rodgling
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 posted on 18/4/11 at 12:32 PM |
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high crankcase pressure
A few weeks after passing IVA, I noticed that there is very high pressure at the crank case breather hose (if I remove the hose and put my hand on the
head, I can feel strong pressure, almost enough that I can't seal it with my hand). There's a big hissing sound of escaping air when I
turn the engine off, and I suspect the same thing while running, except I can't hear it over the engine.
On a friend's 328 (it's an E36 328 btw) the pressure there is very light, I can't really feel anything.
There's light vacuum at the filler cap, as per normal.
What could cause this? My understanding is that crankcase pressure is caused exclusively by blowby, and the less of that the better... does this mean
my engine is shagged? I compression tested it before installing it and got low-ish-but-ok readings with acceptable variation (160 psi +/- 15), I think
Haynes suggest 185 psi is ideal.
I'll compression test it again and see if there's been any change, any other suggestions?
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Mr Whippy
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| posted on 18/4/11 at 12:44 PM |
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Depends on the engine and not necessary an indication of excessive blow by. I just put my mum’s 50k Corolla through it's MOT, which sailed
through the emissions. Yet there is so much "air" coming out the oil filler while running that you can't even place the cap over it
without it flying off. Yet my Bluebird has just a slight draft, again it's another 50k car
Unless the cars running rough or is smoke - Don't panic!!
[Edited on 18/4/11 by Mr Whippy]
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rodgling
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| posted on 18/4/11 at 01:06 PM |
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It's not normal for this engine. The pressure is high enough that I think it will cause problems since it's not supposed to have that much
pressure there; there are also other issues which may be related - my car's currently using too much fuel (about 20 mpg instead of 30+), and
emissions are a bit borderline. So it's clearly not running right - I need to get to the bottom of this.
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Mr Whippy
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| posted on 18/4/11 at 01:28 PM |
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Check the breather then and that it is not blocked. If there is too much blowby then it would also be using plenty of oil and smoking, as it is worn
rings that cause blowby. What are you using to judge the MPG as it is actually quite difficult to establish when it is abnormal due to all the various
driving conditions and can only really be done by comparing what the engine use to do in the past in the same circumstances. Remember quoted fuel
figures given my manufactures are works of complete fiction when compared to real life car driving.
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rodgling
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| posted on 18/4/11 at 01:33 PM |
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I'm measuring pressure by putting my hand on the "nozzle" bit on the head itself without the breather valve attached - so its
definitely not the valve or the hoses going to it. (The valve is new anyway and I have cleaned out the pipes). Could there be a blockage in the head
somewhere?
I'm measuring mpg by brimming the tank, driving 50+ miles, then brimming it again. I haven't driven it enough to assess oil consumption,
but it hasn't used much/any in the few hundred miles I've done with this engine.
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Bare
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| posted on 18/4/11 at 03:33 PM |
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Crankcase pressure Must come from somewhere.
However there is always 'some' even in a fresh engine, ergo: universally fitted breather gizmos.
Excessive pressure is often from
- broken breather system
- worn rings
- head damage that allows combustion gases to escape into the engine.. somehow. (were not Beemers at one point infamous for cracked cyl heads?)
More than few older engines had a comb chamber bleed off passage for airpump/ emissions uses, sometimes blanked off but the castings still retained
the passages.
Dunno if yours is one?
But crankcase pressure it's not a good Omen by any yardstick.
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rodgling
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| posted on 18/4/11 at 03:40 PM |
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It's not the breather system - I've observed the pressure directly at the head (at the point where it would normally feed into the
breather system).
So my guess is that it's either rings, or head damage (presumably valve or head gasket being the prime candidates). A wet / dry compression test
should give me an idea as to which it is.
Plan is to try a compression test and have a look under the rocker cover tonight, and see if I can get any clues as to the cause.
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chrisbeale
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| posted on 20/4/11 at 06:07 PM |
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i have a similar problem on my xflow. spitting out a lot of oil through the breather, and no. 2 cylinder doesnt suck much through the carb, ive had
the carb apart, checked the tappets, done a compression test (85psi throttle closed, will do another tomorrow throttle open) and now wondering if
indeed my rings are going. grrrr
so close to stoneleigh.
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