shortyboy

i think my piston rings are going.symptoms show leakless oil consumption(1 quart every week or so) and white/greyish smoke from exhaust at deacceleration.is this a case of bad piston rings?if so, how can i fix it?at what cost?if not,what else could this problem be?obviously the PO raced this car without do proper maintinence and such.vaccum gage shows ~15 hg. on the boost gauge.need help from all that who knows how to fix this.much thanx in advance!!! -wayne

fast951

Few more things can be causing the burning oil on deacceleration:

* Bad valve guides/seals, especially if you notice oil burning at initial startup after the car sits for a while (Oil drips from head to cyl). On deacceleration you go to vacuum and the engine is sucking oil from the head.

* Bad Turbo. Check turbo for play and for excessive oil at turbo inlet and in hard pipes/intercooler.

* Bad rings. Do a compression and a leakdown test. However, your compression rings may be ok, and have bad oil scrapers rings. The test results will not show this, and the test results will be normal.

To replace rings you have to pull the cyl. head off as well as the oil pan. At this point, it might be better to pull the complete engine and do a full rebuild. At least replace the rod bearings when doing the rings.

If you decide not to pull the engine. Start by removing the head to examine the shape of the cyl. walls...

Jon Moeller

Fast951,
Any comments on the use of a bore scope for a preliminary evaluation? I'm debating between freshening the head on my 951 and going for a full engine rebuild in the next year. The debate would have an easier answer, if I didn't have to pull the head to evaluate the condition of the block.

I've never used one of these, and wasn't sure how clear of a picture that they present.

Thanks,
Jon

fast951

Jon,
The bore scope a nice tool, however you might still miss something. Also, if there is a carbon trace on the wall or a scratch, it'll be difficult to figure out the depth.
If you have severe problems, then you'll be able to see it.
Personally, I would go with the leakdown test first. You might want to do the leakdown at TDC and BDC (2 tests per cyl.)

What caused you to think about refreshing the head or the full rebuild?

Jon Moeller

My 951 has good compression but very poor leakdown. I knew this when I purchased the car, and accounted for this in the price. The shop that performed the test said that they could hear the air leaking past the exhaust valves, and that the bottom end seemed solid (I'm not sure how they could tell this from a leakdown and compression test).

To give you some additional information, the engine burned some oil (~1qt/1500 miles, but highly dependent on driving style) with Mobil 1 15w50. I've since switched to Redline 10/40(or is it 15/40?), and have seen consumption drop to almost zero (<1/2 qt over 2000 miles).

I'm looking for the car to be my daily driver and weekend track car, so I want to perform the correct fix.

Thanks for the help,
Jon
'88 951

fast951

Jon,

The leakdown helps you determine the weakes point in the system (Intake valves, exhaust valves, rings, headgasket).

id the shop tells you the % leakdown you got. If less than 8-10% I wouldn't worry about it, unless it's a race car. For your peace of mind, I would redo the leakdown, some carbon might have been stuck at the valve seat and the valve didn't close all of the way.

Your oil consumption (or lack of it) is very good.

Jon Moeller

Sorry for hi-jacking your thread, Shorty.

The figures from the shop were ridiculous, like 85% leakdown, not 15%, but 85%. I couldn't believe this, but as an indicator, I've seen the car roll on a hill when parked in first gear. Thus, my concern over the head.

The lack of oil consumption has me hoping that a head rebuild is all that I need. Since I would like to go to a TEC-3 setup on the car, I want to make sure that I'm starting with a good baseline.

Thanks, again, for the help. And, John, if I didn't have my heart set on the TEC-3, I'd be all over one of your kits. Excellent work.

-Jon

shortyboy

no problem jon, well fast951 i removed my hard pipes and found some oil around the couplers and a little buy the opeing of the hard pipes.i also checked the play on the turbo and there was a little bit of play.the intake j-boot has a few cracks in them,especially where it connects to the turbo,its a cracked up.so im guess from the information youve given me that its a bad turbo.thank GOD it wasnt bad piston rings.i guess the PO wasnt aware of this and ran the car pretty hard with this problem.well,thanx for all the help fast951,now its a search for a turbo upgrade.

adrial

shortyboy, not so fast...

There should be a thin coating of oil on the IC pipes, but if the oil drips out...its too much.

fast951

I agree with Adrial. A bit of oil (a film) in the intake and in front of the turbo is normal.
All turbos have some play. Excessive play, where you can visually see the shaft move is no good.
You can actually measure the play...

I would think it's time for a leakdown test.
You might even want to do the test on a cold and on a hot engine tests...

m42racer

If you are having a ring problem, and you are using oil, then the oil rings will be loosing tension. If this is the case, the ring is not scaping the oil off the cylinder walls. The Spark Plug will show signs of oil on the center electrode. But wait, as has been said here also, it may well be guides that is the problem. You cannot have a good compression test and a bad leakdown. Although both tests show different information, they do still show the sealing state of the cylinder.

As has been advised here, start with the easy stuff. Look at the Turbo for play etc. Probably need to have the Turbo checked by someone experienced in Turbo's before tearing down your engine. A liitle spent upfront can save hundreds later down the line.

If the Turbo is OK, then next probable cause is guides. Most likely the Exhaust guides as they get the hottest and the wear rates on Exhaust guides is typically higher. A way to check if the Guides are possibly the cause without tearing down the engine is to use a Borescope. Turn the engine carefully over so to put each Valve at full lift, then take the scope and look at the back side of the valve. If its wet, then the guides probably have too much clearance. If the deposits are on the Valve, but not wet, then the guides are probably OK. If the valve head is wet, then this usually shows a Ring problem, or a problem associated with the ring seal. Looking at these areas is usually within the scope of the home mechanic. Looking thro a scope without knowing exactly what you are looking for can be misleading.

I'm not sure what Fast 951 means when he says to do the Leakdown at TDC and at BDC. I am not up to date on 951 Cam design or the Valve Timing, but correct me if I'm wrong, but at no time is the Piston at BDC with both Valves closed. Maybe you must be running some very short Cams or the timing is way advanced/retarded depending upon which lobe your talking about. If by chance I'm wrong here, and the valves are closed at BDC, there should be no difference between the numbers, except for a crack in the cylinder wall.

As for doing the Leakdown, if the numbers are high, it can be caused by carbon on the seat etc. On our 951 engines it is not easy to tap the valves to get them to seat. Maybe try to use the scope to look at the seat and valve for deposits.

If the Turbo is the cause, then you should see oil on both sides. You can normally follow the oil thro the Intake system and or thro the Exhaust system. If you see smoke throughout the engine run, then its a good chance that the Turbo is the cause. If its only on startup, it can be either the Turbo or the Guides etc. But if you see the Oil on the overrun the the guides are most probably the cause as stated in previous posts.

shortyboy

well here another observation.the other day when i was in the garage i started to rev up my engine to see if smoke would come out.sure enough alot of smoke did.but only at about 5k rpm and up.at low rpm everything seems fine but when the turbo kicks in alot of smoke comes out of the exhaust.it was greyish black smoke.another thing is when im crusing under 3k rpm i do not loose any oil,but when i hit 4k and above i notice some oil mising.i took the IC pipes off to examine them and only a film of oil was(as youo guys stated as normal).the front of the turbo was kind of oilly and dusty(which looks a little normal.but when i took off the j-boot it was all cracked,could this cause any damge?air leaks?...im willing to do all the work myself,with the help of my dad(and the help of some tech books) just to keep the costs of labor at a minimum.

m42racer

Two things could be happening here. At higher RPM's your Oil Pressure is higher and you could be loosing oil control thro the center bearing of the Turbo center section. If your Turbo is a plain bearing type, there is no oil control other than clearance between the shaft, bearing and Housing. The bearing is a sleeve bushing really. On the newer type of ball bearing centersections there is still no control other than clearance. If these clearances get to big, then the tendancy to allow oil to escape is possible. Higher Oil pressure will cause this to happen at a quicker rate. If its a boost related issue, you probably have a Thrust issue, which is now causing the main sleeve bearing to wear. I won't bore you on how. Without the proper Thrust, the torque is all directed thro the bearing. If your Turbo is a KKK, then most do not have a full circle type Thrust bearing. They were normally sold only as an update for racing only, or as a manufacturers request on some diesel applications. If its not a boost related issue, high RPM's no boost situation, then it could be an Oil ring tension problem. Turbo engines all have a problem with ring tension as the temps are higher in the Piston and transfer of this heat is required thro the rings. 1 piece Oil rings are more prone to be affected with this problem, and most Porsche ring packs have a 1 piece Oil ring. Start with the Turbo. In most cases its here. Some turbo's can have more clearance than others and have no problem. The main clearance or fault is not usually between the shaft and bearing, but between the bearing and center housing. Have the Turbo checked by a professional who knows turbo's. The best people for doing this are Turbo resellers/service peolple who deal mostly in diesels. The reason is they are not trying to sell you something you do not need. They are used to getting the truck back on the road with the least amount of fuss. Typically hot rod turbo shops will tell you the turbo is junk, and will sell you their hybrid upgrade. Remember, there are oversize bearings available, and your center section can be rebuild, usually for way less than a new turbo. If you email me I can direct you to a place I would recommend. Hope this helps.

Jon Moeller

M42racer,
Please explain how you can't have a good compression test and a bad leakdown. I'm asking because this occurred, and I had the PPI done at a (I thought) reputable dealer. Compression was 140, 135, 135, 135. Leakdown was between 80-85% per cylinder. My understanding is that compression on the 951 is supposed to be around 138psi.

-Jon
'88 951

fast951

If the leakdown is at 80%, then it will be very difficult for the engine to generate compression as the compressed air will be escaping at a rate of 80%. I would redo the leakdown test.