964enthus

Hi
before asking for your advice ,i have carefully sifted through the archives and particularly this thread :
https://rennlist.com/forums/964-foru...warm-up-2.html
-11990/C2
-huge cloud when I started after 4 weeks rest
-lasted 2 to 3 minutes ,then gradually disappeared
it is the very first time in five years it has done that
-no typical fine oil droplets or blue colour at the exhaust
-my impression was one more of gas than oil and I thought it could be a blocked open injector
-No problem with idle speed and /or acceleration though
-last oil change last year and no top up since
car in same place (flat).so,OVERFILLING CAN DEFINITELY BE RULED OUR IN THIS CASE
-stopped and retatarted an hour later :no problem
i would really appreciate your input /comments
Peter

AOW162435

Do you let the car idle for 30 - 60 seconds before shutting it off?

My '95 smoked a bit when I started it for the first time in the PO's garage five years ago. Never since.


Andreas

Steve Weiner-Rennsport Systems

A few thoughts,.....

911's that sit for several weeks will siphon oil from the tank back to the engine and when the oil level equalizes between these two, the bottoms of the cylinders & pistons will be partially submerged in oil, resulting a cloud on the restart. Its nothing to worry about as thats a characteristic of these flat, dry-sumped engines.

Don't assume anything: check & confirm proper oil level with the dipstick and make sure its at or below the middle of its range when the engine is hot. I cannot begin to tell you how many cars I've seen with overfilled systems caused by improper readings from the guage or dipstick. Some were totally filled to the intake ports and hydrolocked.

964enthus

to Andreas and Steve
- idle for 30 to 60 seconds before switching off : no ,not done it until now
I'll try that if problem reoccurs
-overfilling : not a possibility il levels checked and.... ;last oil change last year
-from what you both said ,you do not suspect a gas problem ?but smoke is white ,not blue ish like with oil ?
I understand the siphoning issue ,I think .if this is what is happening ,is there a way to prevent it ?
last question :may this sudden influx of oil or petrol have damaged the cat converter ?
Peter

Steve Weiner-Rennsport Systems

Peter,

JMHO,.........

Its not likely a fuel-related problem, however you need to check this for yourself since your car is a long way from our shop.

Remember, catalyst-equipped cars do not smoke that much as thats precisely what the cat is there to do. Obviously, its not good for these things to see excess oil or fuel as those two things will shorten the life of the cat. In the case of excess fuel, its dangerous.

Finally, due to the physical layout of the 911 engine and its dry-sump oiling system, its not possible to prevent the oil from transferring from the tank to the engine when the engine sits for several weeks or longer. Its simply the nature of the beast. Of course, you can drain the oil from the tank and engine (call it oil change time), however one must remember to replace it before starting.

Generally speaking none of these issues are major concerns except for the possibility of a leaking injector which can be a problem, but only with the engine running. For your own peace of mind, you may wish to remove the injectors and have them all tested for opening pressure and spray pattern.

Indycam

If that was a gasoline cloud you would be smelling a huge amount of gas .
You would also has some back fires no doubt .

That cloud was a good thing , your cylinders were wet with oil .
Wet cylinders are happy cylinders .

964enthus

many thanks yo you all for your precious -and reassuring- comments
If anything develops ,I'll inform the forum
I must say I am still puzzled by this "siphoning "after a few weeks inactivity:
lots of questions like :
-what is exactly happening ,through which pipe(s)..,up to what level
-why does it not happen over a -say 2 to 4 day period.....
-would keeping the oil level just above the minimum improve the situation
-ditto for parking the car at an angle ...
Pretty academic questions ,I know ...

Peter

darth

If the smoke is white and dissapates quickly it is most likely condensation which has formed in the exhaust and boils off when the engine is started.

Bill

964enthus

Hi Bill
defitely not condensation :I am talking about a HUGE cloud os smoke !!!!
Peter

Indycam

My guess is that the oil stored above the level of the oil pump ever so slowly drips back into the case via the oil line from the tank to the oil pump .
When enough oil has dripped back into the case its level gets up to the pistons .
Then the oil ever so slowly makes it past the rings .

It might make an interesting time lapse movie .

"-would keeping the oil level just above the minimum improve the situation
-ditto for parking the car at an angle ..."
I don't think those would work .
Long time ago I read a story about some poor farmers .
They didn't have anti freeze for their tractor so every evening when they thought it was going to freeze over night they would drain the water and take it inside . In the morning they would heat the water and pour it back into the cold motor .

Steve Weiner-Rennsport Systems

Generally speaking, thats very close,....
Fluids always find their own level and the tank always stores far more oil than the engine. Over a few weeks time, the higher oil level in the tank pushes back against the oil pump through its feed line (under the engine-mounted filter) and back fills the engine until the level in the tank and engine are more equalized. Being a horizontally-opposed engine, the resulting fluid level leaves the bottom 1/4 of the piston/cylinder assemblies in a pool of oil which floods the oil rings. On startup, you get a nice cloud for a quick moment until the pump re-evacuates the case and pumps it all back into the tank where it belongs.

I totally agree,...

You are correct. Fluids always seek their own level so absent any fluids, there is nothing you can do about it besides remove it.

crg53

Just pour a pint of Castrol R in the oil tank, that way every time you start up the car you will get a wiff from the past, "the finest smell known to man"

darth

Another source of oil entering the combustion chamber is via. the valve guides/seals. A test which may be able to determine if they are worn is during prolonged deacceleration, pressing on the accelerator is followed by a cloud of smoke in your rearview mirror. If Steve's analysis is correct and I don't doubt that it is. It may be a good idea when orienting the ring gaps on a rebuild, to minimize the number of gaps on the lower part of the pistons.

Bill

964enthus

thanks for the abundance of interesting info !!
one puzzling question still :
Fluids always seek their own level
if my understanding of the engine and lube lines is correct ,the oil in the oil tank has to go up before it goes down into the pump
in normal circonstances ,equalization of levels should thus not happen,unless of course there is a siphoning effect
am i Right?
if siphoning is indeed the culprit ,would a less- or more -viscous oil help?
Peter
att Darth : worn valves :defintely not ,all checked before and ,reminder ,this is the very first time this cloud appeared

race911

One of our regular autocrossers (when I used to do it, up to the mid-90's) used to bring out his 4-cam Speedster sometimes instead of his RSA. You could always tell the newcomers, as they were completely confused by the smell as the Speedster made its runs.