One day when I opened the bonnet with the engine running, I saw the oil cap 'dancing' and rotating slightly. I took it out and to my surprise there was quite a current of air coming out. It can blow a paper towel at 10 inches...

Is this normal ?

Hard to tell - certainly my engine will move the cap about if the rubber ring isn't perfect, but that's as much from vibration as anything. The engine shouldn't breathe heavily, but I think it takes a bit of experience to know what it should be like...

One day when I opened the bonnet with the engine running, I saw the oil cap 'dancing' and rotating slightly. I took it out and to my surprise there was quite a current of air coming out. It can blow a paper towel at 10 inches...

Is this normal ?

sounds like my girlfriend!!!!LOL,just checked mine and the cap doesn't rotate or dance but when i take it off there is a current of air,i would think that your cap needs replacing.or at least the rubber seal.

Edited 10 June, 201115 yr by ALLYZ66

sounds like my girlfriend!!!!LOL,just checked mine...

what?!!! you checked your gf and she needs a new rubber seal ??? bummer...

anyways, now seriously, I guess the answer to my question is this: YES, I need a new rubber gasket for oil cap and YES, a current of air is normal, although I would like to know what moving parts are pumping.

The pistons! There's a lot of movement.... heavy breathing is down to worn rings though.... or a cracked piston in my case once!

Piston rings are in good order. No oil consumption, lots of compression, plenty of power and torque.

So why is it so windy under cylinder head cover ?

This is a bit of a shot in the dark... maybe the breather is blocked? so what normally would get blown that way is coming out the cap making it appear like there is more than usual??

This is a bit of a shot in the dark... maybe the breather is blocked? so what normally would get blown that way is coming out the cap making it appear like there is more than usual??

That's exactly what I was thinking too. The most likely (and cheapest to fix) issue is a blocked crankcase breather.

Worn bores or rings are also possible in principle, but less likely and ruled out by a good compression test anyway.

There isn't a crankcase breather on that engine, it breathes via the rocker cover with an outlet into the induction... As I've already said I don't think there's anything particularly surprising happening if the 'wind' is as little as you say; the oil filler cap on these engines will move about if the rubber seal is worn down to vibration, and there will always be some wind with an engine; there's a lot of movement in there and lots happening temperature wise as well. If you have real breathing problems, you'll know about it as the pressure will be lots.

If you have real breathing problems, you'll know about it as the pressure will be lots.

And the oil cap will pop off I wondered the same with mine. I think it's just the normal compression/decompression and otherwise the small amounts will be taken up with the breather when the oil cap is on.

There isn't a crankcase breather on that engine, it breathes via the rocker cover with an outlet into the induction.

Ok, so how does crankcase pressure, say from oil vapour, never mind blowby from the cylinders, get from the crankcase to the rocker cover if there isn't a crankcase breather? I never said that the crankcase and rocker cover had separate breathers (largely because they're normally linked [and ex-works recirculate vapour through the induction]).

Have you ever even been near a 136 engine?

ok, it's summer (probably in UK too ), but before things get hot, why don't you guys throw some schematics showing the breathing circuit ?

Ok, so how does crankcase pressure, say from oil vapour, never mind blowby from the cylinders, get from the crankcase to the rocker cover if there isn't a crankcase breather? I never said that the crankcase and rocker cover had separate breathers (largely because they're normally linked [and ex-works recirculate vapour through the induction]).

exactly the same way as an A series engine does up thru the pushrod holes which having spent a lot of time studying on Kens engine and his expensive mistake!! are very carefully designed so that returning oil from the head lubes the cam lobes.

there are holes in the block to allow the pumping gasses up to the head and these would never block up (well unless you filled them with paper tissue or something lol.

not only does the engine displace 1298cc with every 2 rotations but those same pistons are gonna be moving gasses about on the underside of them too so as the piston goes down it has to push the air from under it etc, if its really breathing badly you would have found the oil filler cap on the tray under the altenator lol not just rattling a bit

Exactly!

I believe the document below will explain everything in great detail.

Blow-by and Breather Systems

It's just the rubber seal that's gone. Same thing happened to mine. A few times when giving it some beans it had popped off then one day I couldn't find it so got a new one.

It does blow a fair bit of air out the filler without the cap on but nothing to worry about.

Phil