JD 950 Cylinder Pressure

[Dirtdigger5]

Had some blow-by on my Yanmar engine at 1500 hours. Took it to the local JD Dealer and they did a compression test that showed about 430 psi. The service manual says that normal compression for this engine is from 560 to 639 psi with no more than 25 psi among cylinders. It recommends that an engine job be done when the compression reaches 426 psi. They did not do a wet/dry test to see if it was the valves or the rings before they started dismantling the engine Dealer changed cylinder liners, rings lower bearings and sent the head out to have the valves done. When I got it back it was still blowing-by so I took it back to them and had them re-do the compression test. They got around 440 psi on all cylinders. They put it on a dynamometer and ran the tractor for several hours. The blow-by stopped but the compression never improved much. They told me that there must be a misprint in the technical manual that I bought years ago as 560 to 639 psi was too high for a small diesel engine. Has anyone done a compression test on a newer 950 and recorded what that was. I never did a compression test on this machine when it was new so I have no way of refuting them. I have put about 50 hours on the engine since I got it back and it blows a lot of white smoke when I throttle up after the machine has been idling a while. I still have the break-in oil in it that was put in during the winter so that may be a contributor.

 

[dfkrug]

I would expect that a port-injected diesel with 22:1 compression ratio to show
mid 400s psi on the gauge. Somewhat less for DI Yanmars, which are more like
18-19-to-one CR. I have not done compression tests on diesels myself, even
after rebuilding the engines.

 

[jd110]

In my experience, these small Yanmar engines have higher compression pressures than a larger Deere engine. I cant say I have ever seen in excess of 600 psi, but usually newly rebuilt engines don't get checked. Pressures over 500 psi are definitely possible. With that said, there are factors that affect compression readings. Most likely is accuracy of the gauge, and cranking speed of the engine. If the battery, battery cables or starter are not up to par, engine will not spin fast enough to get accurate reading. If engine is starting easily and making good power, chances are good there is no problem since the blow-by is back to normal. Also I think the manual may be off. My manual shows 34 to 44 bar or 560 to 639 psi. Conversion of 34 bar to psi comes to 493 psi. That is more believable.

 

[dfkrug]

Just checked the manual for my last JD4300, which is direct-injection. I believe the 950
is port injection, but I am not sure. Anyway minimum pressure is 370 +/- 14 psi. CR
is 18-19.

 

[Dirtdigger5]

Thanks to dfkrug and jd110. Perhaps the manual is at fault. The tractor does start easily but it did that before the dealer re-built the engine. The tractor did put out about 27 PTO horsepower on the dyno which is what it should be. I am going to change out the break-in oil and see if that solves the smoking issue. The tractor did not do that before the engine work. Good little tractors those old 950s. This one has done a lot of work for me over the years.

 

[dfkrug]

I am going to change out the break-in oil and see if that solves the smoking
issue. The tractor did not do that before the engine work. Good little tractors those old 950s.

I like to change oil and filter after just a few hours on a rebuilt engine. I also wash out the bare block
very carefully with soap and water to remove as much honing grit as possible before reassembly.

Anyway, did you find out if your 950 is a 22:1 CR port-injected engine?

 

[Leejohn]

Your compression might come up alittle and smoke might let up after you get 10 to 20 hrs on it. There might be a case they overlooked the valve guides when the head was done.

 

[Dirtdigger5]

DFKrug; This is a 1983 vintage 950. It has a 3 cylinder Yanmar diesel engine that is rated at 33 engine hp. The injectors go into a precombustion chambers on the side of the head at about 30 degrees from vertical. The compression ratio is 20:1.
LeeJohn: I already have about 50 hours on the engine since the re-build and the white smoke seems to be getting worse only in the last 10 to 20 hours and usually after the engine has been idling. I'm going to change the oil and filter this week and get rid of that mineral oil and if it persists I guess the service manager and I will be having another discussion.

 

[dfkrug]

This is a 1983 vintage 950. It has a 3 cylinder Yanmar diesel engine that is rated
at 33 engine hp. The injectors go into a precombustion chambers on the side of the head at about 30 degrees
from vertical. The compression ratio is 20:1.

OK, 20:1 CR. Your pressures are OK then. And they likely WILL go up as you break it in, as stated by
LEEJOHN.

 

[hsc749]

In my experience, these small Yanmar engines have higher compression pressures than a larger Deere engine. I cant say I have ever seen in excess of 600 psi, but usually newly rebuilt engines don't get checked. Pressures over 500 psi are definitely possible. With that said, there are factors that affect compression readings. Most likely is accuracy of the gauge, and cranking speed of the engine. If the battery, battery cables or starter are not up to par, engine will not spin fast enough to get accurate reading. If engine is starting easily and making good power, chances are good there is no problem since the blow-by is back to normal. Also I think the manual may be off. My manual shows 34 to 44 bar or 560 to 639 psi. Conversion of 34 bar to psi comes to 493 psi. That is more believable.

HI! just found this forum while looking to solve a problem with my 950 I recently bought. the thing runs well but it has blow-back through the radiator overflow tank - you can see bubbles when it is running. Kind'a like blow-by but through the radiator. no water in the oil or out the exhaust so it has all my local advisors saying it is the head gasket. but shouldn't it leak back either in the oil or exhaust? Does anyone have any ideas on what the problem is (or confirm it is the head gasket)? I am not much on diesel engines or tractors so all help is appreciated....