743 crankcase pressure problem

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jstewart3514

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Feb 13, 2024
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Looking for some help with a bobcat 743 it has the 4cylinder kubota engine. The belt broke and my temp gauge was not working so I'm assuming it may of run hot a little bit I didnt run it long like this. But after that It blew the dipstick out twice and blew oil everywhere. I figure it was probably a bad head gasket took the head off and the gasket has no noticeable blow outs and the head appears to be in decent shape. Any idea we're to go from here? Only other symptoms was it did smoke a lot at start up white smoke And was hard to start even with glow plugs. There was no fluids mixing and no scoring on the liners thats I could see.
 

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foton

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those 1702 engines are bad about cracking the head itself when overheated, have the head tested and at good shop. aftermarket heads from what I know have been used with sucess.
 

laurencen

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look closely at the head around valves, they crack easily when overheated, before I took the head off would have done a compression and leakdown test, heads either complete or bare are on ebay at a decent price
 
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jstewart3514

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those 1702 engines are bad about cracking the head itself when overheated, have the head tested and at good shop. aftermarket heads from what I know have been used with success.

look closely at the head around valves, they crack easily when overheated, before I took the head off would have done a compression and leakdown test, heads either complete or bare are on ebay at a decent price
I'll check it thanks yeah I wish I would of done that but didn't even think about it as I don't mechanic everyday
 

Marc Martello

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Looking for some help with a bobcat 743 it has the 4cylinder kubota engine. The belt broke and my temp gauge was not working so I'm assuming it may of run hot a little bit I didnt run it long like this. But after that It blew the dipstick out twice and blew oil everywhere. I figure it was probably a bad head gasket took the head off and the gasket has no noticeable blow outs and the head appears to be in decent shape. Any idea we're to go from here? Only other symptoms was it did smoke a lot at start up white smoke And was hard to start even with glow plugs. There was no fluids mixing and no scoring on the liners thats I could see.
If you are blowing out the dipstick, then you have a fair bit of crank case pressure, I would say the rings are gone, with the head off what are the condition of the bores, are they scored, is there a lip at the top of the bore ?.
If you are having trouble starting and there is heaps of white smoke during start up, you should check the injectors, they maybe dribbling, the excess fuel at low revs will wash the bores of lubricating oil this will cause premature wear of the piston rings causing blow by.
 
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jstewart3514

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there is no scoring on the bores and it's like it randomly blew it out it would do it then I fill back up with oil and it would go a few days worth of running and then do it again
 

Chessie

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there is no scoring on the bores and it's like it randomly blew it out it would do it then I fill back up with oil and it would go a few days worth of running and then do it again
Crankcase pressure can only come from the combustion pressure not being contained in the combustion chamber. So either the rings are loose and leaking (blow by) or the valves are not seated well and some of the pressure goes around the valve seat and into the head and back down into the crankcase through the oil drain holes.

Since the head is already off, it's time to reseat the valves (assuming there are no cracks and imperfections... time to have the head machined flat after it is tested for cracks) and install new rings. I have the same engine, but I haven't taken in apart .. I don't know if there are oversized rings available or if you can sleeve it, but you may be looking at machining the bores... it's worth it at this point of dis-assembly...

That Kubota is a good, hard-working engine that wants to live... you may as well rebuild it.

Good Luck!
 

foton

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I would have the head checked first if there is a problem it is up to you if you want to refresh the engine any more, if the head is good, then pull the engine and rebuild. but yeah a compression test is always good info. to have even just as a reference in the future for wear comparrisons.
 

Micheal

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Crankcase pressure can only come from the combustion pressure not being contained in the combustion chamber. So either the rings are loose and leaking (blow by) or the valves are not seated well and some of the pressure goes around the valve seat and into the head and back down into the crankcase through the oil drain holes.

Since the head is already off, it's time to reseat the valves (assuming there are no cracks and imperfections... time to have the head machined flat after it is tested for cracks) and install new rings. I have the same engine, but I haven't taken in apart .. I don't know if there are oversized rings available or if you can sleeve it, but you may be looking at machining the bores... it's worth it at this point of dis-assembly...

That Kubota is a good, hard-working engine that wants to live... you may as well rebuild it.

Good Luck!
Yes they have over sized rings and cylinder sleeves. I replaced my sleeves when I rebuilt my Kubota 1702
 

dfb

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It sounds like a piston/cylinder problem more than a valve issue, I am not a diesel expert by any means, I am just not following how a leaking valve would allow that pressure to end up in the crankcase...it could get into the cooling system for sure. A machine shop can absolutely check the head for proper valve sealing and the presence of any cracking.
Perhaps some of the more experienced diesel guys can chime in on this thought since the head is already off. Can you put an equal amount of ATF on top of each piston to see if any drain faster than the others, say an inch or so.
 

Tazza

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I work for a diesel engine repair company, we rebuild multiple V1702 engines every week.

The only place you can get crank case pressure is via the pistons/rings, valves can't push combustion gasses into the crank case,

If you engine got hot, yes these heads are prone to cracking between valves or valve to pre-combustion chamber. They are prone, but they do not ALWAYS crack.

When these engines get hot, the rings can loose tension, get stick in the ring grooves of the piston, or worst case, pickup in the bores.

Your head is already off, flip it over and take the sump off and pill your pistons, ensure the rings are not stuck. You're up for a $100 head gasket, no sense putting a new one on and needing to replace it yet again. You can re-seal the sump with silicone without issues.

As for smoke, if you get lots of white smoke when cranking, your compression is low, it's not igniting the air/fuel charge. If this started after the over heating event, it's likely stuck rings. Black smoke would be a sign of bad injector spraying. If it was dripping, you will see evidence on the piston, fuel leaking will erode the surface of the piston.
 
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jstewart3514

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its smoked ever since I got it. Took the head to the machine shop no cracks but was needing resurfacing they said it was off pretty bad going to have them overhaul the head new valves all that. I turned the engine over with the head off and inspected the bores didn't see any scoring but my finger nail won't catch on the top on any off them.
 

foton

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time for rings, bearings , use a macinists straight edge and check the block for flatness . If the head was warped the block I would think is too.
 

Tazza

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As for bearings, inspect them, if they look good, no reason to replace them. If they are serviceable, there is no harm flipping them, con rod bearings have pressure on the piston end, almost nothing on the cap, so flip them on the rod for some fresh bearing material.

Blocks can warp, but they do need to get fairly hot. You also will likely be using a composite head gasket, unless someone has taken a fair bit off the block. They are more forgiving to not quite perfect surfaces.
 
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jstewart3514

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are there any overhaul kits you recommend using I'm looking at aftermarket kits but not sure if there complete junk or not I've heard of some people using them but unsure oem is double the price though I'm trying to stay cheap get it back going and get rid of it as it's just been a money pit from the beginning
 

laurencen

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every motor I have rebuilt all parts came off ebay, full gasket sets, cylinder heads and complete rebuild kits including pistons, rods, cylinder liners and at least half price compared to dealer, last 843 the rebuild kit was cheaper than a single piston with liner

ebay would be my first choice,
 

Tazza

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We used aftermarket parts on most of our builds, for genuine it would cost more than a new engine. Some parts we have no choice but to go genuine though.

We have not noticed our after market parts not lasting as long, the main failure we notice is lack of servicing, or worst yet, not re-tensioning the head gasket and it failing, making the engine over heat.

Just use a kit off ebay, as long as the parts match yours, you should be good to go. I'm in Australia, so if you're over here i can help you out, if not just prowl ebay. I'd suggest you measure your bores or at the very least, check for a lip at the top of the cylinder. If it's out of spec, get +0.5mm over sized pistons and rings and get your block machined to suit the new pistons.
 
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jstewart3514

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We used aftermarket parts on most of our builds, for genuine it would cost more than a new engine. Some parts we have no choice but to go genuine though.

We have not noticed our after market parts not lasting as long, the main failure we notice is lack of servicing, or worst yet, not re-tensioning the head gasket and it failing, making the engine over heat.

Just use a kit off ebay, as long as the parts match yours, you should be good to go. I'm in Australia, so if you're over here i can help you out, if not just prowl ebay. I'd suggest you measure your bores or at the very least, check for a lip at the top of the cylinder. If it's out of spec, get +0.5mm over sized pistons and rings and get your block machined to suit the new pistons.
Your saying to have the bores machined and use over sized pistons instead of replacing the liners?
 

Tazza

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That is correct, please keep in mind, any kit you see with liners on the internet are only semi-finished,. You need to press or machine the old ones out and press the new ones in then machine them to final size.
 

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