28-2017, 04:13 PM
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I would suspect faulty valve stem seals, which would explain the intermittent smokey startup. When intake valves are open on the affected cylinders during shutdown oil leaks down the valve and into the combustion chamber. I had this issue on an engine I built with fresh cut valves on a rebuilt head which tore the stem seals when they were installed.
Old 04-29-2017, 01:30 PM
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fullquack thank you for the break-in steps.
Old 04-30-2017, 05:43 AM
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ChrisD87 this is helpful I've ordered new seals.
Old 04-30-2017, 05:44 AM
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So I was told today that there are three --not just two-- ways oil can get into a 993 combustion chamber:

1) through valve seals and valve guides
2) through piston rings
3) sucked in from the tank through one of the vent hoses on the intake manifold.
#3 happens with oil overfill and is news to me.

I'm investigating!
Old 05-02-2017, 02:30 PM
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Quote:
Originally Posted by full quack View Post
It isn't so much the term "drive it hard", but more correctly, you do need to load the rings in order to get them to seat in properly.

Below is the basic procedure for an American single cam, push V8, so not totally applicable for our overhead cam engines, but still basically sound advise non-the less.

1.} First the obligatory 20 minute run in at a set 2000 rpm {no driving} to break in the cam and settle all the bearings.

2.} Then start the ring seating procedure with a 1/2 dozen 15-30mph 1/2 throttle pulls to start putting load on the rings.

3.} Then let the engine cool down a bit by continuing with a 5 minute easy drive.

4.} Then finish up the procedure with another 1/2 dozen 30-60 mph 3/4 throttle pulls.

5.} Cool her down again, and drive the next 500 +/- miles never exceeding 3/4 max rpm.

This was more or less the standard I was taught in school, and has worked for me for many years and many engines.

Mark
Mark would you still recommend the 20 minutes at 2000 RPM if the only thing replaced were piston rings (everything else except gaskets and valve seals are re-used)?
Old 05-27-2017, 11:39 AM
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No, in your situation I would skip the cam break in step.

That is really just needed for a new camshaft{s}, and in particular, in a pushrod style valve train.

Our P cars do not have pushrod style vale trains & your engine does not have new camshafts.

So you would just be wasting gas :}

Mark
Old 05-27-2017, 09:28 PM
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If #3 is an option you should find plenty of oil in the intake system. My money would be on valve seals too. It is about the only way oil can drain in on a stationary engine (blue smoke on start up).
Alan
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Old 05-27-2017, 09:46 PM
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