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Concerning oil leaks, crankcase ventilation, the "jiggle" test, and Playboy bunnies![200/1991] posted by Don Foster on
Sunday, 21 March 1999, at 8:41 a.m.

Wow! Three questions, and all the same -- or close to it. Personally, I wanna see the jiggling....... No, never mind. Let's talk about crankcase ventilation.


A normal byproduct of a normal engine operation is "blow-by", or a slight leakage of combustion gases by the piston rings and into the crankcase. It is very desirable to release this pressure. Years ago it was simply vented into the atmosphere. Later, it was vented into the intake system through the air filter. More recently, it is forcibly removed using engine vacuum and certain controls. This system burns the vapors, reducing the environmental problems.

On the Volvo B21/23/230 engines, this is accomplished with the contraption we call the "flame trap", but the system also includes a flame arrestor, the oil separator, some hoses, and a fitting on the intake manifold.

Excess pressure can force oil past seals. Excess pressure can damage seals and gaskets. Excess pressure gunks up engines. Excess pressure vents into the atmosphere, which is not good for the environment. On the 240 with the 230, excess pressure can pop the plug outta the back of the cylinder head (happened to my daughter and she won't let me forget it).


Crankcase vacuum is directed through the "flame trap" system. During normal driving, engine vacuum is from the intake manifold (upstream of the throttle), through the top of the flame trap, through the flame arrestor, through the oil separator (a.k.a. "breather box") and to the crankcase.

During idle, when there's insufficient vacuum, additional vacuum comes through a small hose attached to the flame trap and to a small fitting in the center of the intake manifold.


The flame trap consists of a lower hose (which sits directly on the oil separator), the upper hose (which has a large and a small fitting for vacuum hoses), and the flame arrestor. The gases in the crankcase are highly combustible, so it's very important to prevent igniting them with a backfire. (If you did, you'd be replacing all your engine seals and gaskets, or maybe your car.) The flame arrestor will absorb the thermal energy from a backfire and quench a flame front. (Note -- this is a very old technique, used during the last century in coalmines when the miners had acetylene lanterns.) So it blocks flames but passes gas.


The oil separator is bolted to the block and is a single molded plastic unit. I don't know how to tke one apart, although I've successfully cleaned out several of them. They're cheap enough that you might buy a new one without a second mortgage.

The function of the separator is to allow the oil vapor and droplets some time and space to coalesce into larger drops and flow back to the sump. The separator has two openings into the block -- one for vapors to rise through, and one for the oil return. The oil return opening has a hose the MUST remain in place -- don't dislodge it, or you'll be pulling the pan.


The flame trap is located (buried is more truthful) under the intake manifold between headers 3 and 4. It sits directly on the oil separator. Cleaning the system includes cleaning or replacing the top 'n bottom hoses, the brass or plastic arrestor, the large hose to the intake manifold, the small hose, and the small fitting. And the oil separator.

The small hose and small fitting (in the manifold) are famous for plugging. Clean the fitting, replace the hose.

Inspect the arrestor (old style=brass, new style=plastic) to confirm the passages are clean and free. If not, either was or replace -- they're only a buck, or so. Buy a handful.

Volvo sells a "kit" which includes the top 'n bottom molded hoses and the arrestor. It's worth the few dollars.


A coupla quick checks....... With the engine idling, pull the small hose off the flame trap and feel for vacuum -- it should be there.

Pull the trap off the oil separator and observe (or feel) the separator to confirm that crankcase vapors are streaming up.

Do the "jiggle test" -- with the engine idling, loosen the cap but leave it in place. If it sits there quietly then there's enough vacuum to hold the cap. But if it jiggles and bounces, you have insufficient vacuum (and maybe too much pressure). It's conceivable that a partially blocked system might provide slight vacuum, requiring only a little finger pressure to hold the cap down. If so, I'd start thinking about some preventive maintenance.


For the technically-inclined....... I built an adapter and measured the crankcase vacuum at idle. All four of my registered Volvos ('82 245 with 335k; '82 245 turbo with 130k; '85 245 with 235k; and '91 740 with 180k) measured from 1.75 to 2.25" of water, vacuum.
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