• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

O2 sensor and crankcase vent

Share this page

Morris

Morris

Yoda
Offline
I have hooked up a Wideband 02 sensor to my '79 Midget, and I believe the blow by in my engine is corrupting my signal. With the crankcase vent tube plugged into the carb, the car reads a little richer than without. This seems to be regardless of whether or not the tube to the charcoal canister is plugged in.

Is there a way to vent the crank case without plugging into the carb? If I just pull the tube and allow it to vent to air, oil blows out the dip stick when the engine is under load.

morris
 
If you are getting a lot of blow-by from the dip stick when your engine is under load, you likely need new rings or pistons with re-bore (or maybe valve guides). There is no real way to get around this.
For temporary testing, you could try running thicker oil (like the 50W oil used in Harleys), but this isn't a proper cure.
To some degree, these 1500 engines *do* push a bit of oil from the dipstick if they are pushed real hard. Mine used no oil on the street and had perfect rings/pistons/etc. but it still pushed a bit of oil out of the dipstick until I threaded the dipstick and screwed in a plug (I have to unscrew this to check oil). But my car *lives* at 5000+ RPM and is almost never under no-load.
At any rate, here is the system I use on my race 1500. It ran over 6 total hours racing at a weekend at Summit Point and burned no oil, so this sytem works good (if your internal engine parts are OK). The "spacer" is 2" ID PVC pipe. There are no other crankcase vents to the engine...just out of the valve cover.
carb-vent-1500.jpg
 
You can vent the engine through the dipstick tube. It works great. Run it up high to a catch can. Keep the dipstick somewhere safe in the car. Works for me
 
That catch can schematic looks great, Nial!

I am sure that I need to repalce the pistons, valve guides, and everything else, but I also think I figured out why I was blowing oil out the dipstick. I rigged a PCV valve between the carb and valve cover. When I was pulling the tube to check my AFR, the PCV (still connected to the tube) was falling down such that the valve closed and stopped all ventilation. Rev the enging up and squirt!

Also, in my lastest tests, pulling the tube has no effect on AFR. This confirms that my carb is shot (surprise, surprise) but it also may mean that I can still get good readings when I am tuning my EFI system.

morris
 
i have a 1500 mg midget, weber dgv carb.

when you talk about "blow by from the dipstick" what does this mean?

If I remove the dipstick on mine and put my finger over the hole when the engine is running I notice a vacuum is being pulled (then I hear air rushing in when I remove my finger). Is this normal? Or does this mean that valves or piston seals need to be replaced?

Should the dipsick be sealed?
 
Seems to me if you feel a vacuum at the dipstick you don't have anything to worry about as far as crank case pressure. I do wonder though where you are picking up that much vacuum for your crank vent with the weber.
JC
 
My 1500 is the same way but my weber conversion connects the vent pipe to the bottom of the air cleaner. No way there is that much vacuum present to pull vacuum at the dipstick unless I really rev the engine. Oh the the dipstick has a felt pad at the top of it. That is your seal or at least a quasi-filter with the vacuum you are pulling.
JC
 
do you have your instruction on where the connections go for your weber conversion that you can scan in and post or email me?
 
my crankcase vent connects to a port on the intake manifold right below the carb (in the hose there is check valve between the crankcase vent and the intake manifold)
 
Back
Top