Mystery Oil Usage on My 993 - Page 2 - 911/Carrera GT - PistonHeads UK

Hello All, I am looking for help with a confusing level of oil usage on my 993 C2S. The car has 72,000 miles on it. It is using a litre of oil every 400 miles. In good roads with higher revs and wide open throttle this can drop to 300 miles, if bimbling it might stretch to 600 miles. There is zero smoke from the exhausts, zero oil deposits in the tailpipes (both look identical) nor on the rear PU. We have removed the lampda sensor from the exhaust and its completely clean. Leak down and compression tests are normal. So, it doesn't appear the car is burning oil. We have removed all the undertrays, no oil deposits, even looked at the oil lines on the sill that run to the radiator and can't see any evidence of leaks. There has never been any oil on the floor of the garage. So, it doesn't appear the car is leaking oil. Three different well known and respected specialists have looked in detail and can't find anything. So, I am turning now to the combined knowledge and smarts of Pistonheads. Any thoughts / ideas / similar experiences would be very gratefully received. Thanks in advance, Andy

Have you asked on the 911uk forum?

No, I haven't really been there before. I'll give that a go too. Thanks

What you describe is impossible. Unless the oil is only coming out in a jet as you drive along. So there is NO oil on top of the heat exchangers? And how are you measuring oil usage?

It does seem odd that there are no obvious symptoms to show the oil use? Initially I wondered if you were measuring the oil level correctly every time, as in mid way up the temp guage on a flat surface, but I have to assume the specialists you have visted know how to measure the oil level correctly on a 993. If new oil filters were fitted without filling them first, that could bring the level down by at least 500 ml as they fill up and do their job.

Orangecurry said: What you describe is impossible. Unless the oil is only coming out in a jet as you drive along. So there is NO oil on top of the heat exchangers? And how are you measuring oil usage? Thanks. Checked the top of the heat exchangers and initially found a tiny amount of residue but traced that to a weep from a cam cover but it didn't make a material difference once fixed. Measuring the oil level using gauge and dipstick and measuring how much oil of after how many miles to refill to the same level. it's been going on for some time so not a one of incident. I am at a loss to explain it.

If the oil isn't leaking there is only one way it is going out of the engine and that is via the exhaust. Only thing that springs to mind is the oil/air separator in the crankcase ventilation system not working properly, I would stick some white cloth over one of the tail pipes and run the engine to see if the cloth gets oily. You might not be able to see light oil consumption like this, not like a turbo blowing or knackered rings which is catastrophic and very obvious.

Have somebody follow you and watch for exhaust smoke on the overrun from highish revs. You are measuring with the engine hot as in thermostat fully open and driver side wing hot to touch? Edited by ras62 on Saturday 1st November 17:10

Inbox said: Only thing that springs to mind is the oil/air separator.... Not on the 'air-cooled' engines.

Just to exclude a few things: 1 - is it a UK car? And by that I mean has it lived in the UK since manufacture? (fuel RON and quality) 2 - what oil grade are you using? 3 - what engine work has been done on the car? 4 - when did these symptoms start? 5 - did you do anything to the car immediately prior to this? 6 - as you had no obvious symptoms, what alerted you to the lack of oil?

(thinking outside of the box) 7 - manual or auto? There is a pipe from the clutch bellhousing that feeds into the airbox if a manual. If manual - I presume the air filter has been inspected? 8 - has the clutch slipped at all?

Orangecurry said: Inbox said: Only thing that springs to mind is the oil/air separator.... Not on the 'air-cooled' engines. Oh well school day today then, probably best described as a PCV.

Inbox said: Oh well school day today then, probably best described as a PCV. Again.. I don't think there is anything along those lines. Dry sump engine and all that.

Any chance at all that you're over-filling and it's burning back to normal each time?

As I indicated above, the manual cars had a pipe from the clutch to extract any 'dust' and dump it into the air filter, so this is a removal of pollutants, but I think the dry-sump design emissions were clean enough for ROW regs.... I read somewhere the early 911s did have a pipe feeding into the oil reservoir tank? The American 993 engines all had the SAI system to reduce emissions, but ROW 993 engines do not. Are you thinking of that?

All engines cause pressure in the crankcase during running and dumping this oily mist to atmosphere is frowned up so it is vented and typically ends up in the engine intake and burnt in the engine. If this system isn't working correctly e.g. faulty or gummed up then then excess crankcase pressure can cause force oil past the rings or down the valves and the oil gets burnt in the engine. Whatever it is called, pcv or aos the engine is consuming the oil. This has nothing to with a dry sump, there is still presure and an oily mist in the crankcase. I wonder if with the engine running does removing the oil filler cap change how the engine runs? Doing that should cause rough running.

Orangecurry said: Just to exclude a few things: 1 - is it a UK car? And by that I mean has it lived in the UK since manufacture? (fuel RON and quality) 2 - what oil grade are you using? 3 - what engine work has been done on the car? 4 - when did these symptoms start? 5 - did you do anything to the car immediately prior to this? 6 - as you had no obvious symptoms, what alerted you to the lack of oil? Hello All Thanks for the replies so far. In answer to the various questions: 1: UK car owned by me since 2001. Always fuelled on 98. 2: Originally used 0-40 oil. For the last two or three years (after the symptoms started) have used the Porsche recommended 10-60. This appeared to reduce the consumption somewhat but only by circa 20%. 3: Engine is original. Serviced every year, OPCs until 2012 (when they seemed to stop understanding air cooled) and then well known indies. 4: Hard to say exactly when the symptoms started, certainly over 5 years ago. I have only driven circa 1,000 miles per year in 80 - 150 mile stints I recognised that it was using oil but as the filling was sporadic I didnt calibrate until this year after a 1,200 mile trip at pace. it used 3 litres. 5: There is no obvuious correlation between work done and the symptoms starting. 6: I've always watched the gauge and so that is what alerted me. Having said that, I don't trust it, I alway use the dip stick but the correlation is good. The gauge reads a bit low (i.e 3/4s up is "full" on the dipstick). 7: Manual. Air box / filter checked (and changed regularly with nothing to note). 8: No clutch slip. I had gearbox work done 2 years ago (after the oil issue started) and took the obvious opportunity to change the clutch. No significant wear / contaminants on the old one. Yes, measuring the oil only when properly hot, with the thermostat open. No change in engine note / idle etc when removing the oil filler cap. Someone said it should cause rough running but not sure I understand that or why. On the 1,200 mile trip I had company behind me from time to time whilst using the car "as intended". Even though my friends were looking, they could see no smoke, residue etc So far, from the replies, other than the obvious leaks or the engine burning the oil that we can't find any evidence of, the two new ideas appear to be something to do with the breather pipe if any but not quite sure what that would be and a potential small leak from the oil cooler so the oil is misting out of the car onto the road. Any other thoughts really appreciated. Thanks Andy

A D C said: Leak down and compression tests are normal. So, it doesn't appear the car is burning oil. That doesn't necessarily follow. You've got a worn engine and my bet is valve guides. Check this link which is specifically about your symptoms, https://www.pistonheads.com/gassing/topic.asp?h=0&...

Good to hear the clutch isn't full of engine oil. A D C said: No change in engine note / idle etc when removing the oil filler cap. Someone said it should cause rough running but not sure I understand that or why. No - that idea was simply wrong - that poster clearly doesn't understand the drysump system. The oil cap is in no way 'open' to the engine. A D C said: 4: Hard to say exactly when the symptoms started, certainly over 5 years ago. I have only driven circa 1,000 miles per year in 80 - 150 mile stints I recognised that it was using oil but as the filling was sporadic I didnt calibrate until this year after a 1,200 mile trip at pace. it used 3 litres. A D C said: 6: I've always watched the gauge and so that is what alerted me. Having said that, I don't trust it, I alway use the dip stick but the correlation is good. The gauge reads a bit low (i.e 3/4s up is "full" on the dipstick). I don't see how it can it be the thermostat jamming in odd ways to cause keeping too much oil in the engine vs the reservoir, as that would lead to overfilling and burning off when the thermostat did open. If different specialists have looked at it, shirley they can't all have missed a leak, even at the front cooler or pipework-to. So therefore it must be being burnt off - it's only those two possibilities. Or, as I said half-jokingly, the oil is only leaking out on-the-move. Obviously the specialists have had the car up in the air with the engine running. Is the oil low or filled at the moment? Exactly how do you add oil? One way to 'test' that the oil-level gauge is working/moving is to get the car properly to temperature, on level ground, and leave the engine running. Open the rear deck, and add oil whilst looking through the rear window at the oil gauge. As you add oil, there is a short delay before the reservoir level settles on the new total volume. Is that what happens? A short delay? Edited by Orangecurry on Sunday 2nd November 13:38

Orangecurry said: Good to hear the clutch isn't full of engine oil. A D C said: No change in engine note / idle etc when removing the oil filler cap. Someone said it should cause rough running but not sure I understand that or why. No - that idea was simply wrong - that poster clearly doesn't understand the drysump system. The oil cap is in no way 'open' to the engine. Jeez thanks, I do understand the dry sump system but please educate everyone including myself on why where the oil gets stored in the engine affects the oil consumption problem the OP has posted about. I am genuinely interested to hear your explanation and by the way if 'the oil cap is in no way open to the engine' how do you put oil in the engine???

I'm trying to help the OP find the cause of the oil loss. I'm trying to think of any possibilities (including jammed thermostat, or anything) that would mean it isn't engine wear, valve-guides or whatever. If there really are no leaks, then unfortunately it must be being burnt, as I've said, twice. But I'm being as gentle as I can.

Orangecurry said: I'm trying to help the OP find the cause of the oil loss. I'm trying to think of any possibilities (including jammed thermostat, or anything) that would mean it isn't engine wear, valve-guides or whatever. If there really are no leaks, then unfortunately it must be being burnt, as I've said, twice. But I'm being as gentle as I can. Okay so no explanation on my question and trying to divert attention, if you had any stones you would answer the question. Of course it is being burnt in the engine if it isn't leaking, even my Mum could see that. As there are no obvious symptoms beyond oil consumption the most likely cause is a fault in the emissions control system as engine compressions are good.

[quote=Inbox] Okay so no explanation on my question and trying to divert attention, if you had any stones you would answer the question. Of course it is being burnt in the engine if it isn't leaking, even my Mum could see that. As there are no obvious symptoms beyond oil consumption the most likely cause is a fault in the emissions control system as engine compressions are good. there is no special emissions system to speak of. Basically just a pipe connected to the intake system. It sounds very much as though the Exhaust valve guides are shot. Not unusual but can get expensive depending who does the work

Inbox said: ....stones.... ahh Pistonheads... let's gets angry about something. Inbox said: ...the most likely cause is a fault in the emissions control system.... There. Isn't. One.

Orangecurry said: Inbox said: ....stones.... ahh Pistonheads... let's gets angry about something. Inbox said: ...the most likely cause is a fault in the emissions control system.... There. Isn't. One. I asked you to educate people and you didn't want to, very telling. The 993 engine uses a CCV or Closed Crankcase Ventilation system which is basically the same as a PCV system, it does the same job. If this doesn't stop the oily mist from the crankcase getting into the intake you will get high oil consumption. As a first step, changing that is a lot cheaper than a top end rebuild.

These engines are dry sump so oil mist isn't a big issue. Most drops out due to gravity while the vent line at the top of the engine runs to the oil tank and indirectly to the intake manifold. There isn't anything to replace other than the lines.

I ran a 993 for a few good years, switched the cat for crossover pipes. There was some slight smoke with the straight through pipes that the cat seemed to be able burn off. That usage is abnormal and in the absence of leaks it’s getting into the combustion cycle and being burned off, regardless of the results of the compression test. Is there any drop in power when using the car under load? I had no other symptoms other than the smoke myself…

Sorry for resurrecting my own thread. I thought it might be useful or interesting to feedback what I found to be the root cause. With thanks for all the earlier contributions, I think you can summarise the outcome as after eliminating all the possible causes except the seemingly inplausible one, then the inplausible answer must be the answer. So, even though the Lambda sensor was completely clean and there was never any smoke or evidence on the rear PU it turns out there was wear on a number of the valve guides. Having refurbished the whole of the top ends I now have no oil usage at all so to all who said the problem must be that the oil is being burnt in the combustion chambers you were right!