leyland thorneycroft 4/98 jcb engine

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jeff mann
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leyland thorneycroft 4/98 jcb engine

Post #1 by jeff mann » Sun Apr 24, 2016 4:54 pm

hi ...i have a leyland 4/98 normally aspirated thorneycroft diesel in my boat .i think it was a jcb engine back in the day. It has two breathers on the rocker cover . the forard breather has a pipe that connects to the dipstick tube . the rear breather has a pipe that runs down to a plastic bottle, and then to atmospere. i thought it would be a good idea to take the pipe off the dipstick tube (as i dont see the reason to put any crankcase fumes and pressure back into the crankcase) and put a "Y" connector into the two pipes ,thus venting both tubes together and then putting a barbed connector into the air filter,and connecting the "bottle" tube to it, to draw any fumes and small crankase pressure into the inlet manifold ...
I have been told that this could ruin the engine ,i might lose all my oil ,all sorts of stories ...I cannot see the point of pressurising the crankcase more ....this could surely blow the rear main oil seal etc ....has anyone any ideas or advice ?....the breather pipes do not smoke hardly any ,there is a slight crankcase pressure , but this is a thirty year old engine , still runs nicely
Thanks in anticipation ...
Jeff


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Re: leyland thorneycroft 4/98 jcb engine

Post #2 by Mrsmackpaul » Sun Apr 24, 2016 11:11 pm

diesel's can run on sump oil and or all types of other stuff dunno about vapors for the engine I guess you could try but Im thinking it might runaway on the vapors given the right circumstances and then can rev till it falls apart or runs out of oil or vapor
A diesel doesnt suck thru a carby like like a petrol and makes no vacuum at all so I dont know how well it would work anyway
No doubt someone will have the right answer shortly

Paul
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Re: leyland thorneycroft 4/98 jcb engine

Post #3 by jeff mann » Mon Apr 25, 2016 8:42 am

Thanks for the reply
It seems the local "experts" seem to think there is some valve or suchlike that for positive crankcasepressure ...i cant see anywhere this would be !!
they seem to think that that because there is such a saturated mist of oil in the rocker cover, that the oil would pool in the breather pipe and effectively suck all tyhe oil out of the sump ..
I have my doubts , just hoped someone would know where the breather runs on a standard JCB of that age e.g 1985
Thanks , jeff

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Re: leyland thorneycroft 4/98 jcb engine

Post #4 by FOWLER MAN » Mon Apr 25, 2016 8:53 am

Hi Jeff,
Before the Modern diesels were fitted with " Closed Crankcase Venting," (CCV or PCV), systems, as you know, the oil laden blow-by gasses were just vented to atmosphere.
They were usualy vented down through a vertical breather pipe the end of which was cut at an angle to help the slip stream from the cooling fan and or forward motion of the vehicle create a small vacuum in the pipe which assisted the breathing. The oil vapour and gasses just dissipated into the atmosphere.
Clearly this can't happen if a marine engine is enclosed in a boat.
Why your engine has two breather systems I can't say, but the front breather probably incorporates an oil separator / filter .
The pipe linked to your dipstick would be to return the separated oil to the sump. The ones I have seen incorporate a small non return valve in the separator or the drain pipe which is closed by crank case pressure when the engine is running, only allowing the oil to run back to the sump when the engine stops.
There is certainly a powerful vacuum in the inlet manifold of your engine , enough to operate the governor in many older diesels anyway. I've seen burning rags sucked into engines when trying a cold-start with a flame.
I would be tempted to leave well alone if it isn't causing you a problem.
You may not even need the second breather??? Hard to know without seeing it run.
Fred


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Re: leyland thorneycroft 4/98 jcb engine

Post #5 by jeff mann » Mon Apr 25, 2016 10:35 am

Hi Fred
that makes a lot of sense , and i didn't want to feed oil saturated air into the inlet manifold
. There is no non return valve in the pipe to the dipstick from the forward breather in the rocker cover ...I think i will take your advice though and leave thing alone .
it just doesn't look very pretty , but as you say , in a boat there isn't any scope for just venting to the air .
Thanks again
Jeff


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Re: leyland thorneycroft 4/98 jcb engine

Post #6 by essexpete » Tue Apr 26, 2016 12:35 am

I guess you had better take the advice of the proper mechanics on here. I did install a pipe from the rocker cover to the air intake (after the filter) on an International 358 6 cyl. I have read since that it might not be a clever thing to do but I had no grief in the year or so i ran it like that and it sure cleaned up the blow-by vapour on the normal breather. Perhaps I was a lucky fool?


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Re: leyland thorneycroft 4/98 jcb engine

Post #7 by jeff mann » Tue Apr 26, 2016 8:21 am

Thanks pete
I agree. I dont want to coke up the cylinders in the event of a surge of oil mist..There is a lot of oil pumping around the valves and rockers . Its a different matter on a boat when the engine dies or wont start when you,re a few miles offshore . Its been really helpful reading all the experts on here .
Thanks everyone
Jeff
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