Universal M25-XP Overheat Issue - Followup

[Ward H]

Went for a sail yesterday, 2nd of the season. Motor overheated within 10 minutes of leaving slip. Did some trouble shooting on the water and after being towed back into the slip. After my last step I started the motor and it had good water flow. Never found a definitive reason. Here are the trouble shooting steps I took:
1. Motored out into the channel leading from my slip to the bay much more slowly than usual (3 knots vs 5 knots) due to traffic.
2. Just as I entered the bay overheat alarm sounded. A quick look over the stern and I saw no water going out of the exhaust. Total run time of 10-15 minutes.
3. Shutdown motor, unfurled the jib and sailed away.
4. Opened the intake water strainer. Water was good coming out top of strainer. Strainer was clean.
5. Changed impeller on Oberdorpher water pump. (I change the impeller every spring. Never had one with broken vanes.)
6. Started engine. No water flow.
7. Removed hose from strainer outlet. Good water flow. Reinstalled hose.
8. Opened impeller cover. Good water flow out of water pump. Reinstalled cover.
9. Removed anode from bottom of 3" heat exchanger. Started engine. Good water flow out of HX. Reinstalled anode. (HX was removed refurbed by radiator shop in 2018)
10. Tow boat arrived and took me to slip.
11. Started motor and observed flapper on exhaust outlet to ensure gasses were being expelled. They were. Didn't let the motor run long enough for water flow to start.
12. Removed outlet hose of siphon break which is between HX and water injection port on exhaust manifold. Good water flow and pressure into bucket.
13. Used zip tie to check water injection port for blockage. No blockage. Clear into hump hose on water lift muffler.
14. While I was at the siphon break, removed duck bill and flushed with clean water. Reinstalled.
15. Started motor. This time had a good water flow.

I ran the motor for 6 hours today. Water temp was steady at 165 deg temp.

The only other thing of note is the exhaust flapper sometimes sticks open sometimes. We had a NEstr over the weekend. Maybe some water got pumped into the muffler? (I'll be fixing that flapper on Thursday.
Could a stuck siphon break valve cause a hydrolock?
Do raw water systems suffer from hydrolock?

Appreciate any input on what might have caused the water flow failure and allowed the flow to return to normal without the obvious findings. (blockage at inlet thru hull, strainer, HX, failed impeller, etc.

 

[Stu Jackson]

8. Opened impeller cover. Good water flow out of water pump. Reinstalled cover.

You might have just "wiggled" the cover enough for it to reseat. It needs to be really flat and can build up a ring. Whenever I replace an impeller I always sand the inside of the pump faceplate. Of course, this maybe doesn't apply if your Oberdorfer has the O ring, although one of our C34 skippers mentioned just recently that he did so with his O ring pump - I don't have one. Link: Oberdorfer Raw water pump

 

[Ward H]

@Stu Jackson Thanks for the link. I read it through.
I do have the o-ring in the pump cover and I sanded it clean with sandpaper on a piece of glass in the spring when I changed the impeller before launch.
Not setting the cover properly after the impeller install, then having it set properly after I removed the cover and reinstalled is a possibility. I hadn't thought of.

 

[JRT]

I had a similar issue the first time I changed my impeller, my overheat was always the impeller. Also I have always had water flow very soon after startup. You also might keep an eye on the impeller as I messed one up fast with it jammed/not turning correctly.

 

[njlarry]

Hi Ward.
You showed great presence of mind in your diagnostics while under fire!
Cause is a mystery to me. Dont think it was the raw water pump as your opening it showed flow and did not change the situation. If I understand the siphon break check may have fixed it so maybe that was the cause or temp obstruction? Are the exhaust cooling hoses in good shape and not delaminated? Let us know when you solve it.
Good luck!

 

[Ward H]

Hi Larry,
All cooling hoses were new in 2018 and are wire reinforced. However, your idea of a delimited hose does make me think about the hose between the strainer and pump.
It is 5/8" hose about 3' long, has a u shaped bend in it and the bend does get pulled a bit tighter when I remove it from the strainer and stick it in a bucket for test running while on the hard. That hose gets a bit manhandled so being on the suction side of the pump, that would be the most likely hose to have delimitation issues.
The thru hull to strainer hose is a straight 12" length of 3/4" hose and was new this year so I doubt that delaminated.

Regarding the siphon break, I remember last year working in the cabinet where the siphon break is located while the motor was running and hearing it "buzzing". It no longer makes that noise. I have a new duck bill piece on order and will replace it soon.

@jssailem suggested, off line, maybe there was an air lock in the system that finally cleared after my playing with the hoses.

My current thinking is now:
Delaminated Hose (I'll measure length and order a spare hose)
Siphon Break/Air Lock (New siphon break valve on order)
Poor fitment of water pump cover.
Spring Gremlins

Thanks