I've been having overheating
problems and would appreciate anyone's insights on this. I'm pretty sure the problem is inadequate raw water
flow at higher RPMs to cool the engine
. This "conclusion" comes after quite a bit of head
scratching and testing (and a five month cruise
earlier this year with a lot of motoring!).
I should say at the outset that the main problem here is that I do not have (and cannot get from Volvo
Penta) an "official" spec for my engine's water
pump (engine is an older Volvo
2003, 28HP, max. 3200 RPMs).
I did two "bucket tests" yesterday to measure raw water
(1) raw water from exhaust
at wide open throttle/neutral @ 3200 RPMs = 4.5 gallons per minute (gpm)
(2) raw water from impeller pump, same engine
settings as above = a bit less than 4.5 gpm (I think we spilled some water as the hose we attached to the pump accidentally chafed on the alternator
belt, cutting a hole in it! Didn't realize until a bit later)
Essentially, therefore, output is the same both at the end of the raw water system (from the exhaust), and the beginning of the raw water ssytem (from the pump, before it enters the heat exchanger
and then exhaust
elbow). Someone with a Yanmar
3GM30F 27 HP engine (3400 max. RPMs) told me his manual shows raw water flow for that engine specced at 7 gpm @ 3400 --similar sized engine to mine, with slightly higher max. RPMs. So, my raw water output should be somewhat under that --perhaps 5 gpm, maybe 6?
However, I don't know if such raw water flow specs are typically a "minimum" flow or a "normal" flow. A friend on the dock
with a new 28HP Beta Marina engine showed me his engine manual, which notes the raw water flow rate for that engine as a "minimum" (we did a bucket test on his engine at the specified RPM
, and the output was a good bit above the minimum).
So, I'm wondering if my 4.5 gpm output is really low, or just kind of low. In any case, comparing to the Yanmar
specs above, my bucket tests appear to suggest the pump isn't pushing enough water. In gear
while underway, performance is very predictable --we run fine up to 1900 RPMs, but do run a bit warm --then, over 1900, predictably, it begins to overheat within 10 or so minutes. So, my guess --only a guess, since I don't really know how water flow and heat transfer works in an engine-- is that cooling
is fine up until 1900; then the heat produced by the engine at this higher RPM
increases at a faster rate, and the amount of cooling
water isn't increasing fast enough to keep up with the greater increase in heat after 1900 RPMs?
It looks like I need to pull off the raw water pump
and see if it can be determined what brand/model it is, what its output specs are, and replace it? How "easily" do raw water pumps wear out? How do they wear out? OK, granted, my engine is 20+ years old...
I bought it rebuilt, but I don't think (but don't know) the raw water pump was replaced.