Compression loss after replacing cylinder head

[brad8526]

Apologies for the length but I'm at my wits end and I'm in need of some expert advise.

I've got a B8200 with the D950 engine. The original symptom was it seemed like it was loosing compression and didn't have the power it once had. Took off the cylinder head, got a gasket set and changed out the head gasket along with the intake and exhaust gaskets. Put it back together and couldn't get it to start. Did a compression test and all three cylinders seem to have low compression. Took the cylinder head to a machine shop where they found some hairline cracks that they couldn't fix. Ordered new head and the machine shop swapped out the valves, installed new valve seats and had them do a valve job to lap in the old valves to the new cylinder head.

Got the head back from the shop and installed the glow plugs but left out the injectors because I wanted to do a compression test. It barely registered anything on all three cylinders. Connected my air compressor at 125 psi to each of the cylinders to do a cheapo leak down test and its leaking air from the valves. It's worse on the intake valves but the exhaust valves are leaking as well just not as badly. Searched the internet for advise, suggestions include put it back together and try to start it, see what happens. Maybe once it starts up the valves will seat properly and it will start holding compression?!?

Reinstalled the injectors and bled the fuel lines, adjust the valve clearance to .008 and crank it but no joy. Try compression test on cylinder three but through the glow plug hole this time - 100 psi, need 325. Haven't tried compression test on cylinders 1 or 2 with it back together because I figured my time would be better spent typing out this post.

Any advise would be welcome but my questions are

1. Does it matter which port I do the compression test through (injector vs glow plug)
2. Should I try doing the valve job myself?
3. Is there something else I need to do to get the valves to seal up, get it to hold compression?

Thank you for reading

 

[Roadworthy]

Using injector or glow plug hole shouldn't matter if the other hole is plugged. I don't know if the injectors will allow pressure loss or not. I'd probably compression test using injector holes. Glow pugs shouldn't leak by. How are your rings? You can do a wet / dry compression test to get an idea. If compression improves with a bit of oil squirted into the cylinder it's probably rings. I don't know the tractor's history. You could have one or more bent rods, too.

 

[lugbolt]

If it's leaking through the valves, that needs to be addressed. It don't take much leak to cause enough loss to keep it from running on smaller diesels.

 

[Dave_eng]

Who set the valve clearances?
If that procedure was done incorrectly then it would explain the air leakage back through the valves and the low compression.

Dave

 

[brad8526]

Using injector or glow plug hole shouldn't matter if the other hole is plugged. I don't know if the injectors will allow pressure loss or not. I'd probably compression test using injector holes. Glow pugs shouldn't leak by. How are your rings? You can do a wet / dry compression test to get an idea. If compression improves with a bit of oil squirted into the cylinder it's probably rings. I don't know the tractor's history. You could have one or more bent rods, too.

While the head was at the machine shop I tried a low tech method checking the rings. I poured a little diesel into each cylinder and timed how long it took to leak out. All cylinders still had the same amount of diesel after an hour but by the next day it had leaked through into the crankcase. I know this isn't a definitive test but I was hoping to avoid having to do a full rebuild. Anyway I'm pretty sure if I can get the valve issue fixed it will hold compression.

 

[brad8526]

Who set the valve clearances?
If that procedure was done incorrectly then it would explain the air leakage back through the valves and the low compression.

Dave

I did using a feeler gauge on the rocker arms.

 

[brad8526]

If it's leaking through the valves, that needs to be addressed. It don't take much leak to cause enough loss to keep it from running on smaller diesels.

That's the angle I'm chasing. Is it possible the machine shop just didn't do a good job getting the valves lapped in? I don't want to assume they did sub-standard work but this is my first experience with them. I watched a video on doing a valve job myself and it doesn't look too difficult but I figured the pros would do it better.

 

[cthomas]

Did you have each cylinder at top dead center? Did you use the English/metric both times(service manual and cylinder head adjustment)((Had a buddy call me, pulled ATV into garage to adjust valves and now it does not start)). I found the metric adjustment in the manual was translated in English on the feeler gauge. Even after fours beers was still laughing. The valve job is mostly to allow time for the heat in the valve to be dissipated to the head. Yes sealing is important but lapping a valve is to make a good heat transfer point on both surfaces. Otherwise you will burn a valve, I have lapped many Subaru's EJ25 engines for this reason. Pulled head, replaced valve after lapping by hand for ten minutes and reinstalled head/engine.

 

[GeoHorn]

That's the angle I'm chasing. Is it possible the machine shop just didn't do a good job getting the valves lapped in? I don't want to assume they did sub-standard work but this is my first experience with them. I watched a video on doing a valve job myself and it doesn't look too difficult but I figured the pros would do it better.

Yes, it’s possible the valves are leaking. There are a couple ways to check this.

First method: Take each cylinder in-turn to TDC and using a differential compression-tester introduce shop air (80-100 lbs) to that cylinder and see if the cylinder ”holds” 80% of the air-pressure introduced. Example: pumping 100 lbs air IN should show 80 Lbs on the differential tester. ALTERNATELY: you can introduce air into the TDC cylinder an LISTEN for air leaks at the exhaust pipe (exhaust valve leaking) or the INTAKE system (intake valve leaking) or the OIL-FILLER CAP (piston rings leaking.)

Second method: BEFORE INSTALLING THE HEAD when you first get it back from the shop, With the valve-springs installed, (the valves are being held closed by their springs), place it upside down onto the work-bench and fill the cylinder-tops with solvent or mineral spirits (you are making a small puddle of solvent inside each cylinder-top in the head with the valves submerged in solvent.)
THEN using your shop blow-gun ...blow high-pressure air into the intake ports and exhaust ports of each cyhlinder in-turn and see if any air bubbles escape thru the solvent around the valves.

I do this BEFORE installing any head received for installation. No bubbling is acceptable.

Hope this helps.

 

[North Idaho Wolfman]

Took the cylinder head to a machine shop where they found some hairline cracks that they couldn't fix. Ordered new head and the machine shop swapped out the valves, installed new valve seats and had them do a valve job to lap in the old valves to the new cylinder head.

There is 2 statements in here that bother me...
I've yet to see kubota cylinder heads that have hairline cracks, they either have big cracks or none, the big cracks will result in a coolant leak not a compression loss.
Kubota heads don't have valve seats, so that's also a red flag!

Did the tractor start before you started working on it?
If it did then it probably wasn't a head gasket or head issue to start with, it probably was a fuel delivery issues, old worn out injectors do more for poor performance than anything else.

Did you do any initial compression tests on it before you opened it up?

Did it have any smoke, any blow by?

 

[JerryMT]

There is 2 statements in here that bother me...
I've yet to see kubota cylinder heads that have hairline cracks, they either have big cracks or none, the big cracks will result in a coolant leak not a compression loss.
Kubota heads don't have valve seats, so that's also a red flag!

Did the tractor start before you started working on it?
If it did then it probably wasn't a head gasket or head issue to start with, it probably was a fuel delivery issues, old worn out injectors do more for poor performance than anything else.

Did you do any initial compression tests on it before you opened it up?

Did it have any smoke, any blow by?

Kubota heads don't have valve seats, so that's also a red flag!

I think you mean "valve seat inserts". Do the valves in a Kubota seat in the head cast iron?

 

[SidecarFlip]

Who set the valve clearances?
If that procedure was done incorrectly then it would explain the air leakage back through the valves and the low compression.

Dave

Not only that but did you visually ascertain they lapped in the seats and faces? Sounds to me like they lied. I would have checked that first (before installing the new head Easy to do (then). Just compress a valve and observe the seat. Shiny is lapped, dull isn't. Sounds to me like the clearance is way too tight or you got lied to. Don't take much to loose compression o n a small displacement motor.

 

[BAP]

Did they grind the old valves first before lapping them in? Or just lapped them into the new seats with lapping compound? If they didn’t grind them, then they aren’t going to seat correctly because it is unlikely that lapping compound alone is going to do it.