To whom it may concern I recently acquired a 1989 740 GL and have been battling an oil leak on it for the past 3 days, I bought a new valve gasket, a cam shaft seal (original source of leak) and oil cap from O'Reillys. After replacing the gasket there was a little trickle of oil coming from the valve cover in the rear, after trying another gasket the rear leak returned so I used a silicone gasket maker with the new gasket and it seemed to work. Upon my arrival home this evening I pulled up the hood to check and make sure all was well, and the leak on the valve cover was gone, however now there's another leak coming from under the oil fill cap, I don't understand how as this is a brand new cap if anyone has any idea what could be causing this please let me know!
Thanks!

 

Have you checked your flame trap and oil separator? Make sure it's not clogged. It provides crankcase ventilation and if it is clogged you will spring all sorts of oil leaks due to the pressure buildup in the crankcase.

 

+1. I don't trust O'Reily oil caps, just the seal would have been better... But the engine runs a slightly negative vacuum (sucks rather than blows) so the cap should not be a great source of leakage. If you open the cap while the engine is running at idle, there should be no air blowing out. Sounds like your system is indeed plugged. See

 

More the likely its oil the cap that doesn't seal properly. I mistakenly put another oil cap from a newer volvo and it kept spitting oil which it never did before. I found the original cap and put it on and no more leaking. Changing the rubber seal under oil cap would be the way to go.

 

I would also suggest looking at your flame trap...they get clogged over time, and when they do you have positive pressure inside your engine that will push oil out of any orifice where the seal isn't 100%. Just like act1292 pointed out. I did my flame trap the first time, without pulling everything apart around the intake like most folks say you need to do. I have real long needle nose pliers, and with some effort was able to pull and replace that little $1.00 part. Companies like IPD also make a flame trap re-locator kit, to make subsequent inspections/changes super easy. There is also the oil separator that bolts to the block under intake manifold. If I remember correctly, I was able to remove that without removing intake...just took off throttle body assembly to get under there to remove/clean/replace if needed. Read up on it, as there is one thing you don't want to do (can't remember for sure, but something to do with o-ring not getting pushed into block or something...google volvo b230 flame trap and oil separator...tons of info out there). If you do take off intake, make sure to buy a new gasket for it. My separator was pretty dirty. Also cleaned out all the tubes/hoses that go to flame trap, etc. That video shows things pretty well, but lots more out there also. Great chance to clean up that dirty throttle body too...which helps idle quality, etc.

As pointed out, while running, the oil fill hole in the valve cover should suck, not blow. There is an old trick I read about...if you take off the stock oil fill cap, and turn it upside down on the fill hole while car is running...the vacuum that "should" be there should be able to hold the cap over the hole without it vibrating away from it. Mind does!! This video shows what I mean
Again, the idea is that if oil separator box, any of the hoses, or flame trap are clogged...the positive crankcase pressure will manifest leaks that may otherwise not surface with proper negative pressure. When you find yourself "chasing" down those pesky leaks...only to create a new one when you plug another one up, it's time to check the crankcase pressure and rectify that if needed. Most of the leaks will subside. The ones that don't need to be addressed, as you are doing. Keep it up!

 

When checking the flame trap do not forget the small hose and the nipple on the intake. They can also get plugged.