Low Compression

[vollr]

Hello, This is my first post and its about my old Kubota D-950. I don't know what it came out of and the dealer can't tell me either by the block #. It was rebuilt locally by a mechanic who has since left the area. The engine lacks power and I'm guessing its putting out about 10 hp instead of double that. I used to rebuild vintage VW diesels but that was a long time ago and this isn't quite the same, but I do understand diesels and really like the Kubota's. Analyzing for lack of power I ran a compression check and found it was about 100 psi lower than spec. This was the only thing I could find that was wrong. Engine starts easily with glow plugs and runs smoothly; maybe too smoothly. Local dealer's chief mechanic said it might have the wrong pistons. The engine has a new head and didn't need the shim that comes with some C.H. gaskets but I think it was used anyway. How much compression might be lost if the shim was used with the head gasket on a new head? Thanks in Advance.

 

[85Hokie]

Can you tell us what the compression is now?

If you were to drop a spoon full of oil in glow plug oil and ran compression again - do numbers go up or stay the same?

The shim is typically used if head was milled a couple thousand...... so if used anyway and no milling, it would change the compression a bit. The amount of change with be hard to figure without knowing all the numbers ..... and IF the wrong pistons were used , all numbers known are thrown out the window!



Here are specs for that engine:

D950 normal 412- 469 PSI lowest 327 PSI

 

[vollr]

Ok, good idea. I'll do that first chance and get back to you. I'll rerun the compression check as is and then with the small amount of oil. thx

 

[North Idaho Wolfman]

Little confused????
How long since the rebuild?
Why was it rebuilt?
What parts were used for the rebuild?
Were the parts Kubota or aftermarket?
Has it lacked power since rebuild or did this happen over time?

I suspect that your telling us that it's lacked power since rebuild?
Low compression could be a factor, but not always the reason for low power.
Low compression could be from things leaking, like, Glow plugs, injectors, valves.
Low power could be from Injection pump timing ( missing or added shims) or engine timing if the Gears were not installed properly.
Low compression on a newly rebuilt engine is not all that odd, especially if it wasn't honed properly.
Low compression can also be caused by someone not installing the rings in the right direction.
Wrong pistons would be fairly odd, unless the motor was either not re-sleeved and it was over bored, or out of spec and should have been bored or re-sleeved.

 

[JerryMT]

Thee are any number of reasons to be down on power. Power is determined by airflow, fuel flow and compression ratio. How do you know it is down on power?

Let's assume you are correct that the power is lower than it should be. Is there any restriction in the intake system( i.e. clogged air filter, mouse nest in intake pipe, etc? Are the valve clearances set correctly? Is the valve timing correct? Too thick of a head gasket can cause a significant loss of compression but that depends on the relative difference between the gaskets and whether or not the head was shaved. If the rings have not properly seated in the bores, the compression can be down. Anything is possible when someone rebuilds and engine but I'm assuming the the piston's were not changed and the bore was within specs, which may or may not be true.

Is the fuel delivery and fuel timing correct?

 

[Henro]

Thee are any number of reasons to be down on power. Power is determined by airflow, fuel flow and compression ratio. How do you know it is down on power?

Let's assume you are correct that the power is lower than it should be. Is there any restriction in the intake system( i.e. clogged air filter, mouse nest in intake pipe, etc? Are the valve clearances set correctly? Is the valve timing correct? Too thick of a head gasket can cause a significant loss of compression but that depends on the relative difference between the gaskets and whether or not the head was shaved. If the rings have not properly seated in the bores, the compression can be down. Anything is possible when someone rebuilds and engine but I'm assuming the the piston's were not changed and the bore was within specs, which may or may not be true.

Is the fuel delivery and fuel timing correct?

I remember reading that pistons need to go back in the same cylinders they came out of. Not sure why this is, but if the rebuilder just randomly put them back in out of order, could this have any affect?

If this is true, and someone explain why this is?

 

[85Hokie]

I remember reading that pistons need to go back in the same cylinders they came out of. Not sure why this is, but if the rebuilder just randomly put them back in out of order, could this have any affect?

If this is true, and someone explain why this is?

If an engine is NOT rebuilt - all parts need to go back in the EXACT same places .....

This is because all the wear and how the wear was created at the time - those part "mated" as they wore.
When placed back together - the wear is minimal. BUT if parts are not placed back in the same places - those parts will wear AGAINST the other parts they have never touched, thus creating many more wear points. On an auto - this includes rockers, tappets, lifters, pistons and even the push rods.

New parts typically go against new parts or places ..... thus starting the process all over again.

 

[Henro]

If an engine is NOT rebuilt - all parts need to go back in the EXACT same places .....

This is because all the wear and how the wear was created at the time - those part "mated" as they wore.
When placed back together - the wear is minimal. BUT if parts are not placed back in the same places - those parts will wear AGAINST the other parts they have never touched, thus creating many more wear points. On an auto - this includes rockers, tappets, lifters, pistons and even the push rods.

New parts typically go against new parts or places ..... thus starting the process all over again.

Thanks...that makes complete sense...

 

[kubotafreak]

Run the valves and see if any are tight.

 

[GeoHorn]

Hokie is correct... but after a hundred more hours... It no longer makes any difference, IMO.

 

[lugbolt]

I guess I am the only one who thinks a little differently

if a diesel is 100 psi low, what am I gonna do?

#1 check valve clearance. Most kubota stuff is so easy it makes no sense not to particularly if I don't know it's service history. Then re-check compression. If it's still low, pull the head and inspect.

I would not recommend using oil to test compression as you would with a gas engine. Oil will ignite/burn and the engine can start. Secondly if it's enough oil, it can bend the rod.

If it's low, find out why. One can do a leakdown test but it's a little harder with a diesel because there ain't no spark plug hole-one can use the injector hole or the glow plug hole but either way will require a special adapter to fit and seal. I made mine, wasn't too hard but the adapter was like $25 in itself plus time it took to make it all work. I mean there's only 3 ways a diesel can be low compression. Or 4 actually. Head gasket, valve(s), rings, or rod(s).

 

[vollr]

Ok guys, sorry I let this lay for a while but other work doesn't wait.

Compression Test showed 360, 380, 360; a few lbs difference when small amount of 10W oil is added to each.

I'll try to answer all your questions above. The engine was old and I've recently found it was originally in a mower; brand unknown. The engine was used up and a local boy overhauled it. He replaced the head since on the original exhaust gases had burned through the exhaust port. At the time of the rebuild a new injection pump was added along with new bearings and rings. I'm pretty sure all pistons were replaced. After less than 1000 miles the engine failed. It was disassembled and the center cylinder had a large hole and piston was in even worse shape; unknown cause, lots of guesses about too little fuel, then too much fuel, then pistons not fitted properly. No reason was actually found. At this point the engine was in 100 pieces scattered on a concrete shop floor and stayed there for a couple of years. I came on the scene and traded for the engine. There was a time when I would have overhauled it myself but health problems have a tendency to slow me down. I've now got just two speeds, slow and stop. I still wanted to see the engine run and hear that Kubota sound so I found a shop that was willing to overhaul it. I took the pieces to the machinist who would be the main rebuilder; 30 years experience building race engines. We got along great and he proved to me he knew a lot about engines in general. Later, when things didn't go quite right, it was hard for me to believe that he had made a mistake but finally came to the conclusion that everybody makes mistakes and he, after all, was not well versed in Kubota (or any other diesel) engines. After getting the engine placed into its new home and back in my shop I found that the engine would start readily using glow plugs for 5 seconds. It would idle smoothly but the engine lacked power and couldn't pull a load. I went through the entire fuel system, had the injection pump and injectors checked by Diesel Injection Service and all checked well. I checked the compression and there was the problem as you can still see above. I checked the valves; no problem. Rings seem to be seated well considering the engine has just about 20 hours on it and that the oil added during compression tests didn't make much difference. I guessed that it was out on valve timing, but as I found later the engine would not tolerate much of that without the valves contacting the pistons. I finally got smart and went to my local Kubota dealer. Shop foreman and partial owner put me off for about a year because his top mechanic had a serious accident and didn't come back to work until this past month. Mechanic came to my place and listened to the engine; said it sounded "flat", which translates to little power. Dealer didn't want to mess with it. I can understand that and can't blame him; too many fingers have been in the pie. The mechanic, however, said he can take it to his own shop and fix it in his spare time. His first guess was that the wrong pistons could have been installed at the first overhaul. This would make sense and explain the lack of compression, power, etc., and would also explain why the latest rebuilder, the machinist and race engine builder, might have missed the problem, not being a Kubota expert and simply matching the damaged piston with the other two undamaged pistons from the first rebuild. My contribution is that I'm pretty sure the shim and the head gasket both were installed in the latest rebuild and I'm hoping that is the problem since the head has never been shaved. That is the reason for my original question above as to how much compression might be lost if this is the case. That's about the end of the story. Hopefully this genuine Kubota expert will finally fix my engine.

 

[kubotafreak]

The engine is still above service limits.(mid-325's hot, 460ish new) If you have the valve to piston clearance, the shim would easily make this difference in compression. Have you had the timing cover off to verify the timing is correct? Sounds like injection timing is off.

 

[vollr]

The engine is still above service limits.(mid-325's hot, 460ish new) If you have the valve to piston clearance, the shim would easily make this difference in compression. Have you had the timing cover off to verify the timing is correct? Sounds like injection timing is off.

There is no objectionable smoke when the engine is running, but there is some under acceleration just as with most diesels. You just might have hit the nail on the head though because if injection is out of time just a bit combined with the shim on the head gasket causing a loss of compression the performance would have to suffer even if the engine is just inside specs on compression. I may have forgotten to mention above something important I had forgotten about. While there are no "apparent" leaks of any kind when bleeding one of the injectors I could not get the bubbles to stop no matter how long I bled it. This is a relatively new problem and seems to indicate a small air leak into the line of that particular injector. I figure if the leak was in the pump itself or the line back to the tank all injectors would have a problem so it must be confined to the line and fittings for just that one injector. Soooo, among the 3 things we've discussed, compression - possible injection timing problem - and leak before one injector, that may be more than enough to explain the lack of power. I'll pass this info along to the Kubota mechanic and hope his promise is good to take this engine and fix it right.

 

[vollr]

Ok, I'm backkkk. The Kubota mechanic decided he had other work to do so I'm the man. I pulled the head and found the engine had just one gasket, and no shim. The rebuilder had coated the head gasket with black RTV. I was taught that's a no-no and suspect that might be at least part of the reason for slightly lower compression. There were no leaks. Valves are well seated. I poured some diesel fuel around them with the head upside down and also around the pistons. No leaks there either. I was going to pull the front pulley and the gear cover but I'll get to that later. To do that I will have to pull the engine and am trying to avoid it for now. I've never rebuilt a Kubota diesel but have rebuilt older model VW diesels and I know that when valve timing is correctly set there's just .060" clearance between valve head and piston head. I figured it would also be close tolerance on the Kubota and that doesn't leave much room for error. The problem I have that would affect compression does not seem to be head, valves, pistons, or gasket related. What else could account for lower compression? Two things come to mind. If the valve timing is off slightly compression would be affected, BUT it seems the valves would contact the pistons and that would be the end of my problem as well as my engine. Kubota mechanic told me it would be almost impossible to set the valve timing wrong since all the gears involved are plainly marked. Now comes my question. How many teeth, if any, can the valve timing gear be off and still not damage the engine? I'm hoping that "none" is the correct answer but no one I know can answer that question. The only other explanation I know of to explain low compression, if the valve timing is set correctly, is that the engine may have internal friction more than normal and wouldn't be turning up fast enough with the starter to bring compression to normal. I've never run into this on a newly rebuilt engine that had been built right. Perhaps the rods and mains are too tight. Reason I thought of that is that I have a nice long-handled strap wrench to use on the crank pulley to find TDC and the engine seems to be too tight. Sooo, the important thing is can you answer my question? Will the engine run smoothly but poorly if the valve timing is slightly off? Thanks for bearing with me on this.

 

[kubotafreak]

My initial comment about timing being off was more attributed to lack of power running the engine. Readings cold in the 360 lbs for a fresh rebuild is not too bad if the rings have not seated fully and the valves were not lapped. Were the cylinders ball honed? If they were not, the rings will never seat. Also if this is not a tractor it can be hard to run in the engine under load needed to seat rings. Lower ends are very picky, in my opinion the hardest part of rebuilding an engine. With the head/injection pump off, if you think its tight, its tight, and something is not right. Probably a tight bearing, or rod cap backwards... Any metal flakes in the oil? Crank should spin smooth with no tightness at any point of rotation. The only real resistance to turning should be ring pressure(which will be much higher on an un broke in engine). The sound fresh bores make against rings almost sounds like sanding something, but smooth.

I try my best to put a good flex hone to the factory cylinders, pistons, and rings. Many of these engines have so low hours so the cylinders/pistons/rings are not worn much at all. The akelas heel tends to be overheating. They will cylinder glaze easily. Many of the smaller older models had convection cooling only, no pump which only increases their sensitivity to poor cooling systems/clogged radiators. Good concentric cylinder bores, can absolutely be re-honed in frame with pistons at the bottom of the bore. The bottom portion of the cylinder that gets missed does not account for much wear/sealing at all. Middle cylinder up gets 20 and 80 percent of the wear. Thus the reason bores get wider at the top. I know most feel this is shade tree, but I can assure you it is better than some knock off brand of piston, rings, engine bearings, and some inexperienced person taking it all apart. Much wasted effort and $ on these forums with people throwing aftermarket parts, and money at something. Many of the gaskets are reusable, the thick copper is, the fiber gaskets are, and the rubbery/fiber head gasket in good shape can be retorqued several times. The only seals that I never reuse are the thin copper washers, and the O-rings. Most Kubota engines are made to be easily rebuilt. Kind of the trait for having super expensive parts. If you read the service manual, it gives measurements for everything. I would not advise using aftermarket on any hard part of the engine.

As far as the shim" you only need that if the milled cylinder deck becomes such the piston to valve clearance is too tight.(can be checked with solder through the injection hole) As lugbolt pointed out, bent rods are also suspect if any cylinder has a different piston tdc height.

My guess would be no more than a tooth or two before you will have piston interference.

 

[PoTreeBoy]

I'm no expert, but I think you're overly concerned about the compression. From reading on here, low compression causes starting issues before serious power loss, and your engine starts well. And your numbers are within spec.

I was going to recommend checking the valve timing, but you've said that's going to be involved, so leave that for now.

My next recommendation is to check the injection pump timing. I think late timing would cause that 'dead' running. I'm pretty sure that engine has the cassette style pump that uses shims to set the timing. There is a 'spill' test procedure to check and set it.

You mentioned bubbles in one of the injector lines. I don't think a cracked injector line would let air in - fuel would squirt out. You may be aware that there's a lift pump ahead of the injector pump. A leaky diaphragm there could admit air, but why would it go to only one injector. There's also a thumb screw bleeder valve on the injector side that, if left open, will allow the engine run at light load but may prevent full power.

 

[vollr]

My initial comment about timing being off was more attributed to lack of power running the engine. Readings cold in the 360 lbs for a fresh rebuild is not too bad if the rings have not seated fully and the valves were not lapped. Were the cylinders ball honed? If they were not, the rings will never seat. Also if this is not a tractor it can be hard to run in the engine under load needed to seat rings. Lower ends are very picky, in my opinion the hardest part of rebuilding an engine. With the head/injection pump off, if you think its tight, its tight, and something is not right. Probably a tight bearing, or rod cap backwards... Any metal flakes in the oil? Crank should spin smooth with no tightness at any point of rotation. The only real resistance to turning should be ring pressure(which will be much higher on an un broke in engine). The sound fresh bores make against rings almost sounds like sanding something, but smooth.

I try my best to put a good flex hone to the factory cylinders, pistons, and rings. Many of these engines have so low hours so the cylinders/pistons/rings are not worn much at all. The akelas heel tends to be overheating. They will cylinder glaze easily. Many of the smaller older models had convection cooling only, no pump which only increases their sensitivity to poor cooling systems/clogged radiators. Good concentric cylinder bores, can absolutely be re-honed in frame with pistons at the bottom of the bore. The bottom portion of the cylinder that gets missed does not account for much wear/sealing at all. Middle cylinder up gets 20 and 80 percent of the wear. Thus the reason bores get wider at the top. I know most feel this is shade tree, but I can assure you it is better than some knock off brand of piston, rings, engine bearings, and some inexperienced person taking it all apart. Much wasted effort and $ on these forums with people throwing aftermarket parts, and money at something. Many of the gaskets are reusable, the thick copper is, the fiber gaskets are, and the rubbery/fiber head gasket in good shape can be retorqued several times. The only seals that I never reuse are the thin copper washers, and the O-rings. Most Kubota engines are made to be easily rebuilt. Kind of the trait for having super expensive parts. If you read the service manual, it gives measurements for everything. I would not advise using aftermarket on any hard part of the engine.

As far as the shim" you only need that if the milled cylinder deck becomes such the piston to valve clearance is too tight.(can be checked with solder through the injection hole) As lugbolt pointed out, bent rods are also suspect if any cylinder has a different piston tdc height.

My guess would be no more than a tooth or two before you will have piston interference.

Thanks for the quick reply. I drained the oil pan and found no flakes. Lack of ring gap could contribute to friction, as would the main and rod bearings being set up wrong. The rods checked good by machine shop. No overheating since rebuild. New radiator and water pump. Also, the block was cleaned inside out. Normally I would have rebuilt the engine myself but a serious operation and 5 years to get back to normal kept me away from my shop. Too bad because now I don't know if all the good things were done to this engine as expected. Rings appear to be seated ok as shown by the diesel fuel test and there are no vertical scratches on the cylinders. I hate to say this but after reading your opinion I'm positive I'll have to pull the engine and disassemble it to inspect for clearance problems. I always used plastigage to check rod and main bearing clearance. Is that still done these days? Anyway, if I can get the plastigage that's the way I'll do it now. I really hate to pull this engine but we do what we have to do. Thanks for your opinion.

 

[vollr]

I'm no expert, but I think you're overly concerned about the compression. From reading on here, low compression causes starting issues before serious power loss, and your engine starts well. And your numbers are within spec.

I was going to recommend checking the valve timing, but you've said that's going to be involved, so leave that for now.

My next recommendation is to check the injection pump timing. I think late timing would cause that 'dead' running. I'm pretty sure that engine has the cassette style pump that uses shims to set the timing. There is a 'spill' test procedure to check and set it.

You mentioned bubbles in one of the injector lines. I don't think a cracked injector line would let air in - fuel would squirt out. You may be aware that there's a lift pump ahead of the injector pump. A leaky diaphragm there could admit air, but why would it go to only one injector. There's also a thumb screw bleeder valve on the injector side that, if left open, will allow the engine run at light load but may prevent full power.

Thanks for your reply. Good point about the compression. It may be lower than optimum but still not bad enough to cause this problem. If the valve timing is ok then the problem pretty much would have to be internal drag. That would explain a reading of slightly lower compression as well as loss of power. Fortunately I have not run the engine very long; less than 20 hours since rebuild and most of that was idling. It just had no power and was no fun at all. Once I locate the major problem pump timing will be made right and that slight air leak during bleeding will be located. I figured, like you, that if the air leak was before the injection pump then none of the injectors would bleed properly. You are correct in that the fuel would squirt out of the line. This may indicate a leak inside the pump at the injection pump piston. Thanks again for your opinions. They will help a lot.

 

[kubotafreak]

Thanks for the quick reply. I drained the oil pan and found no flakes. Lack of ring gap could contribute to friction, as would the main and rod bearings being set up wrong. The rods checked good by machine shop. No overheating since rebuild. New radiator and water pump. Also, the block was cleaned inside out. Normally I would have rebuilt the engine myself but a serious operation and 5 years to get back to normal kept me away from my shop. Too bad because now I don't know if all the good things were done to this engine as expected. Rings appear to be seated ok as shown by the diesel fuel test and there are no vertical scratches on the cylinders. I hate to say this but after reading your opinion I'm positive I'll have to pull the engine and disassemble it to inspect for clearance problems. I always used plastigage to check rod and main bearing clearance. Is that still done these days? Anyway, if I can get the plastigage that's the way I'll do it now. I really hate to pull this engine but we do what we have to do. Thanks for your opinion.

I still use it, make sure it comes from a fresh stock, and not all dried up.
Rings being too tight would only occur if you had plus size rings in std bores. I cannot remember the last time I had to file a set of rings because they came too tight. I just cross my fingers the gap is smaller than allowable spec when measuring.

1. I would confirm the gear sets under the timing cover
2. run a hone in the bores
3. triple check the cylinder head for cracks, hand lap the valves, and check guide smoothness
4. Check the piston to deck (stand proud) measurement at TDC is within spec.
5. Pull lifters and make sure cam is not messed up
6. Put it back together, because you more than likely have a fuel issue.