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Old 09-12-2009, 13:44   #1
Jud
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Talking Overheating and Raw Water Flow Specs - 28hp Volvo - Help Needed!

Hi all,

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">raw water pump (engine is an older Volvo 2003, 28HP, max. 3200 RPMs).

I did two "bucket tests" yesterday to measure raw water flow:

(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.
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Old 09-12-2009, 14:21   #2
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How are you determining the overheat?
have you cleaned the tubes in the heat exchanger?
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Old 09-12-2009, 14:28   #3
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Have you considered the other side of the cooling circuit? The "freshwater" side?
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Old 09-12-2009, 14:52   #4
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Overheating and water flow

I should have added that the heat exchanger was boiled out by a mechanic about two years ago, but to be honest, I haven't looked at the piping/tubes for the fresh water side of the cooling system. I don't know if the freshwater pump is circulating (and I don't know how to check that). Also -I just changed my anti-freeze the other day, so that's brand new.

I'm measuring overheating from my temp gauge, which I've determined is reasonably accurate (the other day, we ran the boat under load in gear and I pointed a heat gun infrared thermoeter at the area on the cylinder head near the temp sender, and looked at the gauge --very similar temps, so I'm pretty sure the gauge is accurate. (Cylinder head temp measurement slightly lower than gauge, probably b/c was measuring outside engine temp, whereas the sender is measure fluid temp inside? I also measure temp on thermostat housing.)

I'm basing my conclusion that the problem is the raw water pump by the fact that we're only pushing 4.5 gallons per minute at max. 3200 RPMs wide open throttle, which seems pretty low compared to raw water flow specs I have for a Volvo 2030 (29 HP, 3600 max. RPMs), which is 8.3 gpm at max. RPMs, and a Yanmar 3GM30F (27 HP, 3400 max. RPMs), which is 7 gpm at max. RPMs.

I haven't looked at the freshwater side of the cooling system yet since someone suggested to me the other day I should start with an easy check --measuring raw water flow out the exhaust and from the pump itself. So, it seems (only seems...just a guess at this point!) that I'm not pushing enough raw water.

Overheating is predictable --fine up to 1900 RPMs in gear, then overheats after that, within about 10 mins.
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Old 09-12-2009, 15:07   #5
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If the flow is the same...before and after the heat exchanger....

Have you pulled the thermostat?.....Checked the raw water impeller?..

What temp thermostat is in there? vs..your operating temperature..?...

It could be sticking....try changing the stat...before digging further...if you haven't done that already
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Old 09-12-2009, 15:13   #6
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All the work in checking the flow. You could've already pulled the pump apart for a look, see. Seems at higher rpms you should get more water? Is the belt slipping? I would snug the belt, and then look inside the raw water pump.......i2f
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Old 09-12-2009, 15:20   #7
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Originally Posted by Tempest245 View Post
If the flow is the same...before and after the heat exchanger....

Have you pulled the thermostat?.....Checked the raw water impeller?..

What temp thermostat is in there? vs..your operating temperature..?...

It could be sticking....try changing the stat...before digging further...if you haven't done that already
I'm going to check the impeller again...just to be sure.

I changed the thermostat last summer so I assume it's not sticking (but very well could be). I assume the thermostat was the right one! Unless I'm mistaken, if I was accidentally given a stat for sea water-cooled engine, it would open at a lower temp (according to the specs in my manual). Says freshwater stat opens at 174F, fully open at 189F (sea water stat temps are lower). We're overheating around 199F, which is where I have the alarm set at.

I've checked the belt --definitely not slipping.T he overheating has gone one for many months, and is always predictable. That is, I've tightened belts, changed fluids, etc. and done those kinds of regular periodic maintenance things at certain engine hours over the past 8 months...still overheating.

I keep thinking about the raw water flow, which seems much lower than for similarly sized engines (in terms of HP and max. RPM). I don't want to focus on one thing, but I'm curious if anyone has any insights or experienc with this kind of raw water flow...seems low to me.
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Old 09-12-2009, 15:24   #8
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All the work in checking the flow. You could've already pulled the pump apart for a look, see. Seems at higher rpms you should get more water? Is the belt slipping? I would snug the belt, and then look inside the raw water pump.......i2f
When you say, "look inside the raw water pump", any tips as to what I should look for? Besides, say, broken or bent impeller vane (which I'm nearly sure isn't the case)?
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Old 09-12-2009, 15:27   #9
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have your tried going methodically through the entire raw water circut and checking for blockages? intake, elbows, mixing elbow. Otherwise I'm not sure that 199 is overheating..........unless the manual says so.
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Old 09-12-2009, 15:29   #10
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Possibly at higher rpms the impeller starts to slip. Is the impeller worn on either side. Since it's apart why not slip in a new one? Seems to me at higher rpms there should be more water flowing. Possibly a restriction internally, such as build up in an elbow, heat exchanger? I once had shells after the pump in the hose. This caused over heating at higher rpms..........i2f
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Old 09-12-2009, 15:49   #11
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Jud...Gotcha,

I've always found it best to start with the simple solutions 1st...then work up...

1st...coolant levels...oil levels.....
2nd...raw water strainers before the engine...through hull and in-line strainers
how long has this been going on..gradual..or sudden.?..you could have crap on
blocking the flow through into your through hull strainer..or growth..
3rd...impeller
4th...Thermostat
5th...Belt tension/slippage
6th..heat exchanger......flow is equal in and out..so..I'd set that aside for now..

If you are 100% sure all the above are operating correctly then I'd look further.

Good luck!
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Old 09-12-2009, 17:26   #12
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Jud...Gotcha,

I've always found it best to start with the simple solutions 1st...then work up...

1st...coolant levels...oil levels.....
2nd...raw water strainers before the engine...through hull and in-line strainers
how long has this been going on..gradual..or sudden.?..you could have crap on
blocking the flow through into your through hull strainer..or growth..
3rd...impeller
4th...Thermostat
5th...Belt tension/slippage
6th..heat exchanger......flow is equal in and out..so..I'd set that aside for now..

If you are 100% sure all the above are operating correctly then I'd look further.

Good luck!
Thanks for everyone's thoughts on this. Just to clarify, the problem does appear (only appears, but strongly appears) to be a lack of raw water flow --this is based on the apparently low amount of raw water I measured both from my exhaust (i.e., water that had through the whole raw water system) and from just my pump (before entering the cooling system), as described above. Amounts collected at both locations at wide open throttle for one minute were basically the same, suggesting that there are no blockages anywhere downstream of the pump (pipes, heat exchanger, elbow. It just seems like a very low flow, compared to similarly sized engines.

All this being said --I'm going to go right back to basics and check it all, even though I'm pretty confident everything is in order. And check to be sure all connections, and sea strainer, are tight (not sucking air at high RPMs).

Then, I'll do another water flow bucket test at the exhaust and pump, to see if there is any change from the first test. Then have a pump shop check the raw water pump to be sure it's pumping to spec

Then I think I'll be able to confidently narrow it down. I'll keep everyone posted...it's been a bit of mystery for quite some months.
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Old 09-12-2009, 17:50   #13
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Lightbulb A couple questions and comments.

How long have you had the boat and did it ever NOT over heat above 1900 RPM's

Is you inlet thruhull the same size or larger then the inlet to the pump? (I'd recommend one size larger then the pump inlet)

Have you checked your temp gauge? (I had a bad one last year that cost me a thermostat and a bunch of fittings I didn't need, and ended up only being a sender @ $12.
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Old 09-12-2009, 17:55   #14
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I would have changed the impeller a while ago to rule it out.

Your test #2 - water into and out of the pump and no other part of the system connected is telling. 2 possibilities 1 - 4.5 GPM is OK. 2 - 4.5 GPM is not OK. Change the impeller because this is a butt simple thing to change and you should be carrying spares anyway. Then the pump should be working as designed and the flow will be the flow. You can compare to other engine designs but that really isn't conclusive. If the engine is assembled to spec - i.e. right pump and the right impeller - you get what you get.

Now let's assume the system is performing normally in terms of water flow and cooling efficiency.

Has this always overheated on you or is it a "new" problem? If the cooling system is good, and I suspect it might be then, overheating = overworking. Do you get any smoke?

New problems could be:
Impeller key failed - impeller slipping
Exhaust system restriction
Prop fouled
Bottom fouled
Low compression
Shaft packing gland too tight

Old problems could also include
wrong size/pitch prop for the boat
engine undersized for the boat
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Old 09-12-2009, 18:21   #15
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How long have you had the boat and did it ever NOT over heat above 1900 RPM's

Is you inlet thruhull the same size or larger then the inlet to the pump? (I'd recommend one size larger then the pump inlet)

Have you checked your temp gauge? (I had a bad one last year that cost me a thermostat and a bunch of fittings I didn't need, and ended up only being a sender @ $12.
I don't remember if it ever overheated before...the boat was being refit for a long while and rarely used. FYI, bottom is clean (painted in March), prop clean. I don't think it ever heated, as before we had a very undersized prop that barely moved the boat!

We re-propped last year and it took three tries to get the right prop --first two props made us overheat, so went down in pitch and diameter; the third and current prop appears to be the right size, since we can almost get to max. RPMs (I'm told by the prop shop that getting within 10% of max. RPMs underload is close enough to be properly sized). 2900 is within 10% of our 3200 max., and we overheat at just over 1900.

Inlet through hull is 3/4"; inlet to the pump is 1/2".

Temp gauge has been checked against cylinder head area around sender with a infrared thermometer gun; gauge appears to be fine/accurate. IAnd it's fairly new, only 3 years old.)
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