The o-ring at the dipstick top and how the tube bends, bending the dipstick when in-place, both serve to retain the engine oil level dipstick in place. If excess crankcase pressure, the oil level dipstick in the tube would pop-up exposing the o-ring so excess crank pressure can escape. An old o-ring has shrunk, and may not help seal / retain the dipstick in place.

A new o-ring can help maintain the oil level check dipstick in place with normal crankcase pressure.

The engine oil seals are pressed into place and are secured more tightly into the engine oil seal bores. Is oil leaking from the engine oil seals you replaced?

Securing the engine oil dipstick into place won't cause the engine oil seals to pop out with normal crankcase pressure and proper PCV system.

As oil heats to near and above water boiling temps, combustion byproducts including moisture in the oil boil out. The PCV as you know scavenges this crankcase vapor in a push - pull action. Push from the sump vapors boiling out of the oil, some very slight combustion passing the piston rings, and pull as engine vacuum pulls the vapors through the PCV and into the air intake.

The white block engines employ very tight tolerance in design. Combustion product passing the piston rings happens, yet is very slight.

You used Volvo or OEM quality parts when replacing the PCV assembly on the 2.9 L B6304 I6 in your 1998 S90? And I'll guess you used seal presses for oil seal installation?

The B6304 PCV crank case ventilation uses no moving parts, contains no valve or moving parts. Merely the flame arresting sieve in the flame arrestor assembly. Two vacuum connections draw crank case pressure into the engine through the air intake port manifold, one for idle and the larger PCV vapor hose when throttle is open.


Some vapor will escape from the running engine through the PCV and, if unsealed by a not in-place oil level check dipstick tube, the oil level check dipstick tube.

Link to the Volvo Parts Webstore B6304 Crankcase Ventilation Assembly. Opens in new browser tab or window:

Crankcase ventilation 6-Cylinder. Fits Volvo 960 (1994-1997); Volvo S90 (1997-1998); Volvo V90 (1997-1998).

Link URL
https://www.volvopartswebstore.com/showAssembly.aspx?ukey_assembly=238482

If the engine is cold, and you start the engine, and vapor begins to immediately escape the unsealed dipstick tube, suspect engine combustion (detonation) escaping into the engine oil sump or crankcase. Either passing the piston rings or through a combustion pressure leak into an engine oil drain passage in the form of a cylinder head gasket failure.

So far as you know, the B6304 engine timing belt has not ever broken during the engine lifetime?

You have verified that the oil trap, or oil breather separator box mounted to the engine block is in no way clogged. Vapors escape yet oil vapor condensate drains back into the engine?

One way to check for proper PCV action is to perform the unsecured engine oil fill cap. With oil fill cap unsecured, yet set in place, with engine started, cold engine is best, the combination of running engine intake vacuum stays the unsecured cap in place. The other method is the glove test, yet the B6304 does not offer a rise on the valve cover to secure a glove to as with say, redblock four cylinder.

You use synthetic or mineral oil?

Another way to test with cold engine off, remove the large PCV pipe at the intake, pinch or stuff closed the smaller PCV vapor at the intake end, wipe the large PCV vapor end and blow into it. With oil fill cap off, easy to displace air through the engine sump space and out through the uncovered engine oil fill hole. With oil cap in-place, about impossible to blow through.

You've checked compression or performed an engine leakdown test?

Sorry for the armchair speculation. Been a long time I've had my ham-handed-hands on the B6304. Would love one in an S/V90 with M90L behind it. All about torque. No need for Turbo.

I'm sure Spook and other brickboard 900-series owners with recent or current direct experience can better help you.

Hope that halps.

Questions??!?!

Ham-Handed-Hand Boyeeeeeeeeeeeeeee!!!!!
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Jonathan Harshman Winters III: The Mightiest, Greatest, & Most Powerful North American Comedian & Comedic Actor in Perpetuity!