Oil in exhaust?
Boat has Universal M25XPA diesel.
Testing a new jib in a minus tide in SF bay the other day, we got up to 19kts wind, but got stuck in the shallows for 20-30 mins. The wind was headed to deeper water, which made it easy getting out. After taking down the sails, the bow pivoted away from the wind and I unfurled a patch of jib. The tide started coming in. Once we got pulled to where we showed a wake and were making about 1.5kts, I turned on the engine and we motored out to deeper water. At some point, I think when I put on a bit of power to 2500 RPM, I noticed dark grey/black stuff in the water exhausting from the stern sea water heat exchanger pipe.
About 10 days prior, I had done all the routine engine maint. (oil & filter, fuel filter, zinc, impeller, clean intake strainer, etc.) and she had tested fine. There was no appreciable sediment in the bottom of the fuel filter.
I cut back on the power to 2000 RPM and the coloration stopped. When I tried to go back up to 2500, the blackish stuff was in there again, and I swear I felt I had less power than normal. So, I stayed low and we motored back through the channel at not to exceed 3.5 kts, when I normally do 6.
We had to run after docking, but I came back the next day and checked the oil level. When I first pulled the dip stick, it was bone dry. However, my experience is that the stick vibrates up a bit when the engine is used, so I put it back in and it came out coated a touch under full. I wiped it off, put it back in, and again got the full reading.
The mud in the bay is blackish dark grey to medium gray in many spots. Crew suggested I was showing mud sucked into the heat exchanger from when we were in disturbed shallow water. However, I don't trust anything at this point except the cost to replace a damaged engine. So, before I do anything more, would welcome pointers to:
1. Can I trust my dip stick? Is there another way to confirm oil level?
2. Can mud really appear through heat exchanger system, being sucked in through the sea water intake? (I've never seen it in the 2 times I've changed the impeller)
3. Why would the grey/black exhaust color only appear at higher revs?
4. Can engine oil get into the heat exchanger and what gasket or other parts am I facing inspecting and replacing?
For reference, I found the following on Wikipedia for generic diesel engines, but not specifically for seagoing:
"Diesel engines can produce black soot (or more specifically diesel particulate matter) from their exhaust. The black smoke consists of carbon compounds that were not combusted, because of local low temperatures where the fuel is not fully atomized. These local low temperatures occur at the cylinder walls, and at the outside of large droplets of fuel. At these areas where it is relatively cold, the mixture is rich (contrary to the overall mixture which is lean). The rich mixture has less air to burn and some of the fuel turns into a carbon deposit."
"The full load limit of a diesel engine in normal service is defined by the 'black smoke limit', beyond which point the fuel cannot be completely combusted. As the 'black smoke limit' is still considerably lean of stoichiometric, it is possible to obtain more power by exceeding it, but the resultant inefficient combustion means that the extra power comes at the price of reduced combustion efficiency, high fuel consumption and dense clouds of smoke".
What I'd love to hear is "relax, everything is fine", but I'll take any expertise you all can provide.