RKD in OKC

Got my GTS at 42,000 miles.
The previous owner put less than 2000 miles on the car the 3 years he owned it.
The first 3 years it was my daily driver and racked up another 25000 miles of daily driving, road trips, and auto crossing.

it has no drips or leaks, the intake is not oily, and it does not smoke under heavy acceleration (but I do run cats.) The exhaust tips are sooty, there is a black watery spot behind the car from oily condensation on cold starts, and the rear of the car does gather a sooty film.

It's oil usage was very sporadic but always used at least 1 qt every 400 miles. Sometimes it didn't use any oil for almost 1000 miles, then all the sudden it would use a quart or two very quickly, like in one tank of gas. The jump in usage had no correlation to driving type. The oil level never dropped when auto crossing. Some road trips it would use oil and some it wouldn't. And the same use/no use with daily driving around town. Got into the habit of checking the oil every time I filled the gas tank. However, it usually only needed oil every other gas stop.

During this time I tried different brands of oil, and additives. Tired Lucas Oil Stabilizer and it seemed to hold off the oil usage for a short time but would still suddenly gulp down a quart.

Once I topped up the oil and drove 2 miles to the corner gas station, gassed up and drove it home. When I got home I smelled raw oil. When I popped the bonnet found oil splattered all over the front of the hood liner and engine, and the oil filler cap wedged into the front of the passenger intake tube. So I was thinking that was crankcase pressure from blow by and I tried some Restore with the same results as the Lucas Oil Stabilizer, a little longer with no oil use then suddenly gulp.

When my cam crashed and Sean pulled the motor had the heads redone. The engine looked clean for 68K. There was some time delays with getting the heads done, an powder coating the intake and cam covers. So the engine sat dry for a while. I asked Sean about drilling the oil ring relief holes and at that time he claimed it wold make it burn even more oil so he didn't pull the pistons out. All he did was check the crank and rod bearings for wear and said they looked good. Put the car back together with the new valve job, GT cams, the Greg Brown filler neck baffle and the stock filler neck baffle.

I've only put another 4000 miles on the car since getting it back. That included the trip to the Wichita OCIC, a couple of trips from OKC to Dallas and a couple of trips from OKC to St Louis. The car did sit a while during that time to present due to my foot ordeal, but it has consistently been using a quart of oil every tank of gas, Oil usage is now very consistent.

Every once in a while (4 times in the last 1000 miles to be exact) when the car is cold and I accelerate hard after about a mile it will fog the area with grey/blue smoke that smells like RAW oil. AND then it stops within a block or so. Most of the time, cold it does not smoke on hard acceleration.


I've been intently watching the burning oil threads with the oil filler neck baffle, oil separators, crankcase venting systems, vacuum pump, missing oil ring relief holes, etc. Also done some googling on oil burning and not specific to the 928. It seems a couple of other cars have oil burning problems like the Mazda and the Saturn. They have a problem with the ring grooves, specifically the oil wiper ring groove filling with gook. At some point the rings get stuck and don't rotate providing a better passage of both oil and blow by through the ring gaps. Normally the rings rotate keeping the gaps moving enough to provide a good seal. The cure for oil burning on those cars was to remove the pistons clean the piston ring grooves, put in new oil scraper rings and no oil burning. Seems that once it starts it just slowly gets worse and worse.

They also discussed how this buildup in the oil ring groove was worse on cars that ran dino oil because of the varnish created in the dino oil, but how the carbon from the gas burning would build up and get sticky just as bad.

So my question to the engine guru/rebuilders with 928 experience is when taken apart are the GTS ring groove dirty and sludged up? And would having the engine sit apart and dry, but without the pistons pull case the sludge to harden and be more of a problem causing my increased oil usage?

Until someone explains differently the sticky oil ring theory makes since to me for my erratic and sudden oil gulping AND the additive helping a little but not curing, how the driving style didn't change the usage, and the consistent increase after the engine sat semi-disassembled for a while.

IcemanG17

okay its a simple ??? why does GTS burn SO much more oil than other 928 S4's................what is the single biggest difference between an S4 that burns almost none and GTS? Both share the 100mm bore & the GTS has a slightly different breather, possibly worse.... The rings never changed.

So why such extreme oil consumption.....1L per 400 miles in excessive....extreme really.

while I am no engineer the longer stroke of the GTS with the horrible S4 oiling system clearly exposes even more weaknesses in the design.... I don't know how much higher crankcase pressures the GTS will see vs an S4 for its extra 7mm stroke...but clearly it overwhelms the already marginal system....

I like to think its not about "sticky" rings....its more about rings designed in the 70's and hitting certain resonances that cause them to flutter.....hence the gulps and smoke....

RKD in OKC

fBefore I had the GTS I also had a 90GT. The 90GT used oil unless I drove it like an old man, never revving over 4000 rpm and never accelerating full throttle. Seemed to burn more the more aggressively I drove, both in acceleration and lifting off the throttle. Unless driving very easy it burned 1 qt in a little over 500 miles. It did not smoke under hard acceleration except when cold and then only occasionally like the GTS. The major difference being the 90GT would predictably burn more oil the more aggressively it was driven where the GTS was much more random and not associated with how the engine was loaded, unloaded, or revved.

Was told at the time the 90GT oil usage was the crankcase vent system, even though the intake was dry. And the 90GT collected carbon both on the valves and in the cylinders. I even had the 90GT walnut hulled once to clean the carbon out of the top end. Don't remember exactly why we de-carboned it, but had the injectors cleaned and balanced at the same time. After the cleaning the 90GT ran much better with more power.

So in my experience, the S4s are not immune to oil usage. And while the longer stroke of the GTS may aggravate an existing design shortcoming it does not explain to me why some GTS's don't use oil and some do, OR why the usage while consistently high is sporadic and does not seem to be effected by driving aggressiveness like on the GT. Or why my GTS oil usage increased but became more consistent after the valve job.

Ring and groove sizing tolerances and ring gap tolerances? Change in the spring strength of the rings themselves? In the Mazda/Saturn discussions they talked about the oil scraper ring spring design and getting weak along with the sludge buildup.

Alan

This I think is the interesting question - why do some GTS cars burn excessive oil and others do not - what is the large variable here? and why so consistent over time for different cars. To me its clear this is not just a high mileage thing. My car used excessive oil from the time it had 32K miles (when I bought it) all the way to over 100K with all kinds of work to try to address it.

Others have high mileage and never any issues.

Also I am convinced my oil was predominantly going into the intake, since that's the path I have now eliminated and my oil loss is dramatically lower. You on the other hand are fairly convinced yours is primarily ring loss. So it seems there may be two mechnisms driving usage. Could the the two methods even coexist - it seems plausible to have components of both.

To me sticky rings can make sense as the major variable for the ring losses not sure its so clearcut an explanation for intake losses. But then why gunked rings in a few 10's of K miles on some but not on others...

In this thread Greg suggests a difference between engines could be the cam vent elbow restrictors/not in use across GTS models:
https://rennlist.com/forums/showthre...eferrerid=6055
However the issue doesn't seem to be demarcated by build date - so this needs more thought (unless they really mixed and matched elbows at random....)


Alan

andy-gts

I do know that when Greg made my oil control system, the belt driven 3 or 4 stage scavenging pump has sure helped and I have "0" oil issues now....5000 miles and dip stick shows 1\3 drop from top line down....just amazed as I was burning 1 qt every 1000 miles.

I do know Greg made multiple changes to the suction ports and what is below what is seen on the cam covers. what ever he did works and you cannot beat success. I wonder how much vacuum his pump scavenge system makes in the crankcase.....as I am following the threads intensely myself wondering if the old gts engine sitting there has any use with out major work????

glad to see okc up and at em !!!

Alan

I don't think a scavenge pump with otherwise stock like intake plumbing will make any appreciable vacuum - the whole system is too wide open.

But anyway here the question is what are the variables in the cause - let's not get off into vacuum pumps and alternate solutions - other theads for that already, just the question WHY the differences?

To RKD:

You ask why is the GTS oil usage more constant Vs. only when driven hard for the S4? This one may come down to the check valve in the filler neck on the S4 that isn't there on the GTS. The GTS will do more flushing at all speeds, if this has an oil component much of the time anyway - more will be ingested.

Alan

Hilton

RKD and Alan,

Can you both go look at the casting number on the elbows on your cars? You can use a mirror, or just take a pic with a digital camera in macro mode might be easier on the eyesight (its what I did for the 2R pic below)

There's at least 3 variants of elbow, possibly more.

0R - has restrictor, keyed for left-side of cam cover
1R - no restrictor, keyed for both sides (at least, the one I removed from a stock '85 motor has cutouts on both sides)
2R - no restrictor keyed for right-side of cam cover

(Note: "keyed for left-side of cam cover" means driver-side front, or passenger side rear in LHD cars).

S4 has: 0R and 2R on the right-side cam cover (at least, my 87 and 89 both do)
The 1R I have came off an '85 32V engine (in the same place as the 2R on an S4)

I think Greg B's comments indicate that with a bit of a survey of GTS owners, we'll find a mix of different elbows. As you've both got cars which have significant oil consumption, it sounds like we'll find some 0R parts on your cars where maybe other GTS owners won't have them?

edit: old thread on elbow variants: https://rennlist.com/forums/928-foru...-variants.html

the flyin' scotsman

If we expand the discussion to include the other Porsche models I'm curious if they too had oil usage issues?

The 3.6L H6 engine 911 used 100m pistons/rings as did the 944 4cyl engine.

Perhaps the issue is the choice of engine components of the era? and if so what is available today to resolve the issue?

Havent read of GB strokers using oil.

Manfred

For what it's worth. My '91GT will consume a lot of oil (1 quart per 500miles). It seems it consumes more oil the harder I drive it. Since I drive hard all the time, it consumes oil consistently. The only way I have ever seen the oil consumption drop is if I stay below 4000rpms consistently--so yeah, drive like an old man. If I stay below 5k or 5.5 or 6k it doesn't seem to make a difference, meaning I don't have to redline the car to consume oil, just approach the top end and it drinks.

Dan

Lizard928

Rob Edwards posted in one of the threads that prior to the scavenge pump being installed on his engine he could smoke a city block with the stroker engine.

The rings are partially getting stuck in the ring lands yes. But the moment the rings lift and you get a higher rate of blow by it will lift oil and air up sucking it in to the intake. The moment you get oil in the combustion chamber the octane is reduced and the car will detonate, moment it detonates, more ring flutter more oil by the rings and through the intake.

There are numerous reasons that the cars do not consume oil below 4k rpm. One of the reasons is that the timing is not nearly as advanced/close to the edge. If you back off the timing a few degrees on an engine that consumes at higher end you will lower your rate of consumption.

the flyin' scotsman

thx Colin.....do you have an opinion for an alternate piston/ring set that would work with the stock bore?

I set the timing to stock when doing the cam belt project last winter using Porkens tool and recommendations.

Lizard928

I'm talking about ignition timing, not cam timing. Though retarding can timing can help to lower the dynamic CR which will make it less prone.

As to the ring packs, no. I use stock 951 ring packs as they work well on the alusil bore and are more resilient to detonation than a stock S4 ring pack.

But I honestly believe that the pressure pulses caused by the piston movement do have an effect on this as well. That is why I like low vacuum levels to counter these pulses. And this is a portion of the reason as to why the GTS suffers greater than the S4. Longer stroke, stronger pulses.
The more the rings flutter, the more the hot combustion gasses pass the rings, the more it burns the oil on to the piston and in the lands.

the flyin' scotsman

thx for your input Colin.

My GTS has stock ignition timing with all stock engine internals and does not have the oil consumption issues as many others do hence my question wrt what was available to Porsche at the time of engine build when the same size of bore was used in many models.

No matter as that's all very old history and was looking for a solution to the problem with ring/bore material that you answered with usage of 951 rings.

It would be an interesting experiment to only change the rings in a high oil burning engine and see results albeit not practicle.

Alan

Well here is my data but caveats here, my configuration is changed, for a start all 4 possible vents are plumbed and I'm not 100% certain what the original stock configuration was - I only added one new port, the other three stock ports were reused & moved around, all have been rekeyed as required to non-stock angles.

As it is now:
----------------
Drivers Rear: 928.107.733.2R (port not used in any stock configuration)
Drivers Front: 928.107.733.0R
Pass Rear: 928.107.733.2R
Pass Front: 928.107.733.1R

The differences between these actually aren't critical now (once were) since they are now all used for quite different purposes.

3 of these elbows are stock - Pretty sure what I added was the second 2R

I think by deduction this means that the original config was:

Drivers Rear: not used
Drivers Front: 928.107.733.1R
Pass Rear: 928.107.733.0R
Pass Front: 928.107.733.2R

Highly likely but not certain...

Alan

FredR

My S4 motor has always burned quite a lot of oil no matter how hard I try to test as many variables as I can. I associate oil burning with rpms and I agree with the linkage to 4k rpm and higher.

Regards

Fred