Is it safe to use silicone vacuum hoses for crankcase/fuel vent?
 

Hi,

I bought and have been using silicone vacuum hoses from Mazdatrix (3.5mm & 6mm) to replace my vacuum hoses on a stock '82.

When I got to the ventilation valve - which connects to the oil filler tube and crank case, I started to wonder whether the silicone would stand up to fuel/oil.

(Incidentally, I found some of the hoses don't fit that well - I think I should have ordered some of the 8mm hoses... In particular, the idle compensator hose really needs to be 3.5mm on one end and 6mm on the other?)

Anyway, any thoughts about suitability of silicone for those vent hoses?

Yeah those hoses that Mazda uses which have different sizes on each end are annoying. I usually find one that will still be snug on the small side and stretch it over the bigger size on the other end. Not ideal but it seems to be holding up and has been for years.

As for the crank vent, officially no, silicone vacuum hose isn't meant for oil/gas, however I think a lot of people do it (myself included) and haven't had any issues. I replaced all the vacuum lines on my T2 swap back in 2006ish and they've held up really well so far. I think if the hose were filled with oil all the time it would eat it up pretty quick, but it's just vapor and some residue that it has to deal with in our case, it should be fine...

I've been using silicone hoses on the vent lines for over a decade now - the same hoses. Durability is not an issue in vent-line use, I'd say

The hoses stand up just fine, but if continually exposed to fuel or oil they tend to 'leach' - - they will build up a residue of oil/fuel byproducts on their exterior. It's a weird thing; the hoses still hold vacuum, but they get a greasy/sticky goo on the exterior. Wipes right off. Doesn't seem to impact their strength at all.

I for-certain wouldn't use them to carry any liquid petroleum product.

OK thanks for the testimony :)

82transam - the idle compensator hose is a real bastard - the 3.5mm will not fit on the air cleaner end, but the 6mm is loose on the carb end!

Starting up today, that wrong sized hose on the idle compensator caused a vacuum leak. Had to put the old hose on.

Are you talking about compensator for the a/c? I actually used some RTV to glue the bigger hose into it on mine. Actually held up for many years with no issues. Just gotta be careful not to get the silicone in the hose or it'll clog (obviously)

No I'm talking about the idle compensator (not the A/C throttle opener) - this is the bimetal switch inside the air cleaner that opens above a certain temperature. The air cleaner end is 6mm whereas the carb end is 3.5 mm.

The 3.5mm hose fit snugly onto the throttle opener, that wasn't a problem.

Probably I'll order a new stock one from Mazda next time I put an order in (unless it's $563.98 or some other randomly overpriced figure!).

I used a slightly undersized hose and forced that mofo on there. You could also use a line clamp to hold it on..similar to how they used to attach the rad hoses way back in the day...I've seen a few with zip ties too lol I forget what size I actually used but it took some effort...

check out this link
Fuel / Vacuum Hose Low Pressure Braided Cloth - 4mm
This is what I'm gonna try.

In my experiences fuel/oil causes the silicon to swell and soften.

Interesting. For now, I'm just interesting in eliminating vacuum leaks. I think I'll replace the silicone with rubber for those hoses when I get around to it.

It's so satisfying replacing the hoses on my stock emissions though, and to have solved some issues I had with that. There is no excuse to remove emissions - aside from the hoses, everything just works, just takes patience :)