rwestervelt

Since acquiring my 1997 turbo the passenger side exhaust has smoked on start up. This got worse over time, to the point that oil was leaking from the turbo. I went through all the normal fixes.

• New OEM filter
• Lowered oil level
• Inverted p-traps on the oil return lines (Andial)
• Check valves on the oil supply side (Kevin)
• The check valve between the throttle body and the top of the oil tank functions properly

The motor was rebuilt 5,000 miles ago and leak down is 0% to 1% across all cylinders.

After all of this, the turbo continued to leak oil and smoke on start up, so I figured it was probably the turbo seals. I sent the turbo to Todd at Protomotive and learned that the center section and seals were fine. The turbine seal had nice tension and had not worn into the center section. The backing plate was not grooved or damaged from the thrust bearing. Everything was well within spec. However, there was an oil level line on the backing plate and the compressor housing. This is an interesting clue, as you will see in a moment.

I reinstalled the turbo and decide to run a test to be sure the scavenging pump was pulling oil from the sump. I removed the sump drain plug and put a hose on the sump-end of the oil return line. This way I could catch oil from the sump drain and see if the scavenging pump could keep up. I pulled the DME relay (so the motor would not fire) and fuse 26 (to prevent washing down the cylinders with fuel) and cranked the motor for about 60 seconds. During that time very little oil drained from the sump (this seemed strange to me) and the scavenger pump could easily keep up. My helper stopped cranking and 8 ounces of oil poured out the sump drain. What caused this to happen is a complete mystery, but things started becoming clear.

On a hunch, I pulled the sump vent line and ran the same experiment. This time during cranking, oil ran out of the sump drain at a steady clip and the scavenging pump kept up, never leaving any oil in the catch container. My helper stopped cranking and no oil exited the sump drain. So this explains why there was an oil line inside the turbo and why, after all the fixes, the turbo smoked on start up and leaked oil. With the sump vent in place, the sump would fill with oil after the car was shut down. This is not oil from the supply or return lines.

Here is my question to the members of this forum: Why would having the sump vent line attached prevent oil from reaching the bottom of the sump, only to release the oil when the motor is not turning?

Thanks in advance for your assistance.

-Rick

Basal Skull

I believe the vent line goes to the bottom of the throttle body. Do you have a different throttle body? I suppose it's possible that it could be seeing vacuum for some reason and it's sucking the oil from the sump up...

Basal Skull

Just re read your post and saw that your engine is freshly rebuilt. These cars have many vacuum lines (actually quite confusing), it is conceivable that when rebuilding, it is easy to get the lines mixed up and your vent is not hooked up properly and seeing vacuum too...

Philement

Rick, Hi our mutual friend Ed sent me here. I'm no Porsche expert but I want to share my experience with my twin turbo LS3 FRS race car that has remote turbochargers mounted behind the driver and passenger seats. I had originally built the system with a pair of Borg Warner journal bearing turbochargers, 1x -4AN oil feed line from the dry sump oil filter housing split into 2x -4 AN to the turbo feeds, 2x -10AN drain lines, drain scavenge box, and a Tilton pump scavenging and returning the oil to my dry sump tank. The first go ended with oil just absolutely pumping out the rear exhaust and of course massive smoking at start up. So I added a pair of check valves and frankly that didn't do much of anything. Given we built this engine for road racing it had pretty high oil pressure and we weren't pulling the feed from the heads like most do so I ran a pair of oil restrictors. These were still pretty high flow and this helped a ton but it still smoked at start up and oil was still present out the exhaust under load. A good friend of mine who builds many remote mounted turbocharger setups on V8 stuff insisted I remove the scavenge box. I argued with him over this but honestly I was stumped so dropped a T fitting in, eliminated the scavenge box, and wouldn't you know problem solved. No more oil dripping from the exhaust, no more smoke under load, and well he was right.

He said I should even drop the hose size down to -8 on the drains as the scavenge pump will pull more vacuum. At this point I have no reason to doubt him. LOL!

Now onto your why the vent line is causing an issue. Having a vent line could allow a siphoning effect similar to how a gas can vent works. If your vent line is attached to your motor like how a PCV works, you're pulling vacuum in that scavenge tank and that could be greater than your scavenge pump's ability to pull the oil. We see this mistake with dry sump tanks when people don't vent them to a catch can but plumb back to the intake of the motor. The vacuum applied to the dry sump tank results in less flow to the pressure stage where you actually want a little positive pressure in the dry sump tank to help prime the pump. The same concept applies with a scavenge pump especially if it's a mechanical system. I'm sorry but I'm not an expert on the Porsche setup so I can't directly comment on how that pump is designed but if it needs to be primed (wet) that can be part of the issue.

Your freshly rebuild motor may also have much more oil pressure so limiting pressure/flow to the turbochargers may be required. Restrictors can be a little tricky to dial in but Turbosmart actually makes and oil pressure regulator to take the guess work out of this.

Good luck and hope my experience sheds some light on this issue.

rwestervelt

Good point. I will check the vent line routing. I have never been able to find a diagram for the vent lines. Thanks!

rwestervelt

Hey Phil. Thanks for the detailed response. The idea that the sump needs a little positive pressure makes sense.

I initially suspected that the vent line was pulling a vacuum, but during my test the sump drain and return line were both open. I figured the vent line couldn't pull enough vacuum to prevent oil from draining out the bottom of the sump, but I suppose it is possible. It will be easy to see if the vent line is pulling a vacuum, and if so, that will be the next (final?) fix.

Philement

You're welcome. I wish I could speak from specific knowledge of this car but typically if what "Basal Skull" said is correct and the vent line runs to the bottom of the throttle body, I can't see it working properly. At idle and cruise your intake manifold is seeing vacuum so you would have a maximum of 8-9 psi of vacuum but under boost you would have + whatever psi you're running which would pressurize your scavenge tank preventing the turbos from draining well. Oems often place a check valve in this throttle body location vent so make sure it's there so you are at least not pressurizing the tank.

Given I don't know the design of your scavenge pump it's hard to explain the phenomenon you're seeing but I ssuspectair in the tank reduces the effectiveness of your scavenge pump. That's the primary reason there is a tank so that oil fills the bottom of the sump, sealing the drain, and the scavenge pump can pull mostly fluid and not air. In my application the Tilton pump can pull air but geared pumps aren't so great at that.

Ultimately your biggest complaint is smoke at start up which you believe is from oil pressure in the turbine section pushing out the turbine seal when the engine is off. This would only happen if your tank or drain lines are full at shut down and or your check valves aren't working well enough (I run my scavenge pump for a few seconds after shut down).

That's what you think but I'm starting to think you're pulling oil into your intake manifold from the scavenge tank via the vent line. All this oil in your intake runs down the ports and sits on your intake valves or goes into the cylinder bores. Because the motor is flat it doesn't drain into the pan. Fire it up and voila, smoke. We see this all the time with improperly designed catch cans that vent back to the intake of the turbocharger. A proper air oil separator along with a properly restricted vent that matches the flow of the turbocharger solves this. Most just run an unrestricted hose after elimination of the factory pcv system which is restricted from the factory.

I would try leaving your sump vented to atmosphere and see what happens. I believe the reason for the vent is to limit how much vaccum the scavenge pump can pull to prevent sucking the turbo dry. I find that hard to believe it's actually possible though.

​​​​​

tacker

Philement

Yeah I'm running a pair of .060 restrictors. Vibrant Performance makes an AN to NPT restrictor (so you can thread it into your check valve) with .060 as standard, plus a .045 and .030 options.

​​​​​​Restrictors always scare me in anything but a ball bearing turbo though. Just have to be careful not to take it too far with a journal especially if running high boost.

rwestervelt

Thanks tacker.

rwestervelt

Philement

The driver side sump vent leads to a check valve on the throttle body, so it can pull a vacuum, but it cannot create pressure.

I do believe the drive side sump is filled with oil right after shut down, and it is very possible that I am pulling oil into the throttle body from the turbo sump via the vent line.

The rest of your comments make sense. I will check the passenger side turbo sump vent to see how it is piped. I am not having problems with the passenger turbo, so perhaps it is pipe correctly, and the driver side is not.

The next step beyond that is venting to the atmosphere for a test.

Thanks.

morsini

Having the same problem right now - right side exhaust smoking with oil smoke, so I'm following. I'm going to replace the check valves first as everything worked fine before I parked it.

Philement

Hi any luck venting to atmosphere therefore eliminating the possibility the oil is from the intake side of the motor?

DRPM

rwestervelt

I found the sump vent hose diagram in the 993 Turbo Workshop Manual, and my hoses were routed correctly. I can elaborate if anyone is interested.

I cut the sump vent line high in the engine bay and plugged the ends (one for each turbo) running to the intake manifold. I installed a small filter at top end of the sump vent hoses and zip-tied it in the engine bay.

There is no smoke from either exhaust at start up, long idle or hard driving. Woohoo!

I am not sure why the properly routed sump vent hose set up was pulling a vacuum. I will continue to investigate and report back.