My 89 745 Turbo is burning oil (Mobile 1) in a very strange way. This summer while climbing Fancy Gap, VA on I-77 on cruise the tranny downshifted into third - a bit later the engine went boom. Turned out the piston in #1 cyl. had melted,the oil was down two qts, and the radiator was nearly dry. Before this I was burning about a qt/2000 mi. So I get the engine rebuilt (including a new mitsubishi cartridge) and everything is great, no oil or coolant consumed in 1000+mi; I even made it up Fancy Gap again on cruise with no downshift or problems. The oil and coolant levels remained constant from N. Carolina to CT during a Thanksgiving trip. However upon returning down I-81 thru the mountains, I hit resume on an upgrade after slowing down for a semi and naturally the tranny down shifted into 3rd and held it for maybe two minutes at 4000 rpm (75mph set on cruise). Shortly after I stopped for gas, noticed a burning oil smell and found myself down over a quart, coolant was fine. Luckily I checked it, topped up, and had no engine damage. From that point on the oil level has not changed in 1500 miles around town. I feel certain that a load (uphill), higher rpm (4000), and boost (10-11 lbs w/Saab APC) cause me to loose oil in a hurry. This last (post rebuild) happened at night so I don't know about exhaust smoke. What is happening here? My crank breather assembly was replaced about 5K ago and cleaned during the rebuild so it should be functioning fine, all hoses have been checked - but what else can be causing this oil consumption if not this? Anyone have any ideas or similar experience? Oh yea first time I had oil leakage on the firewall from the dripstick tube, second time I could see no sign of leakage. Would a check valve on the line from the intake manifold to the "Y" fitting that closes under boost be of any use? I'm stumped.

"Would a check valve on the line from the intake manifold to the "Y" fitting that closes under boost be of any use? I'm stumped."

Other turbo and n/a cars have this... Its called a PCV valve and prevents the crankcase pressure from rising under boost. I don't know why Volvo chose not to install one.

In addition to the air filter, check the turbo oil return line for cleanliness/blockage. Oil must be able to drain from the cartridge faster than it enters. (that is why its bigger) Otherwise, the oil will gather in the cartridge and leak past the seals.
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bagle, 87 745t

Big hill, Big load. Lots of air being sucked into turbo. If there is any restriction,ie: dirty/blocked air filter, trash blocking inlet to air box, collapsed/collapsing air hose, mouse nest in air box? ......, oil can/will get sucked past the compressor seal even if it is new. Or create a greater vacuum on the crankcase vent system. Didn't sound like that this happened too many miles post rebuild. 2K, maybe?? You didn't say. Takes awhile for things to seat/seal after overhaul. You idea sounds like a pretty good one. Can't say whether a check valve would be of benefit or not. Hope you figure this out.

Thanks for your thoughts, I hadn't thought about blowing oil past the seals before. Due to the loss of oil ( a quart+ in less than two minutes) I can't see how it can be anything else but seal blow by. It is the only thing that makes sense. My higher than stock boost pressure, the higher rpm and perhaps the fact that going uphill had raised the oil level in the rear of the pan may all be contributing factors. If it blew past the rear main seal while I was going up hill it probably would not have left a trace by the time I checked the oil. The only way I can comprehend of the crankcase becoming that pressurized is through the small line from the intake manifold to the breather hose running from the flame trap to the intake air pipe leading into the turbo. I today installed a PCV or one way valve in this line. Went to AutoZone and looked for a PCV that would fit the hose ID and installed it so that it is closed under manifold pressure. I can tell no difference at idle or under boost. Probably I won't know if the problem is solved for awhile, but if I feel I have a conclusive answer, I will post it in this forum. Thanks again, hope it is the answer.