Smoking (Not on Overrun)

[John Clarke]

On our journey To and From the Porsche Club event at Hedingham at the Weekend, The 356 Engine (616/40) /Late 912 Has started to Smoke again.. But only at the Mid to High rev range. (3500 and Above) On the way to Hedingham I got Smoked out from Cabin fumes !!
On a Trip of Only 117 miles On the Return Journey Home, Oil Level Topped Up to the High Level Mark before departure, after taking a Coffee Break at approx 100 miles, I checked the Oil Level Again. It was exactly down to the Low Oil Level !!!
The Engine runs fine and pulls strongly.
I didn't notice this problem on the French trip down to Le Man 24 There is more history on this, but I will come to that next. What causes the Oil to be sucked out of the engine into the exhaust on a High Speed Overrun ?
TIA for any help Jay

[Wes Bender]

Sounds like it's time for a leakdown test, Jay.

[David Jones]

Do a leak down test. It's possible that ring gaps have lined up on one cylinder. Happened on my cabrio. I pulled the engine popped off the cylinders and realigned the rings on all cylinders although only one cylinder was at fault. My issue though was excessive smoke on start up. Very embarrassing.

[John Clarke]

Thanks Wes and David for your very prompt replies
I would have to research the Timeline but Probably only in the last 3 years. We had ring wear problems So We had the Barrels and Pistons replaced with Shasta 1620 cc larger Barrels. Can't really call these Big Bore. But after only a few hundred Miles It had to go back cos it was still smoking. After a Leak Down Test, One of the Cyl Heads which were replaced at the time had a fault.Not sure why but One Head was replaced under warranty.
There does seem to be some debate regarding the use of Valve Stem / Guide Seals or Not using them !
It has got to go back again ! as at 74 I just can't heave out and replace Engines anymore .
Does the Leak Down Test "Tell All" ? Do they pressurise and see how long it holds (Or Doesn't)
That Pressure. Much better than a Compression Test ? We have only put about 1500 miles on the Pistons Barrels and Heads.I chose the Shastas over the AA barrels that were to be fitted ! Disappointed doesn't Cover It !!
Excuse my lack of knowledge
Jay

[David Jones]

A leak down test will pinpoint where the problem lies except it will not tell you if you have worn valve guides. It is limited to pinpointing leaks from valve sealing and ring sealing, or head to cylinder seal.

[Martin Benade]

Leak down is useful but is more about compression than oil sealing. Loose valve guides is the likely suspect here since you you have young top quality p/c. And most of us get reasonable oil consumption without using valve seals.

[Geoff Fleming]

Worn valve guides will cause the exhaust to smoke after the car is taking off from rest, say, at a traffic light.
Unless seriously worn, usually not much smoking at higher revs.

[C J Murray]

One quart every 100 miles is a dangerous degree of oil consumption. I wouldn't even drive the car in that condition. It will only get worse and possibly catch you out with no oil. I am surprised it isn't fouling plugs constantly.

Do the leak down and then take the engine apart because whatever is wrong will be obvious.

You just had it apart so... Did you scrub the cylinder walls with soap and water until they could be wiped with a white paper towel and show no dirt? Did you carefully set the piston ring gaps? Was the cylinder head rebuild done by a shop that has extensive air-cooled experience? They didn't do any old-school idiotic repairs like knurling the guides or using guide liners did they? Were the valve stems carefully measured for wear? "Saving money" on cylinder head repairs is a very bad idea. Are your carburetors ultra rich to the point that they have washed down the cylinders and ruined the ring seal?

Either the machinist made a mistake or the assembler did.

Even if everything is done correctly the vehicle operator has to break in the rings correctly which means a series of full throttle accelerations keeping within the mid range rpm.

[John Clarke]

Hi Cliff
Don't know Anything, as I didn't do the work !
All I do know is I payed a lot to have the work done.
And it Still Smokes !
It Went back and One Cylinder Head was replaced..
Still not right !! On the way to the Porsche event at Hedingham I opened It up some as I got on the Freeway, up to about 3500 /4 rpm looked in the rear mirror and saw a haze, thought it was someone else! and as I lifted off the throttle the smoke billowed. Thought it must have been a broken ring
Will find out , as it's going back to the Shop !
The outfit that does the head work is a trusted source apparently ! Will get that Leak Down first and see what that shows up. To get back home I fed it's oil habit. Not good obviously but the car drives and pulls very well! No noises, just getting poisoned in the cab !
Just waiting on transport back to the workshop.
Thanks for the input.
Cheers Jay

[Harlan Halsey]

There are two competent Porche shops I know of even from over here. Andy Prill (I met Andy at the MHAR) or PRI-Mike Smith (Whom I know through Carrera stuff.) I would go to one of these if I were you. (If I didn't do all my work myself.) 356 Porsches are not something commonly understood anymore as you seem to be finding out.

[Vic Skirmants]

Mike Smith is PRS.

[Mathieu Lecomte]

Any chance you have an oil leak that would deposit on the hot engine or exhaust tubes, creating that much smoke (and the high oil usage/consumption) ? Have you checked your engine compartment and lower part of the engine to see if there is oil ?

[David Jones]

If that were the case it would smoke all the time not just on the overrun.

[John Clarke]

Hi David. I was only going by what I was seeing in the rear view Mirror regarding the Pluming smoke. It seemed to be more intense as I came off the throttle. But on further investigation, there is loads of Oil on the Bursche Exhaust and Silencer which on burning off at high temps would give the impression of a major engine problem. Think it's a leak from the Crank Pulley Oil Seal as Mathieu suggested. Hope So, as that Is an easier Fix
Will report back
Jay

[Olivier Auvray]

If that were the case it would smoke all the time not just on the overrun.

Except if it leaks at a certain value of oil pressure (or engine case air pressure).