Taking the plunge. My journey as an O/O.

That's how it was on the series 60 also. And the kubota in my bobcat. And the massey Ferguson in my tractor. And the 3204 in my dozer. And the engine in my w18 case loader. Thinking I see a pattern here.

 

So it's not just me that thinks it's strange that the coolant was mixing to the fuel, but not the other way around? Good. I think.

If there was fuel mixing with the coolant, I'd hope I would be able to smell it all those times I opened the coolant to refill it. I'm not a diesel mechanic, but I am a steel mill mechanic, and I know what diesel smells like

EDIT add. The idea of hot coolant being at a higher pressure than the fuel after engine shutdown does make sense. I wasn't considering the conditions when the engine was shut down. I'm still a bit surprised that the leaking seemed to be happening only one-way, but I suspect that if I had let it go a few more days it would have started mixing both ways as the damage got worse.

An any case, the shop said they should have the diagnostic teardown finished in the morning sometime, so we shall see what they say then.

 

You miss understood me.
When the engine is running the pressure of the fuel system is greater than the coolant system fuel goes in the coolant. When you shut it off you have the reverse. The coolant is still under pressure yet the fuel isn't. It's like that on every single diesel engine I've ever had to open up to put injector cups in.

 

I understood what you were saying, I think. I now understand how the coolant might have been at a higher pressure than the fuel. When the engine shuts down, coolant quickly becomes the high pressure side of the fuel/coolant interface because coolant pressure is driven by temperature in a sealed unit, not mechanical compression. The fuel pressure drops almost instantly because it is mechanically pressurized.

What I still don't understand is why the engine didn't seem to be pushing fuel into the coolant when the engine was running and the injectors were pressurized. If it was pushing fuel into the coolant, it was pushing too little for me to smell with my nose almost over the cap, looking down into the reservoir to check coolant level. My reservoir is nearly opaque and I have to open it up and look into the cap to see the level.

I can imagine scenarios where seals might leak one way, or at least mostly one way, when pressure differentials flip-flop as a machine changes states, but that sort of gasket or seal damage usually gets worse rapidly in my experience with gearboxes and hydraulic systems.

*shrug* If it doesn't seem strange to you, then I'm probably having some sort of fundamental misunderstanding about how injectors work.

 

After a bit of googling it looks like the dd15 has a fuel/coolant module. That may be the cause. Hopefully the dealer gets the diagnosis right and gets you back up and running.
Have you had any extended cranking or rough idle at start up?

 

No, there has been no difficulty starting the truck, and it has been running smooth. Since my truck is an opti-idle truck, I definitely would notice the engine having difficulty starting or idling rough. I have to sleep through it.

 

Well, I have received an update from the Freightliner dealer.

The injector cups test good. The next step is removing the cylinder head and pressure testing.

I will also be calling the shop and asking if they have looked at the fuel/coolant module @062 mentioned above.

 

I dont know the parts cost on a dd15 but on the older 60 series over 50% of the total cost of an inframe is removing and installing the head. That's a solid 75% of the total labor involved.

 

Good Point.

I also verified today that they checked the fuel/coolant module before they went into the engine.

Hopefully they will be able to get the next stage of disassembly and testing started tomorrow.

 

Did they test for exhaust gases in the coolant....?