Cheburator

Went to the Nurburgring over the weekend with the intention to test the car on the full VLN circuit yesterday. That was not to be - on Sunday morning as I was lapping the Nordschleife, suddenly blue smoke started coming from underneath the engine bay and from the exhaust at the same time.

Stopped the car immediately and popped the bonnet. Oil everywhere and the redundant dip-stick was slightly disloged. Hmm, I thought, that was strange, but it could have just happened by accident. Got the car recovered off the track and then the investigation began.

Took the LHS intake cover off and lo-and-behold the intake was full of oil. Mopped it up with rags and then started the car again - lots of blue smoke from the exhaust. After idling for about 10mins, the smoke started to clear. Then it be only present at 4000rpm and above. We took compression test and these are the results 1: 12.2bar, 2: 12.3 bar, 3: 11.8 bar 4: 12.2 bar 5: 12,2 bar, 6: 12.3 bar, 7: 12.2 bar and 8: 12.3 bar. We put a boroscope down the cyliners and it was all nice and clean with no carbon build-up or oil, the spark plugs were clean too.

Finally, thanks to my 928Intl headers we pulled the exhaust appart and determined that cylinders 5 and 8 are the smokers. At 6000rpm all other 6 are clean as a whistle, but 5 and 8 smoke as if the 928 is a badly running 2-stroke motor.

The car still makes awesome power and there are no funny noises from the engine as I managed to nurse it back from Germany to the UK - 350miles trip and all was good. No higher running temperature either.

I am thinking broken oil control rings? The engine was completely rebuilt 40,000 miles ago and has approximately 25,000 track miles on it...

Thoughts would be appreciated...

Thanks in advance.

Alex

John Speake

Problem with the dry sump system ?

Cheburator

I can't see the dry-sump being the problem - if all cylinders were burning oil, I would definitely think that the intake was gulping oil due to a problem with the scavenge pump and the breather system, but only cylinders number 5 and 8 are burning oil. All the others are running clean. Paul Anderson told me he had a similar problem at Silverstone last year - two broken oil control rings, but on diferent banks. The car is going to him this week and we will pull the engine apart. Great excuse to get the GTS lump with the fancy heads finally installed

This brings another question - until Coling is ready with my cams, I am planning to install my GT cams in the GTS lump. What is the best combo cams wise - GT all round/ GTS intake with GT exhaust/ GT intake with GTS exhaust?

S4ordie

Valve guides? What is strange is the dipstick being blown out, suggesting, er, blowby. All clean sparkplugs makes no sense. If oil is getting into the combustion chamber the plugs in those cyclinders should look at least partially fouled. The coincidence of two cylinders developing simultaneous oil ring failure seems unlikely.

I am no engine builder but this is a curious conundrum.

Cheburator

Valve guides were rebuilt with the rest of the engine, moreover, worn valve guides manifest themselves on the over-run when the engine is pulling maximum vacuum. I am getting blue smoke at WOT... But you are right - the other symptoms somehow don't add up...

Mike Simard

I had similar oil mess issues with the old stock engine in my racer. It was always a mystery why the engine would throw so much damn oil out. I took to tie wrapping a 'diaper' of absorbent cloth to the engine before going out.

When I later took it apart there was a broken ring in one cylinder.

If you do have ring issues, it wont take much to overwhelm the scavenge pumps. Especially on long straights.

Let me know if you need custom billet cams, that's what I do.

ptuomov

Not that I am an expert or anything, can you describe the mods to your engine? If you do that, people are going to have more precise (not necessarily more accurate) opinions.

My bet is that it's going to be either GT intake / GT exhaust or, in some weird cases, GTS intake / GT exhaust. GTS intake / GT exhaust gives LSA of 117.3, which is probably too wide.

Vilhuer

GT intake/GT exhaust for NA engine. GT intake/GTS exhaust for SC /Turbo I think.

AO

Sorry for your troubles. . It's interesting that compression looks good on 5 & 8. GT rings are pretty robust, but of course are suspect. Valve guides would probably show on a compression test.

What kind of oil/air separator are you using? Could be that you're ingesting a lot of blowby - but then that wouln't explain why 5 & 8 show the burning of oil.

Will be watching this one.

mark kibort

who rebuilt the engine and was he familar with the 928 way of doing things?
what rings did he use. were the bores touched?
40k miles ago, is really odd to have something break like an oil ring.
bummer.

IcemanG17

Odd with a dry sump.....I'm assuming that all of the original breathers are routed to the drysump....so the oil MUST have come from the rings or valves (guides)....but your compression is good....have you tried a leakdown?

I would think it you had a ring failure it would NOT get better...only worse.....same for valve seals...was there any obvious oil in the intake ports-valves?

ptuomov

25,000 race/track miles, if not driven like an old lady, equal maybe 125,000 - 250,000 regular road miles. If an engine starts letting go after 25,000 race/track miles, then there's nothing necessarily wrong with how it was rebuilt.

Cheburator

To answer all questions:

I will not do a leak down test since the heads are coming off anyway. I am transfering the GT cams into the GTS lump, then it is only a question of 8 bolts to take the heads off and have a look inside the bores.

The engine was rebuilt by the previous owner, who went into financial difficulties and sold the car with 0 miles on the fresh engine to me. He used genuine Porsche 944 Turbo rings. He was familiar with the engine as he was a 944 Turbo owner prior to buying the GT. I don't suspect his workmanship - the engine has been trouble free since 2005.

While I am a smooth driver, I don't hold back, thus in a way I am pleased that I saw 25,000 trouble free miles out of an engine which was driven very hard without a dry sump for 18,000 of them.

The car still uses the original breather system as we checked the intake after 5000 miles and it had no oil in it and we figured out that the scavenge pump was doing a decent job of venting the crank. To plug all the holes in the stock breather system the intake needs to come off and I was much keener on testing the rest of the car, rather then having it sat in a garage with the top of the engine off...

The GTS lump going in has 968 intake valves, ported, polished and skimmed heads. The intake has also been port- mached to the heads, the rods + pistons are matched to 1gr and it is all Porsche bearings, which again were all matched to be in the middle of the factory tolerances. Intake spacers will also go on, as well as 928Intl exhaust manifolds, a custom X-pipe and a dual 2.5" exhaust.

Wish me luck

Bill Ball

Maybe stating the obvious, but if oil went all over the motor and filled the intake, you had/have more than an oil control ring or valve guide/seal problem. Something pressurized the crankcase, if only temporarily. Have any idea what went on there?

Also, I'm not sure what you will see by removing the heads other than a portion of the cylinder walls. At 25,000 track miles. this engine deserves another rebuild.

ptuomov

How about the following possibility:
- broken oil control or second rings
- because the top ring is intact, the compression is ok at low rpm
- at higher rpm, broken oil control or second ring fails to control oil from getting under the top ring
- excessive and uneven oil film under the top ring causes the cylinder pressure to make it between the ring and the cylinder wall
- massive ring flutter and blowby
- pressurized crankcase pukes the oil into the intake manifold

(Caveat lector, I don't know what I am writing about.)