oil consumption in 1985 Porsche at 171K

[glenncof]

Vehicle: 1985 Porsche 911 – at 171K miles w/engine never opened note: 12 qt oil capacity Background: OCI was 7500 miles under 100K. Consistently used Castrol 20W50 since 100K miles, OCI of 3000 or less. Using Mahle OC-54 filter (factory).. This engine is known for strong lower end but soft valve guides (which leads to oil consumption). Current oil consumption after oil change; 1 quart for initial 1200 miles, 1 quart for subsequent 800 miles, thereafter 800 or less miles per quart. I’ve done this check many times including checking every 100 miles after standard course. Porsche says 600 miles per quart is typical. This is the company that recommended 15,000 OCI for this car ! My questions: 1) What is the likely cause of this increase in oil consumption from the initial 1200 miles to the subsequent 800 miles ? 2) It appears 7500 miles OCI did not completely prevent wear. How could a company like Porsche recommend 15,000 OCI ? Comments ?

 

[bighead]

Sorry, I have ZERO knowledge on Porsche's, but, I'm flippin over the 12 qt sump Maybe they know it will consume so much oil that you'd have to constantly topoff, therefore able to extend the OCI to 15K... Again, sorry but I don't have much if anything to contribute, but rest assured, there will be more than a few who know more and will be able to help.

 

[IslaVistaMan]

Shearing and fuel dilution would be my guess. Have you done a UOA to see the viscosity and dilution values?

 

[Ken E.]

Pardon my asking but would you expect 7500 OCIs to "completely prevent wear?" I would expect some wear after more than 170K miles. It's also entirely possible that that oil consumption would be exactly the same with 15000 mile OCIs and less recycled oil.

 

[double vanos]

First I have to ask how well does your car run in its' current condition (171k)? If it runs well I'd say you are doing pretty darn good considering (I'm willing to bet) you drive the pi$$ out of it. I have no ill will against Castrol products and in fact know several that swear by the GTX series with 300k+ vehicles to prove it. Here's what I think are the answers to your questions: 1.)The first loss of a quart in 1200 miles represents approx 8.3% loss of lubricant. Thereafter the 8.3% loss only requires 800 miles to happen (I assume you top up when a 1 qt loss is observed). And it appears that as you go along past that, the consumption increases albiet in very little increments. Although Porsche says that 1qt/600 is typical, I wonder if your oil is enduring a slow fuel dilution (injectors drizzling the fuel instead of atomizing it). You add a quart when down but that new qt is being added to an increasingly fuel diluted original oil pool. I have seen engines that appear to be running great when in reality they had injectors that were shot. This was discovered when new injectors were installed and "whoa nelly, what have we got here???!!" The car was transformed!! It turned out the loss of performance happened so slowly it wasn't noticed. I would have the oil analyzed when the next OCI comes up and pay particular attention to the fuel dilution aspect of it. 2.)Porsches' recommendation of 15k oil changes I think reflects back to the manufacturers exuberence with the revolutionary synthetic oils of the time. 15k OCI's are a terrific sales point (cheap to maintain) and I'm sure in some instances, in the lab, the engineers actually got the engine oil to make a 15k OCI without harvesting asphalt out of the crankcase. But I'm willing to bet the sales arm of the company won out over the engineers and the 15k OCIs were initiated with the engineers getting to add "600 miles per quart oil consumption is normal" as a sop to the techies that built the dam thing. But let's fall back and look at the big picture: you've got a Porsche that never had to be cracked open ever with 171k miles on it. On a 12 quart sump over 7500 miles BOCI your oil consumption is quite managable - it is a Porsche after all, meaning that it is tuned to run and if it's anything like the Porsches I've ridden in it begs to be flogged. In retrospect I think that 5k oil changes would have yielded a lowered wear rate on the engine (perhaps - who knows?) and if you decide to get an oil analysis done perhaps a top end overhaul will be indicated by it and if you get that done you probably won't believe the difference in performance. Let me close with this: Porsche now recommends 20k oil changes on it's current crop of cars and I think that is insanity!If I were to shell out the kind of money that Porsches are going for now a days you can bet your booty it would get 5k oil changes with the best syntheic oil money can buy! Sorry for the long diatribe but I think you've gotten terrific service out of the car so I just had to chime in.Cheers DV

 

[Thatwouldbegreat]

If this were my car, I'd try a fresh fill of Castrol GTX 20W-50 and 2 bottles of Auto-RX. See www.auto-rx.com Run for 1500 miles and then rinse with Castrol GTX 20W-50 Then fill the engine with Redline 10W-40 or 20W-50 and change every 5K to 6K. If you don't like Redline then try Mobil 1 15W-50.

 

[PulMax]

quote:

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Originally posted by bighead: Sorry, I have ZERO knowledge on Porsche's, but, I'm flippin over the 12 qt sump

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If I'm not mistaken, the oil in an 'air-cooled' 911 basically acts as lubricant and coolant, hence the huge capacity. I think later ones held 14+ quarts.

 

[Steve S]

Porsches are more like a race car than a daily commuter car .Do a compression test dry and wet then go from there . Bring the car to a good shop for diagnosis . Porsche 911's are neat cars.

 

[ChrisW]

Valve seals aren't too hard to check either...

 

[mechtech]

Porsche's come with Synthetic oil now, did yours in '85? That massive capacity sump also adds to oil life. This is why they recommend long oil change intervals now. A 21 year old air cooled engine with 176,000k miles on it that uses some oil is no mystery to me.

 

[vizvo]

I vote with ChrisW on this. Valves seals are more often a cause of oil burning than ring wear, because the vacuum created in the cylinders very easily sucks oil past worn valve seals.

 

[02Pilot]

BMW 2002s (I have a 1974 with a heavily modded engine) have similar issues with the valve seals/guides. My car does not use a measureable amount of oil between changes, but it does smoke a bit on overrun or when engine-braking, especially downhill. FWIW, I have seen notably reduced oil smoke using a mix of 4 qts of Delo 15w40 and 1 qt of 20w50 (Castrol, Valvoline, whatever I have handy) over straight 20w50. Higher oil pressure across the board with that mix as well.

 

[Tosh]

I believe valve guides are inherently the first parts to wear out on any normal engine. The small surface area, high temperatures, reversals of motion, (high velocities?) and strong vacuum all contribute. Something has to be first to go...