Author Topic: Amount of engine blow by?  (Read 820 times)

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Offline FNQ TRX

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Amount of engine blow by?
« on: Jul 10, 2015, 09:28PM »
Hi

Fired up the TRX and just for fun took the oil filler cap off to have a look.  I was amazed at the amount of air-flow out of the filler hole - its quite a steady breeze and blows oil drips out.

Then when putting the cap back on, the engine idle will drop off significantly (not quite a stall but almost...) and open and closing the oil filler will make the engine rev then drop, rev then drop etc. (I guess from the positive airflow going into the intake manifold from the oil breather).

I pulled the hose off the PCV valve on the other side of the throttle body / manifold.  The PCV valve is working fine (sucking air etc), but there isn't much in the way of airflow from the hose that comes from crankcase.

I am not sure as to how much blow-by there should be.  Engine starts and runs well, idles smoothly and has no signs of a blown head-gasket, so I am wondering if the crankcase breather could be blocked?

Apparently there is a oil/air separator below the hose coming from the crankcase - does this get blocked?

Anyone got any ideas?

 Thanks
Old guy who should know better than to mess around with old cars.

Offline FNQ TRX

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Re: Amount of engine blow by?
« Reply #1 on: Jul 12, 2015, 06:22PM »
Short update...

Put compressed air through the crank-case ventilation hose, no blockage and I could hear air 'hissing' through the engine.
 
Took the oil filler cap off and with each squeeze of the air gun I could feel the air blowing up through the oil filler.

Don't know what the deal is, but the car runs well, blows no smoke, starts easily, idles smoothly, so I guess the blow-by through the rocker cover is just the way things are....!   (134,000 kms on the engine)
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Offline squizz taylor

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Re: Amount of engine blow by?
« Reply #2 on: Jul 15, 2015, 09:26AM »
Short update...

Put compressed air through the crank-case ventilation hose, no blockage and I could hear air 'hissing' through the engine.
 
Took the oil filler cap off and with each squeeze of the air gun I could feel the air blowing up through the oil filler.

Don't know what the deal is, but the car runs well, blows no smoke, starts easily, idles smoothly, so I guess the blow-by through the rocker cover is just the way things are....!   (134,000 kms on the engine)


For comparrison, do the same tests you did on any other car. You will find exactly the same thing.

What you are actually feeling is the air inside the engine being moved around by the pistons and crank.

If you excessive Blow Bye it will show up in the intake plumbing. The signs are excessive amounts of oil in the intake just after the air filter.

Offline FNQ TRX

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Re: Amount of engine blow by?
« Reply #3 on: Jul 18, 2015, 07:30PM »
Thanks for the response.  I guess I thought most of the 'wind' would go through the crankcase breather rather than come up through the engine galleries and through the rocker cover.

On other news....got the front struts out at the moment for some new rubber / plastic covers for the shocks.  Hope to get a roadworthy on the machine late next week.

I also note that the fog lights have cracks in the glass lens covers.  Is this a roadworthy item?
Old guy who should know better than to mess around with old cars.

Offline deviate

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Re: Amount of engine blow by?
« Reply #4 on: Jul 19, 2015, 11:43AM »
i wouldnt worry too much about the foggies, but if your'e concerned just pull them out for now.

Offline squizz taylor

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Re: Amount of engine blow by?
« Reply #5 on: Jul 21, 2015, 02:33PM »
Thanks for the response.  I guess I thought most of the 'wind' would go through the crankcase breather rather than come up through the engine galleries and through the rocker cover.

On other news....got the front struts out at the moment for some new rubber / plastic covers for the shocks.  Hope to get a roadworthy on the machine late next week.

I also note that the fog lights have cracks in the glass lens covers.  Is this a roadworthy item?


The sump and the rocker cover area are conected by default. The oil out of the head drains back through all of the cavities in the head and the block into the sump.

When the oil cap is on the air moves through the smaller diameter PCV and breather lines as it has no where else to go. When you take the oil cap off, all the air flows out the filler cap.

This occurs because air, like all other things especially humans, is lazy. It will take the least restrictive path. The Oil filler hole is a lot easier to rush out than to push through the PCV and the smaller breather lines. All liquids, gasses and electricity will flow down the easiest path, it is just how things are.

Offline FNQ TRX

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Re: Amount of engine blow by?
« Reply #6 on: Jul 21, 2015, 03:15PM »
Fair enough!  Great explanation. 

Appreciate the input, I am a mechanical fitter by trade and at best only a 'bush mechanic' so I not sure what is tolerable or not!
Old guy who should know better than to mess around with old cars.