So I cleaned the oil filler out today and moved the CPD valve closer to the filler so that less oil escapes into the catch tank. CPD valve working as expected, compression is good at 160psi on both sides.

I was cleaning up the crank case today having solved an oil leek. There was some residual 'gunk' at the bottom and when `i turned the engine over `i noticed bubbling around the points box. I guess this show air escaping from the crank case, but why....?

http://youtu.be/k9AhVZKiIkw

cpd valve will have minimal effect on the top end compression - by top end were talking above the pistons 160 on that engine in the vid sounds ok given that its cold and cranking slowly rings and valves in good order.

Bubbles show that there is slight pressure in the cases a manomiter test is what you need

DSC00079 by Sean602, on Flickr

down and dirty "redneck" manometer

Sound similar to the problem on my thread - oil leaking from camshaft - There was positive pressure in the engine because of faulty Reniflard - YES do manometer test. Even easier way - clear tube, glass of water on floor, tube into dipstick (sealed with tape or tapered bung) start up and measure water surface to water up the tube - I had 40cm so tube has to longer than around 80cm for safety.

I'll have a go at a down n dirty manometer - what will doing that prove?

It'll confirm whether you have positive pressure in your crankcase - which it sounds like. It should be a partial vacuum.

The 'top-end' pressure (cylinder compression) is a different thing entirely - that shows how well your piston are sealing against the cylinder walls and how well your valves and guides seal too, and 160psi is pretty darn good.

'Bottom end' pressure is also caused by the pistons flying up and down. When the pistons travel down, this increases the pressure in the crankcase; when they go back up, this causes a slight vacuum - so, it pulses. The rubber valves in the Reniflard open when the pressure is 'positive' (allowing the unwanted +ve pressure to escape into your air filter) and close when the pistons travel back up causing the vacuum below. This has the effect of creating an overall vacuum in the crankcase - which is what you need to prevent bubbles and leaks!

So, a faulty Reniflard is the likely cause.

(You knew all that, didn't you... )

No thanks for the explanation! Thing is, the van doesn't have a reniflaud - I've got a (tested and working) CPD valve, or at least, it expels air and doesnt let it return, so I'm slightly confused.

Is it possible it could be fitted the wrong ?

Cpd valve definately round the right way!

Vans jut done 120 miles without a hiccup so can't complain.

Sam,
at engine cranking speed, the crankcase breather ( wot's a reniflaud? anyway?) will not have had time to get into its stride and establish a constant partial vacuum, since at such low speeds the pulses of 'pressure' & 'partial vacuum' are not just more pronounced but also of low frequency.

Even at a normal idle speed of around 900 rpm, it's possible to detect pulsing of the column of *liquid* in the manometer despite its movement being damped through enclosure in relatively small bore tubing.

*Hints & tips time again; Use LHM, then there's no problem if any is ingested into the engine, it's easier to take a reading, divide column difference by 1.1 to obtain equivalent water gauge*

ken

What's the CR of your pistons, btw?
For 8.5:1, a ( hot engine and wide open throttle) compression reading should be 150 psi, for 9.0:1 it should be 175psi.

I believe I'm on ye olde 7.75:1 pistons (could that be right), or that's what she'd have had from the factory, wether they've been 'upgraded' or not I don't know - but I know the pistons rings aren't available anywhere (weren't you there when we measured them?)

Cheers Ken, so nothing to worry about then I wondered if it took a while for the CPD valve to 'kick in'.