1996 850 20V Non-Turbo Why Not Drill Out Flame Trap

[PeteB]

Has anyone tried opening the hole in the flame trap show by the red arrow in this pic?

FLAME TRAP.jpg (27.8 KiB) Viewed 1577 times

I am doing the oil grade 5/8" hose upgrade to the PCV system, but thought why not
open that hole up a bit also? I have already removed the white plastic flame trap
insert and am leaving it out.

Looking at the PCV system it can be seen that the small nipple goes to a manifold vac
fitting for when the throttle is closed and it is idling, that hole shown above goes to
port vacuum for when the throttle is opened. This is a bit complex since
when the throttle is closed the small fitting will draw from both the crankcase via
the large hose but also from the leak to the air box. On the other hand the crankcase
becomes pressurized by the blow-by so those gasses arrive under pressure up to the
flame trap. As the throttle opens the vacuum increases at the port side and since the
hole is much larger than the smaller line the gases primarily take the path with the
largest flow and vacuum through the hole that I'm asking about making bigger.
It seems that there is a balance between the manifold flow and flow through the
port side.
Another consideration is that from the factory the manifold vac goes through a small
hard plastic line that has a rough OD of 3.5 to 4 mm and we increase the flow when
we replace it with rubber hose with an ID of about 4mm - probably a good thing on
an older engine.

Thoughts, should I open that hole up a bit?

[rspi]

No.

[erikv11]

Go for it, I don't think it can hurt. Let us know what you find. You can replace the plastic flame trap housing piece for a couple bucks if it doesn't work out. As you may have read, the turbo models have an analogous hole, in the PTC, that people drill out. Fortunately most of these engines don't have much blowby anyway.

Hopefully you have thrown away the actual flame trap disk, that white thing in the pic.

[cn90]

1+,

Agree with erik. Throw away that white "flame-trap" disc, nothing but trouble.

These engines are now 150K-200K with probably more blow-by gases.
I think it is a good idea to enlarge the hole a bit.
Just do it slow, drill it to the next diameter size.
Then check it in 6-12 months. It gives you something to do.

[PeteB]

LOL - gives me something to do ... Dont' want to look at it ever again.
I'll give it a try.

I did throw away the white flame trap disk, but I think that leads to:
What I did find was that the small nipple was completely blocked with
that burnt dried up oil and I had to drill it out to clear it. I think the
purpose of the white disk is to avoid that problem and just replace the
disk every couple of years. Even if it was clear it would not have mattered
since the small hose was completely broken off. I'm tempted to drill a
hole for a 1/4" nipple and epoxy in a brass one. I expect the flow would
be too high but perhaps it could be metered down somehow.
Has anyone put a vac gauge on a healthy (rebuilt stock PCV) stock engine?
The larger hose would act as a vac leak, in a sense, and probably lower
the vac level, possibly throwing off vac at the FPR. A larger line should
then be metered down to maintain the correct idle vac. The meter point
would probably clog so it should be in an easy place to clean out once a
year or as often as needed.
Thinking more about this the metered down spot should have multiple
holes so that if one clogs, there are still other paths and we are back to
the flame trap. But, the difference with this idea is to only put it in the
small line path to protect it from clogging. Then if the flame trap clogs
in this location it only blocks the idle path and the main path is still open.

My gut feeling is that the idle path is for emissions only not a critical
thing for avoiding blown seals - I could be completely wrong. So even if
it did clog it would not be a major issue as compared to the large main
path clogging. The large, unclogged, main path maintains a pressure
relief path for the crankcase.

[PeteB]

The small passage way, line #3, being broken or blocked anywhere explains
the smoking dipstick at idle since manifold vac is very high at idle and it is
the main path for blow by.

[erikv11]

The disk does nothing to prevent clogging, in fact it only makes clogging worse because the disk itself gets clogged up.

The disk with the holes in it is the actual "flame trap." Its intended purpose was to block or slow down flames, or flammable liquids, from traveling through the line. It was over-engineering and Volvo stopped using it.

[PeteB]

Makes sense, also the old flame trap was supposed to have enough flow for
even the large hose and hole so it would not meter the small path correctly.
Yes, I agree with the suggestion to chuck the white flame trap, especially in
the path of the large hose.

The worst thing on our car was having a huge vacuum leak, so I'm going to
do this with a strong hose to avoid another break and will inspect it often.

[rspi]

I would leave it. Not sure why we have desires to re-engineer these cars.

Have you ever seen an air filter SCORCHED? I have seen a couple.

[PeteB]

I mentioned how there is probably a balance between the manifold vac
and port vac, so I don't think I'll open that up because I don't have the time
to do more if it doesn't work well.
Some of us like to try to fix things that don't work very well, this PCV
fails far too quickly, some report 2 years after new parts are installed.
I removed the flame trap and probably broke some of the lines in the
process. I hope not to touch this again for 5 years, if possible, if we keep
it for that long.
I think that the large hose problem is fixed with the large oil rated hose,
and thanks very much to the people here who figured that out. But I expect
the smaller tube to clog again within the year, so it seems to need some work.

No I've never seen a scorched air filter.