Blow by ????

[RedMeanzGo]

Just wondering if anyone can give me some information about "blow by" ? I am a complete novice that has started taking my 951 apart piece by piece looking for mysterious oil leaks. I'm finding an oil type substance around all of my vacuum hoses, and on the inside of the intake manifold, some in the intercooler, and the idle control and surrounding hoses are also saturated. Is this "blow by" ?? If so any ideas how to clean it up, and stop it from coming back ? My guess is the air oil separator needs to be replaced, but what do I know...

[notny41]

I don't know a ton about "blow-by" but I always thought that was internal to the engine when oil gets past the piston rings and gets burnt and sent out the tail pipe.

[Tom]

Right, blow-by is when combustion gases blow past the rings into the crankcase. Compression and leak-down tests can help diagnose excessive blow by. All motor have "some" blow-by, but as the cylinders and rings get old, it gets worse. Blow-by can contribute to oil in the intake. Blow-by creates pressure inside the crank case, and that pressure can cause leaks, turbo oil draining/flooding issues, and will send oily crank gases out the AOS and straight into the intake j-boot. The more boost pressure, the more this tends to happen, even on a reasonably healthy motor. If you've modified your car and are running more boost, lots of people (myself included) have added a catch-can to separate out the oil before the rest of the AOS pressure is delivered to the J-boot. In my case, I had lots of oil in the intake and my dip-stick tube kept popping up under the pressure in the crankcase. I ended up drilling the top of the AOS a bit so the pressure wouldn't back up in the crank, and added a catch can to minimize the oil getting to the intake.

[RedMeanzGo]

Thanks for the thoughtful replies ! My 951 is completely unmodified… Do you think a catch is a good idea regardless?

[Black944 turbo]

Out of all of the turbo cars I have worked on, every single one of them has some oil in the intake track (some have been fresh builds with fresh turbos). As Tom stated this is somewhat normal, unless it is excessive I would not give it a second thought. If excessive, check all the lines to the aos, I have seen people swap that hose with isv hose and cause this type of issue, check the hose on the top of the aos they tend to crack. That’s my opinion and others may disagree but it comes from many years of observation.

[Tom]

Out of all of the turbo cars I have worked on, every single one of them has some oil in the intake track (some have been fresh builds with fresh turbos). As Tom stated this is somewhat normal, unless it is excessive I would not give it a second thought. If excessive, check all the lines to the aos, I have seen people swap that hose with isv hose and cause this type of issue, check the hose on the top of the aos they tend to crack. That’s my opinion and others may disagree but it comes from many years of observation.

Agreed on all counts.

If the oil mist is getting over the outside of the motor, I'd consider changing hoses -- starting with the little elbow hose on top of the AOS. It turns to rock over time and stops sealing well...

[Thom]

In view of my recent experience with the turbo sucking oil from the crankcase through breather hoses I would consider deleting the routing back to the J boot. I always thought that providing vacuum to the crankcase was needed as per the stock set up but I have not seen any adverse effect (yet) without it, and the significant amount of oil that the turbo always throws into the IC pipes and intercooler does not help getting a clear picture of how much blow by is taking place.
Deleting the line to the J boot and regularly checking how much oil residues are drained from an added catch can should give an idea on the amount of blow by.

[RedMeanzGo]

Thanks again for all the info !