Hi everyone,

not long ago my engine started burning oil, smoking and soon after, rattling. The engine is now apart and being rebuilt. Two of the cylinders and pistons are scored. A few hundred miles before the engine failed, the revs would dip below 400 and then shoot over 1000 before settling, but only when coming to a stop. This gradually got worse, and once the engine even stalled. I have not been able to get any codes out of the car, but do you think that this could have caused my engine to fail?

Thanks!

PS the engine has not overheated and has always been topped up with fresh oil.

 

This is often a symptom of worn main bearings. I have not seen this happen in a Porsche yet, but it is conceivable, and would explain your cylinder scoring.

 

Hard to know in what order things happened. Knowing the condition of the various internal engine components, bearings, crank/rod bearing journal diameters, any taper, the condition of the oil pump and its housing, etc. can provide a clue.

In short a close inspection of the engine's internals can go a ways to providing a clue as to what happened in what order.

Since you report the engine did not overheat then I will assume the answers to the following questions will not shed any additional light, but I'll ask them. Was any water (coolant) found in the engine oil? Was any oil found in the coolant? Was the water pump impeller intact? Was the cooling system leak free? Did you run a good oil? (IOWs, no 5w-20 oil.)

The engine was burning oil and smoking (I assume all the time?) thus it must have been burning a lot of oil. Some owners have reported their cars using around 1 guart of oil in 1000 miles (or even less miles) and AFAIK not one has reported any signs of any oil smoke other than the brief puff (cloud) at some engine starts.

Let's see:... burning oil, smoking (related) and rattlling. This rattling probably comes from one or more (you said there were two scored cylinders/pistions) worn pistons/cylinders.

How might this have come about?

Well, just plain old wear. Some engines can run nearly forever while others can't. One area where wear can occur in fairly short order is of course at the piston/ring/cylinder interface.

Now the wear might have had some help. If the conditions were condusive to accelerated piston/cylinder wear it could have also led to wear in other places. Since you did not mention cam/lifter issues, wear could have happened at the main or rod bearings.

If one or more bearings developed some over the limit wear this can represent an internal oil leak.

Oil pressure can drop and the oil pump may not be able to supply enough oil to keep the piston oil jets fed. If the oil pressure drops too low these are shut off to reserve oil for other more critical areas.

Thus is might have been possible due to low oil pressure -- but not too low for I assume you never saw any oil pressure warning light? -- the pistons could have been 'starved' for oil.

This means that not only do the pistons receive less (or no) oil spray when the oil pressure is too low and they have less lubrication they also run hotter, adding insult to injury.

This could account for the scoring of some cylinders/pistons.

Sincerely,

Macster.

 

I might have had a few ideas to banter around but wow, I'll defer to the gentleman from Livermore. Great synopsis.....

 

Helping to tear an engine (or tearing one down on my own) was always a good learning experience for me. Since I no longer do this about all I can do is talk or read about it.

Post your ideas. We can all learn something.

Sincerely,

Macster.

 

Was it a water cooled Porsche engine? Were you able to determine the year?

 

I just recently had this issue with similar problems, with the exception of the burning oil, then I found my thrust bearings as twisted pieces of metal in the oil pan!

 

I just recently had this issue with similar problems, with the exception of the burning oil, then I found my thrust bearings as twisted pieces of metal in the oil pan

 

Thanks everyone for the replies!

Macster:

There was no water in the oil or oil in the water. Engine was running in 5w40 Motul X-cess, water pump was fine and there were no coolant leaks. Up until the engine failed oil consumption was around 500ml per 10,000 km. Very suddenly it started smoking and rattling at 160,000 km. I am hoping that the problem with the revs was a result of the engine failure and not vice versa, as I do not want to kill my newly rebuilt engine while getting to the bottom of the problem.

 

The fluctuating RPM was likely caused by your worn bearings.

 

OK I'll take a guess that your 996 has a 3.4 since they have the thinnest cylinder walls making them prone to excessive cylinder wear. Regardless of which engine be sure cylinder wear is properly measured & if out of spec it will need to be sleeved to restore the cylinders.

 

Yes, the engine is a 3.4 ...

I'll contact the shop doing the rebuild and ask them about the condition of the bearings and let you guys know.

 

Doubt the bearings will have anything to do with the problems you posted. How many miles on the engine?

 

Huuuuh????? These are EXACTLY the symptoms I had when my thrust bearings failed!!! IDENTICAL!!!

 

100,000 miles