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MG MGB Technical - blowing oil

Hello All,
I was trying to find my old thread about oil loss so i wouldn't have to start a new but alas I was unable to find them.

I have a 79 18V engine with an alloy head and a weber downdraft. I was looking for the source of oil loss and deduced it was being pulled into the downdraft because I wasn't using a PVC valve. secondly , I saw evidence of oil in the fresh air line coming from the charcoal canister, and since I am running smog control free and because I do not know the age of the charcoal I disconnected that line and also disconnected the purge line from the fire trap/crankcase vent to the carb. I was still experiencing oil loss but I was damed if I could see it. I assumed I had valve issues BUT never saw smoke or any other evidence of oil thru the exhaust. Well today, after a 250 mile day, I see this om my inner fender and heater box (see photos. It looks to me like the pressure in the engine is forcing oil out of any orifice it can find. So the questions start coming fast and hard in my head and some how I think all of this is related to the alloy head. In an earlier post I asked if there would be consequences in changing from alloy to steel and one of the responses did talk about compression. Can i assume the present compression is wrong and is creating this unrealistic buildup in my crankcase? This server will only allow one photo. I will start another thread with just the second photo.

Peter Murray

second photo

Peter Murray

first photo, wasn't showing in my post

Peter Murray

Peter,

It looks like you haven't hooked up a proper PCV system. Is that so? The oil on your heater looks like it is coming out of the inbound port on the valve cover. Presumably this would be because there is no vacuum on the outbound side.

Your engine may be making too much crankcase pressure, but it is kind of hard to know that unless it is ventilated properly.

You can run a compression test to see if the head made a significant change, but if the rings are in good shape, the cylinder compression should not be in the crankcase anyway.

Charley
C R Huff

When you say "oil loss" are you talking about a measurable amount that is evident on the dip stick after a short drive? - because in the attached photos, the oil I see is what I would expect from two open crank case ports. Just appears to be vapor blown back by the engine fan.

You need a way to provide negative pressure on the oil separator/breather hose, which you can do with a PCV valve if you modify the intake manifold, or attach the hose (without the PCV valve) to the underside of the air cleaner (there's probably a knockout there for that purpose). The hose on the valve cover needs to be connected back to the carbon canister or put a filter on it (see http://store.summitracing.com/egnsearch.asp?Ntt=crankcase+filter&searchinresults=false&Ntk=KeywordSearch&DDS=1&N=700+115 for examples)or plug it and use a breather-type oil filler cap.

Wayne
Wayne Pearson

Peter,

Yes, your first photo shows evidence of oil blowing out the intake orifice. I'm familiar with this problem as many years ago I drove from Virginia to Minnesota in an extreme cold spell during winter. Condensation in the oil separator line froze and blocked it resulting in oil blowing out the intake at the rate of a quart per 60 miles. The oil separator hose in your second photo is a vacuum hose that was originally connected to the ZS carburetor, enabling crankcase fumes to be burned in the engine. Note that the end of the orifice on the valve cover is very small. This intake is designed to be restricted such that there is always a slight vacuum in the crankcase.

Normally, a pressurized crankcase would be unrelated to engine compression, but perhaps a blown head-gasket or cracked piston could cause compression to relieve itself into the crankcase. Have you run a compression check?

Check to make sure the oil filler cap on your valve cover is of the NON-vented type. It's possible a PO replaced it with the wrong one. It doesn't seem too likely because I can see the "rubber band" on yours - which looks like it connects to a rubber non-vented cap.

Next, hopefully a Weber downdraft guy will post to tell you how/where to connect the oil separator line and whether or not you need a PCV valve. The stock Bs used them from '64 til '69 (plumbed to the manifold), but from '70 til '74, the vacuum hoses connected directly from the oil separator to the HIF carbs themselves.

FWIW,
Allen
Allen Bachelder

There are lots of threads about this in the archives. Crankcase venting is one reason I've held off buying a Weber. These are universal kits so you have to modify stuff yourself.

The crankcase vents are drawn off the front tappet (on the side of the engine)cover. On a 79 this would be a hose to a port on the Zenith that provided vacuum. On late 60's models a diaphragm PCV (Smiths) connected to the intake manifold.

Whatever carb you using this tappet vent has to be open not plugged off. Solutions I've read for Webers are:
1. Hook the hose up to the bottom of the aircleaner without a PVC and rely on the vacuum there. Most common.
2. Install a draft tube like the early MGB's had. This will likely mean lots of oil drips on the floor with a worn engine.
3. Install a PVC (preferrably Smith's) and pipe into the Weber intake manifold. Some tap a hold in the phlenum others have used the booster tap on the manifold. This would give the most positive venting.

The valve cover vent usually get's piped into the canister (make sure there is a restrictor in it) and the canister outlet goes to the aircleaner.

If your engine is badly worn you're going to get more blowby especially at higher loads. If the volume exceeds the system capacity the crankcase gets pressured and oil gets pushed out in all sorts of places.

Good luck.
Robert McCoy

Installing a PCV valve on the inlet manifold and connecting it to the front tappet chest cover is the best option, the other methods (and what you have now) are non-positive and can result in internal condensation and corrosion. The port on the back of the rocker cover should have a filter on it, otherwise it will be drawing dust and moisture into the crankcase.
Paul Hunt

Trying to be politically correct, I didn't mention what, IMHO, you really should do: lose the Weber. If your engine is in good condition it will run better on good HIFs (or even HS4s) and it will breathe as it is supposed to.

Remember, IMHO!
Allen
Allen Bachelder

This thread was discussed between 11/06/2008 and 17/06/2008

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