Low Compression Identification

[Celticnorse]

Hey there. I’m new here but did do a bunch of searching and didn’t see the answer for my situation.
Have a B6100 with the D650.
Engine runs but doesn’t idle well and pours out white sweet smelling smoke. Pretty sure the rear cylinder is dead. When I crack the injector line there’s no change in running but the white smoke goes away.
I did a compression check and got the rear cylinder at only 200 psi.
I added about a cap full of oil and compression only went up to around 220-230 psi. Normally I’d suspect something else like valve/head/gasket issues. But is it possible the rings/cylinder are so worn even added oil won’t improve compression much? Is there a procedure to rule in or out the head side components?
The other two cylinders aren’t great (~340 psi) but they fire.

 

[Dave_eng]

A Sweet Smell
In most cases, it is due to a broken or damaged head gasket, which, in turn, leads to hot coolant leaking into your exhaust. On contacting your engine, coolant emits billowing clouds of white smoke that come out of the tailpipe. If left unattended, a coolant leak might cause your engine to overheat.Jun 1, 2022
Dave

 

[Celticnorse]

Would coolant in the cylinder prevent combustion and result in a dead cylinder?

 

[Runs With Scissors]

Would coolant in the cylinder prevent combustion and result in a dead cylinder?

Enough of it would.

The "sweet" part is a dead give away that it is burning coolant at a significant rate.

Bad rings will not cause that.

IMHO, You have either a bad head gasket, cracked/warped head or a cracked block.

Pressure test your cooling system. I will bet that it will not hold pressure.

Edit: be cautious if you pressure test cooling system, you do not want to pump a significant amount of coolant into the cylinder and cause a "hydro-lock" situation. I mean that would be a lot of coolant to pump but "just sayin".

 

[Celticnorse]

Interesting, some of my searching revealed that unburnt diesel hitting the hot exhaust could cause a sweet smelling white smoke. But if there's a shot its not rings I'm more than happy to pursue that! I'll do the free rental cooling system tester this week and get back to you guys.

Are head issues common with these engines? I did some googling on this and found some posts/videos that suggested if you have head issues don't even bother with machining/repairs just buy one of the many heads online that are fully rebuilt. I guess it's about the same cost or less than machine shop + new valve stuff etc. Thoughts on that?

I really appreciate the help!

 

[RCW]

There’s a big difference between the “sweet” smell of coolant from unburnt fuel.

Lack of a better way to put it, I’ve often described the unburnt fuel exhaust as “rich,” but still certainly smells of fuel.

If you boil a couple drops of coolant on a hot surface, the smell is very distinctive. I’ve opened car hoods and immediately gotten the coolant smell. Once it was a bad head on my kids Ford Taurus.

Not helpful, but wanted to point it out.

 

[Celticnorse]

There’s a big difference between the “sweet” smell of coolant from unburnt fuel.

Lack of a better way to put it, I’ve often described the unburnt fuel exhaust as “rich,” but still certainly smells of fuel.

If you boil a couple drops of coolant on a hot surface, the smell is very distinctive. I’ve opened car hoods and immediately gotten the coolant smell. Once it was a bad head on my kids Ford Taurus.

Not helpful, but wanted to point it out.

That's a good idea! I'll heat up some plate over a torch and cook a little diesel and coolant and compare to the smell I'm getting!

 

[RCW]

That's a good idea! I'll heat up some plate over a torch and cook a little diesel and coolant and compare to the smell I'm getting!

Understand that it’s not unusual to get some fuel smell.

It would be unusual to get a coolant smell, even if “mixed in” with the smell of fuel if that makes any sense.

Sometimes head or head gasket problem will prevent the cylinder from firing, thus mixing coolant and fuel odors.

 

[lugbolt]

pull the head and look. Also measure each piston as to how far out of the cylinder it is-they should all be the same, and it needs to be per the WSM's specification. I suspect that there may be more going on based on the comments, and I'm not going to say "what" because half of the readers will disagree and start a war about it, and I'm avoiding that.

 

[Celticnorse]

pull the head and look. Also measure each piston as to how far out of the cylinder it is-they should all be the same, and it needs to be per the WSM's specification. I suspect that there may be more going on based on the comments, and I'm not going to say "what" because half of the readers will disagree and start a war about it, and I'm avoiding that.

Thanks lugbolt, I ordered a leak down tester and just havn't had time this week to get to it this week. Want to clarify are you saying the pistons should protrude above the block surface at TDC?
Also what's WSM? I do have the full Kubota service manual if that's what you mean. Thought for a service manual it seems a little light on details in some regards.

I appreciate all the great suggerstins and feedback thus far. Just want you guys to know that I'm planning to get to it this weekend and will report back findings. I hate when people ask questions then disappear into the ether.

 

[lugbolt]

Thanks lugbolt, I ordered a leak down tester and just havn't had time this week to get to it this week. Want to clarify are you saying the pistons should protrude above the block surface at TDC?
Also what's WSM? I do have the full Kubota service manual if that's what you mean. Thought for a service manual it seems a little light on details in some regards.

I appreciate all the great suggerstins and feedback thus far. Just want you guys to know that I'm planning to get to it this weekend and will report back findings. I hate when people ask questions then disappear into the ether.

WSM=work shop manual

pistons have a spec as to how far in our out of the cylinder they protrude or recess. I do not remember on your engine what that spec is. Been far too long since i've had to go into one of those and my memory is going away these days too.

 

[Celticnorse]

Well looks like I’ve got problems. I did a leak down and got most air leakage through the intake on the middle cylinder and leakage through the exhaust through the rear (dead) cylinder. So I was thinking valve or valve seat or something that was head based. Yay.

but then I took off the head and found the real issue. Looks like the head was off previously (intake, exhaust gaskets looked new) and maybe they didn’t shim right or something. But issues were obvious by looking at the imprint on the rear cylinder. I think it hit the insert for the pre-combustion chamber and bent a con-rod. See the pics of each cylinder at TDC. I haven’t measured but it’s visibly different.

 

[lugbolt]

Well looks like I’ve got problems. I did a leak down and got most air leakage through the intake on the middle cylinder and leakage through the exhaust through the rear (dead) cylinder. So I was thinking valve or valve seat or something that was head based. Yay.

but then I took off the head and found the real issue. Looks like the head was off previously (intake, exhaust gaskets looked new) and maybe they didn’t shim right or something. But issues were obvious by looking at the imprint on the rear cylinder. I think it hit the insert for the pre-combustion chamber and bent a con-rod. See the pics of each cylinder at TDC. I haven’t measured but it’s visibly different.

definitely one bent rod, or wrong rod, whatever. Pull the pan and knock the pistons out. #2 cyl looks to have been hot in a previous life as well unless my eyes is that poor.

 

[lmichael]

Have never been inside one of these engines. Gonna sound like a stupid question but what is that "shinier" circular area that shows up on the head and the pistons?

 

[Celticnorse]

definitely one bent rod, or wrong rod, whatever. Pull the pan and knock the pistons out. #2 cyl looks to have been hot in a previous life as well unless my eyes is that poor.

Yeah unfortunately I think I'll be tearing into the whole thing. My guess is this engine has been mistreated regularly. I'd imagine something happened and they replaced or reworked the head due to other issues and maybe didn't get the pre-chamber insert thing fully seated and/or didn't get things shimmed right and #3 wacked the head introducing the current rod issue. Based on the very clear imprint of the pre-chamber on the piston and the fact that based on "fingernail analysis" that #3 pre-chamber insert seems to be seated the same as the other two now.

Actually this raises a question:are those pre-combustion inserters normally machined down to the head surface? Mine are all prtruding a tiny amount. I'll measure at some point but I can definitely catch a fingernail on them.

 

[Celticnorse]

Have never been inside one of these engines. Gonna sound like a stupid question but what is that "shinier" circular area that shows up on the head and the pistons?

First, I am NOT an expert! These are not direct injection motors so the injector and glow plug are not "in" the combustion chamber, they are in a small chamber off to the side which has a passage way to the cylinder. Something about this aids combustion apparently. I'm not super familiar with all the variations but in these engines have a larger opening that has this "plug" that establishes the pathway between the two chambers.