Fuel in Coolant, Cummins TSB170004

[Hamish]

Quick update on the blue O-ring 'fix'. It lasted about 2000 hrs, then fuel in the coolant again. Didn't pooch any injectors this time likely due to low pressure rad cap I had retrofitted when installing the blue O-rings.

I would think that anytime coolant gets past the cup and into the cavity surrounding the bottom end of the injector it is only a matter of time till it creates a steam bomb, or hydraulic force, that blows past the bottom injector O-ring destroying it a little more each time. No o-ring can take this, so I just want back to the original green ones this time. Bottom line is that fluids have to be stopped before getting down into the cavity, not afterward on their way up at which point the cup has likely re-sealed like a one way valve.

I still only have 500k on the rebuild which was done with a reman head (before new heads were readily available) so not feeling like another inframe is due yet.

This time I choose additional o-rings to seal against the upper face of the injector cup and the wall of the bore just above the injector cup. I choose a 121 size EPDM against the lip of the cup, backed by a square cross sectioned nitrile 026. Obviously this is not an engineered O-ring gland, so compromise was made (ie. excessive stretch of the 121 to reduce cross section size). Squeeze had to be checked with each cylinder as most cups were 72mm down, but one was 73 on one side, and 72.5 on the other... yes the one that leaked the most. Squeeze was checked by monitoring how much the injector retaining bolt turned from initial contact to tight with only the additional o-rings installed - just over 3/4 of a turn. This vs. no o-rings at all and just over 1/4 of a turn. I did not want to exceed half a turn of squeeze (1.5mm between bolt threads) because this would not leave room within the 'gland' for 7% expansion at running temp. I imagine packing this area too tightly with rubber could lift the injector out of the head at temp, so definitely some risk with this injector by injector custom fitting attempt. On the offending cylinder I had to add a backup ring between the square and round o-rings and hopefully the happy medium fit works for this crooked cup during 7% expansion at temp and 3% contraction sitting outside in winter.

Worst case I lost a day and have to rebuild the engine anyway. Best case the extra o-rings solve the leaking cup problem for another 1.1 million km when the next inframe is due...