[Public-List] Overheating diesel engine-- what I found

[Randy Katz via Public-List]

Greetings,

I sent in the note below a few weeks ago. Since then I did a little digging
around in the cooling system of the Universal M3-20B diesel and seem to
have resolved the problem--for now.

The main symptom was overheating by about 10 degrees at higher rpms. (The
temp. went up from a normal of about 160 to 170 after 30-60 min. running at
2500 rpms.) I noticed that dropping rpms = cooling temps. This made me
suspect the heat exchanger (not serviced in the 1250 hrs. since the engine
was new).

I thought if it were a thermostat problem, the temp would remain high.

To that end, I removed the heat exchanger end cap and checked inside using
a mirror and flashlight. It was very tight but I could see clogging of a
number of the the tubes in the upper half of the exchanger. (I ascertained
that sea water from the impeller enters the bottom of the exchanger, runs
its length to the other end, goes up to the top set of tubes, and exits out
the top and to the exhaust system. A one-pass arrangement, I believe this
is called.)

Given the cramped conditions at the open end of the exchanger, I managed to
poke a couple of flexible tools (copper wire, a narrow SS band) and
carefully ream out what I could of the clogged tubes. I understand that one
must be very cautious because the soft tubes are relatively easy to damage.
Tubes at the bottom of the exchanger (in water-in end) seemed clear.

During last weekend's trip to Cypress Island (a couple of hours motoring
time in flat calm conditions) the temp stayed put at about 162 the whole
time.

My conclusion is that at some point pretty soon, I'll need to have the heat
exchanger properly cleaned. But I was very relieved to find that my
tampering apparently resolved the overheating problem for now.

Next up: What's causing the engine missing, which is especially heard at
around 1900 rpms shortly after the engine's warmed up.


Best Regards,

Randy
#249 Simple Gifts
Seattle/Bellingham


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Greetings, Folks,

On the last trip, my Universal diesel showed an increase in temp. from the
normal 160 degrees to 170 after running for a couple of hours at cruising
speed of about 5 kts.  This hasn't happened before. (1200 hrs. on the
engine).

I assume this temp increase is significant.

I cleared the sea water filter feeding the engine. (It was clogged the
first time, and cleaning it resulted in lower temp. though with another
half-hour of running the temp rose a bit again.)

The exterior hull screen should be all clear (hauled out a month ago). The
seawater filter is now clean. So I'm wondering if it's the heat exchanger
calling out to be cleaned. Or perhaps the thermostat.

QUESTION--Is this likely to be a thermostat problem? That would be great as
it would be easy to replace. I understand the basics re: testing that part
and a replacement would be simple.

QUESTION--Should I expect a clogging heat exchanger could be the problem,
given the 1200 hrs. on the engine? Mechanics suggest there's no need to
bother with this until "it's really necessary." How does one know when that
is?

Is it reasonable (as recommended by a dockmate) to try clearing the heat
exchanger by running vinegar (or acyliatic acid-- splg?) from a bucket into
the engine, letting it sit for an hour or so, and then flushing the thing
out with seawater?

Any advice would be appreciated. I'll start with the thermostat and see
what happens.

I can imagine spending plenty of time trying to pull out the exchanger with
the engine in place--if the thermostat doesn't do it, can I assume that the
heat exchanger is the problem?

Many Thanks,

Best Regards,

Randy
#249 Simple Gifts
Seattle/Bellingham