Flawed understanding of smoking 993TT turbochargers
 

It's been a while since we've had a technical discussion. Not sure if any forum members are still interested in tech discussions but here's one for you. It is understood that 993tt's are prone to smoking on startup and we even have a sticky in this subforum addressing this. Kevin @ UMW describes this as as oil backing up in the turbocharger sumps, the sealing rings in turbos having "gaps", oil leaking onto the heatshield and into the turbine area which subsequently burns off resulting in both the smoking effect we are used to seeing plus carbon forming at the seal area "cutting" into the seals further and resulting in failing turbochargers. Various fixes have been suggested including using the right filter, maintaining low oil level, P trapped scavenge lines and check valves on the turbo oil feed line. I'll preface this with the fact that Kevin @ UMW has far more technical knowledge than I have and I appreciate his knowledge and contribution to this forum. This by no means an attack on his expertise and I know he has solved this issue for many so experiences are not likely the same for every car.

That being said, this theory never really made sense to me. Every performance car I have owned has been turbocharged. I'm not a professional mechanic but have built turbocharged vehicles for 20 years. I've never before experienced any other turbocharged vehicle where oil under gravity feed can bypass the turbocharger oil seal. The CHRA is fed pressurized oil in plain bearing turbos and don't leak so how can oil fed by gravity leak past the seals even if the sumps were backed up? On initial startup, the turbine housing is cold. Even if it were coated in oil, it shouldn't smoke until it gets to operating temp. 993tt's that are smokers blow a consistent puff of smoke right on startup before the turbine housing gets hot. This must be coming from the combustion chamber and not from oil leaking into the turbine housing.

With this in mind, I can say my 993tt has been a chronic smoker from the passenger side turbo. I finally decided to investigate and fix it. After a long drive I parked it for weeks. I can almost guarantee smoking on startup in my car if parked for a few weeks. After waiting a few weeks I pulled the exhaust (easy since my car is straight piped) to inspect the turbine housing. No oil in the housing even looking with a scope between fins at the bottom of the turbine housing. Starting the car, I get smoke. This implies the smoke is not coming from oil burning off in the turbine housing. Starting the car after inspecting the turbine housing I get smoke. This implies the smoke is from oil burning in the combustion chamber.

Step two I pulled the turbo sump breathers and vented them to atmosphere. No smoke whatsoever. Reconnected them and I get smoke. I then adapted a clear plastic container which I fitted between the turbo sump breather hoses and its regular connection to the vacuum nipples just past the mass air sensor. On starting the car I get a small slug of oil being sucked up the turbo sump breather hose which is then dumped into the plastic container. The plot thickens...

In short, I theorize that our understanding of turbo oil smoke may be flawed. In my car it does not seem to be burning off in the turbine housing. I believe it is backing up in the turbocharger sumps, and ending up in the sump breather line. On startup, vacuum in the intake is sucking a small amount of oil up the sump vent hose, which then travels through the intake piping, turbo compressor housing, and intercooler and burning off in the combustion chamber to cause the smoking on startup. This also accounts for why folks see oil in pre turbo intake piping and intercooler.

It seems that running a lower oil level, check valves, correct small oil filter and P trap scavenge lines reduce the chance of oil backing up in the turbo sumps which therefore reduces the incidence of smoking on startup. These are all reasonable fixes. I'm still cynical of the idea that carbon build up from oil leaking into the turbine housing will destroy oil seals by "cutting them like diamonds". Turbine shaft oil seals are not soft rubber/synthetic. They are steel and many report smoking with newly rebuilt turbos even after upgrading to step gap seals. Coking of seals from hot shutdown is well described as is severe coking when excess oil pressure is run due to poor restrictor sizing however gravity fed leakage is not well described. Clearly I'm not a foremost authority on this topic and don't have all the answers however this is my experience on my car. Hopefully others will chime in and we can develop a better understanding of this issue.

Sump Vent Theory
 

Your sump vent theory makes sense to me. The logic is sound and it fits directly with my experience. It should be noted that I have a sample size of one, so it may not be a universal fix.

I fought startup smoke for 18 months, following all the suggested solutions you mentioned, including rebuilding both turbos two times. Nothing I did fixed the problem. Finally, I figured out that oil was migrating from the sump to the intake manifold, via the sump vent hose. I did an experiment to confirm this.

Opening the sump vent hose to atmosphere (and plugging the open hoses on the intake manifold) solved the problem. If you have smoke on startup, I believe this easy and reversable fix should be the first thing to try.

I wonder how many turbos have had rebuilds due to this. It doesn't seem like there are too many folks interested in wrenching on these at home now since they've passed into the realm of collector vehicles however searching the forum from back when enthusiasts did work on these, there are so many past posts about continued smoking despite owners rebuilding both turbos. Most report large amounts of oil in the pre-turbo plastic intake pipes. Although the UMW thread suggests this is oil leaking past seals and into the intake pipes, I can't see how this is possible. For oil to move from the compressor housing to the intake pipes would mean that the compressor housing would have to be half full of oil to spill out into the intake pipe.

It seems the only sensible source for oil in the pre turbo plastic intake pipes is either from crankcase blow by or from the turbo sump vents. Blow by oil should just be a light oil film unless the engine is on it's last legs. The only real source of large amounts of oil in the intake pipes therefore must be from the turbo sump vent lines.

When the turbine seal fails, (exhaust gas pressure) pushes oil (lube oil) inside the bearing housing past the compressor side sealing ring and into the compressor housing/inlet ducting.
I will post a picture of the burnt oil/solid carbon built up in the turbine/bearing housing heat shield..

https://cimg5.ibsrv.net/gimg/rennlis...9a13ad28ab.png

Awesome. Thanks for joining the discussion. Appreciate you adding your knowledge and experience to this. Photos would be great and it would be interesting to see the extent of carbon build up since we don't get much of a chance to examine turbo innards. My understanding is oil is fed under pressure to the turbo bearings but drains to the oil return which feeds the turbo sump and then scavenge pump. The oil return should not see significant pressure since it is vented to the intake and crankcase. If exhaust gas pressure pushes past the exhaust seal it will simply vent into the oil return/crankcase. The only way for it to push oil into the turbine housing is if the oil return was obstructed which shouldn't be the case since it is vented to the crankcase/scavenge pump and the intake piping through the sump vent.

Any chance you can explain how the intake pipe is often filled with oil? I figure that the volute of the compressor housing would have to be half filled with oil before it spills into the plastic intake pipe.

You are making incorrect statements..

"If exhaust gas pressure pushes past the exhaust seal it will simply vent into the oil return/crankcase."

Turbine side seal (piston) ring failure will push oil past the compressor side seal and into the intake inlet piping (first) and then into the compressor housing, intercooler, back thru the bypass diverter valves and recycled back to the inlet piping and compressor housing.

Here, look under "oil leakage at compressor

https://www.borgwarner.com/aftermark...roubleshooting

Oil scavenge issues

Turbine Piston ring failure OR high crankcase base pressure (ie bad valve guides , piston rings)



https://cimg4.ibsrv.net/gimg/rennlis...eebb723404.png

I completely forgot about the diverter valves. That definitely provides a conduit for oil to travel from the post compressor to pre compressor piping. Since they are only open momentarily, I didn't think they would really account for the large volumes of oil commonly found in the intake piping however I've not tested this.

I'm no expert in turbo rebuilds but have disassembled a few. My understanding is exhaust gas that pushes past the turbine oil (aka piston) seal will vent into the oil return chamber of the CHRA. I stole a cutaway diagram of a turbo to reference below. Exhaust gas that pushes past the turbine oil seal on the red (turbine) side will feed into the oil return chamber in the center of the CHRA. This chamber is not pressurized and in most cars feeds the engine sump. The only pressure it sees is crankcase pressure which should be minimal. In the case of the 993tt this oil return feeds the turbo sumps which are emptied by the scavenge pump. Even if exhaust gas pressurized the red (turbine) side bearing, it would just push oil out of the bearing and into oil drain. It would only be if the oil drain was blocked (ie scavenge pump failure or high crankcase pressure) that the oil return chamber would fill with oil and then potentially push past the blue (compressor) side seal. Most of these 993tt's that smoke are otherwise healthy engine wise and have working scavenge pumps and not a ton of blow by. In these cases I can't see how turbine seal failure would push oil out of the compressor side oil seal. I must be misunderstanding something..

https://cimg5.ibsrv.net/gimg/rennlis...bc4a3a7dee.jpg

The bold quoted phrase makes sense. I still getting a sever-chronic LEFT turbo leak, through the compressor side and exhaust side as well, flooding sometimes the exhaust/CAT and sometimes goes away and get back.

993tt got turbos rebuilded, but always the one that smoke through the tail pipe is the left turbo (right side pipe).

This is what we have done so far:

• Check valves installed (oil feed) long time ago
• Custom return lines with check valves, long time ago
• New OE oil filters, not fully torqued (small one) , tried with hands only and later (change of oil) with the torque specified on the filter ,( long time ago and recently did again a new oil filter change)
• Mobil One V-Twin 20W50
• Oil level below mid section of of the machined part of the dipsticks ALWAYS

After a chronic leak on the left rebuilded turbo, we opted to buy a NEW borg warner turbo that came from europe. Installed and seemed to not leak, yet it was smoking, perhaps oil pools still inside the exhaust system burning....then after a few drives, the smoke went away and seemed to be all fine and fixed. Days later it started to smoke again at the tailpipe (left turbo-right side pipe), also noticed slight leak of oil between the cartridge and turbine housing and exhaust turbine side & exhaust tube (downpipe) and very little leak in the compressor side as well. You could see some oil cooked between the cartridge and housing as well. Cleaned it and took for s another spin and smoke went away. Figuring the "turbo did a break in" and self sealed the rings....after a few days of use, the owner started to get tons of smoke (left turbo -right side pipe) again and flood of oil in the exhaust.

Then we decided to clean everything again, and tested the left turbo scavenge pump, by disconnecting the sump return line and hook up it from a clean can of its own oil and transparent line to check and confirm the oil was being sucked into the scavenge pump and was working fine. Then took for another spin to clean up all and was fine. The next few days started to smoke badly again. This time ,after trying to cure with engine running the remaining oil in the exhaust, found a chunk of oil (not the first time I have seen this though) while taking apart the left turbo INTAKE tube (see video): After this I decided to inspect the turbo sumps vent lines.

The vent lines were free of clogging , flowed perfectly. I studied the line and they has in each side a T connection, one goes to a CHECK VALVE that is fitted BEHIND the throttle body, which prevents to pressurize the turbo sump vent line from positive pressure (when turbos kick in) and the check was perfect working as we took it off and tested outside. And the T lines continues at the back of the "Y" or elbow for the intake of the turbo, just behind the MAF sensor, all were free flowing, because at some point I thought it was clogged or somehow it was pulling too much vacuum and the oil from the turbo sumps were sucked and dumped in the intake side (I even video scoped the left side and I could see a thin oil film in the intake tract of the "Y" but couldn't be sure or precise if it was all coated in oil which seemed to not be the case (as why I got such chunk of oil from the left turbo intake pipe in the video).

We also did compression test and made a "Y" fitting to read oil pressure at the top of the turbo oil feed in each turbo as noted in the below photos

Now...we are going to re-install the rebuilded old turbo from the left side (which is sitting in the box, as it was replaced for a new turbo) and will install it in the right side, and see if it leaks like it did in the left side and report back. I have even run the engine with the turbos downpipes off and the left new turbo, cured the remaining oil into the exhaust turbine (as photo shown) and didn't leak anymore......WEIRD.


VIDEO url https://www.youtube.com/watch?v=va_q...ature=youtu.be

https://cimg0.ibsrv.net/gimg/rennlis...f7c65705bb.jpg
https://cimg1.ibsrv.net/gimg/rennlis...33cc81718b.jpg
https://cimg2.ibsrv.net/gimg/rennlis...5215a967d6.jpg
https://cimg3.ibsrv.net/gimg/rennlis...1cf4237217.jpg
https://cimg4.ibsrv.net/gimg/rennlis...bffb01c462.jpg
https://cimg5.ibsrv.net/gimg/rennlis...d6977f23c1.jpg

That's a crazy amount of inlet piping oil DiegoR. I'd take the intercooler off and flush it but it's going to take a while to blow out all the oil in the inlet piping (and probably exhaust/cats) with regular driving so it might still smoke a while even if the source of the oil is stopped. Why not just try venting the sump lines to atmosphere and see if it stops?

Ah - this brings back some fond memories. Back in the mid 2000's, I twin-turbo'd an Infiniti G35 (VQ35 engine same as Nissan 350Z) and had so many smoking issues. It was aftermarket, I get it, but basically because of where the turbos were mounted (below the engine) and due to gravity, sometimes the oil would not be able to drain quick enough from the turbos and cause white smoke. It didn't take long to fully damage the turbos from this so they had to come up with a fix for this as well as replace the turbos. The fix was carefully concocted oil drain lines to the engine oil pan and as you said ensure oil levels were PERFECT. Since then I've seen aftermarket turbo kits use pumps to pull the oil out of the turbos instead. Probably not the same issue as here of course, but just wanted to share some nostalgia. Guessing you were a Nissan guy from your alias :)

Pete

We always clean the exhaust , piping and intercooler before installing them back to keep testing. What we have found so far, is that while it's running, it doesn't leak oil through the turbine wheel or aparrently compressor wheel. Seems like when the engine is toped and left to cool down (night), we find leaks or flood of oil all over the turbo (intake/exhaust). Seems like there is "high" residual pressure left in the system that is being feed into the turbo, since the scavenage pump only works when engine is running, there's no other place to escape for the oil but only through the compressor/exhaust seals-rings.

Yes, these tests are being done with the turbo sump lines VENTED at the atmosphera. We have switched between rebuilded turbos and the new turbo, all tested in the left side (and from right to left side as well, i.e. swapping them fromside to side) and always leak the left turbo side ONLY. Right side is perfect. We even lowered the custom scavenage pump return pipe with check valve, so it seats at the same level or lower than the turbo sump ventilation line of the sump, as we thought it could do something with it being higher than it. No luck..

Honestly, I'm running out of ideas at this point....

I had the dreaded smoking turbos and I did all the easy stuff you can try before rebuilding the turbos. The stuff you should do regardless. Check valves, lines, filter, 20/50w, correct oil level, etc. None of it got rid of the smoking. I sent the turbos off to Kevin and he rebuilt them. After I put them back on, the left side (as I recall, but it might have been the right) still smoked badly. How depressing. Kevin offered to warranty the rebuild because I had completed all of his suggestions prior to removing the rebuilt turbo and sending it back to him for inspection: thoroughly cleaned the intercooler and the muffler, etc. Welp, even though I thought I had cleaned the muffler, including with solvent, there was so much oil residual left in the baffles that when I'd run the car it smoked terribly. I finally got rid of all the residual oil by putting the muffler in a BBQ with the lid shut on 500 degrees until the smoke stopped coming out. I guess I could have driven the car around and burned it off that way but it was somewhat embarrassing to drive with a trail of constant smoke. So yes, when these turbos give up the ghost, you can expect oil in the intake and exhaust - lots of it.

I hear you, I know how much oil collects inside the exhaust (CAT & MUFFLER) and take good time and few hard runs to fully cure or get rid of the smoke. But in my case the left turbo is still smoking/leaking badly. Today I applied a old "technique" I used in my early turbo conversion engine (BMW m20) I did many years ago, added a valve to regulate the oil pressure going into the turbo that fixed the smoke/leak in that engine. I have set the oil pressure lower and I will keep testing and see if still leaking oil (seep) through the turbine exhaust and center section plus through the turbine wheel.

Allright, more data after keep testing, scratching my head to find the culprit of the severe oil leak on the left turbo side. I must say, ALWAYS leaks in the left side ONLY. We have switched the 03 turbos between both banks, and all 03 turbos leaked bad in the left side only when engine is resting, i.e. if turbo "1" leaked in the left side and then it was installed on the right side, it won't leak anymore, and vice versa when installed the right side turbo in the left side.

I have concluded that the severe leaking (left side always) only happens when the engine is left resting after fully working hot condition. So, parked overnight, next day you will find all the oil in the compressor side (like the pic below) , but not ALWAYS such amount, leak through the exhaust turbine and drops of oil in on the sump as well as previous pics shows in earlier posts.

The first photos were PRIOR to the next following test described below (So you can see how bad is the oil in both sides)

https://cimg7.ibsrv.net/gimg/rennlis...066fbc76f0.jpg
https://cimg8.ibsrv.net/gimg/rennlis...56b72a8b5c.jpg


Then we cleaned all, installed back in place and after running the engine, we left it to rest and installed in our "Y" connector, a empty bottle can to collect any possible RESIDUAL PRESSURE left in oil system and here is a data we collected:

Hour ---------Oil in ML

11:30---------- 0
12:05 ----------200
12:35 ----------250
1:25 ------------350
2:10 ------------375
2:40 ------------400
3:10 ------------425
3:45 ------------450 +


First pic bottle full and had to dump it into the quart bottle to keep checking if more oil would keep comming out of the oirl pressure system (oil turbo feed line)....and second pic shows waht extra oil dumped and that was all.

https://cimg9.ibsrv.net/gimg/rennlis...7ffa47336b.jpg
https://cimg0.ibsrv.net/gimg/rennlis...5ca14ad4c8.jpg


Then we moved our "Y" fitting into the right turbocharger and did the same test. We left engine rest overnight , ran the engine and we collected the following data (Keep in mind this time the left rubo didn't leak a drop of oil maybe because the RESIDUAL PRESSURE was released in the right head/turbo side ?



Hour -----------Oil in ML

10:00 -------------0
10:35 -------------200
11:05 -------------250
11:35 -------------275
12:05 -------------300
12:35 -------------325
1:35 ---------------350
2:05 ---------------375
4:45 ---------------400


https://cimg1.ibsrv.net/gimg/rennlis...66390aa9e3.jpg




So right side turbo , as you see, does recieve residual oil pressure as well, YET, the turbos (03) tested in that side NEVER leaks which is very confusing. By the way, turbo sumps breathers were connected to the intake as factory in these last test we made.

Any ideas? :crying:

The nightmare continues...parked overnight

https://cimg1.ibsrv.net/gimg/rennlis...b0e1dc821e.jpg