Water Pump Impeller Question

[jibes138]

I have a new Oberdorfer water pump with the original impeller, very few hours on it. I forgot to open the seacock and started the engine and when I observed no water coming out quickly troubleshot and opened the valve but the pump may have run dry for maybe a couple minutes. Another time I had a clogged exhaust elbow fitting so the flow rate was very restricted. How long can you run a standard impeller dry before doing permanent damage? I have a o-ring style pump so I have taken it apart and the impeller looks fine but I am concerned about the efficiency. Not that all the restrictions are gone it doesn't seem like as much water is pumping as when brand new. At slow speeds temperature is 160 but as I crank the RPM's it starts going towards 180. This is a Universal M25. I have a replacement impeller but not sure if I need to look elsewhere like using a larger fitting on the exhaust elbow.

 

[Stu Jackson]

Most people will say the impeller is toast, replace it. I forgot one day, the impleller's still in the pump and that was three years ago. Water still comin' out? The pump's still workin'.

The larger issues we've seen have been failing globe impellers and loss of prime due to tolerances on pump impellers and the need to fair the faceplates very carefully.

With your temperature rises, I'd check the inlet ports of the HX. Have you upgraded to a new larger HX yet? That'll solve those "creeping" temps we all had with the old 2 inch HX on our M25s.

 

[jibes138]

Heat Exchanger

Thanks Stu...no I am still using the OEM 2" and have never had a problem with overheating until recently when the fitting on the exhaust elbow was clogged almost shut. My guess is debris from the zincs got in there and clogged it. I blew through the heat exchanger and it didn't offer any resistance to flow so I figure it must be pretty clear. I did take off the end plate and didn't notice any significant debris. The motor always ran a steady 160 and coming in to port the last time against a strong current I cranked up the RPM's and noticed it went up about 10-15 degrees. So I figured I'd ask about the impeller. Someone metioned the fit between the plate and the sides of the impeller as causing a loss of efficiency, the impeller blades are all intact and not cracked. I can replace it with my spare and keep that one for a spare as it is easy to change.
Any good sources for the 3" HX ? I'm too cheap to upgrade for the 800.00 or so from the usual spares source. I've seen them on E-bay for a lot less but not sure of the pedigree of the lower price units. The heat exchange will still be dependent on the flow of water through it so the restriction to flow may be the fitting size on the exhaust elbow if it is smaller than the tubes or hoses from the pump. In short a larger HX may not be the answer, good heat exchange depends on the flow of coolant and flow of raw water and the heat exchange between them in the HX. The more raw water flow the cooler the engine.

 

[sailingdog]

Generally, I'd say that replacing the impeller would be wise. You can't always see the heat damage from running it dry, but the blades do become more brittle, less flexible and far more likely to break.

When was the last time you disassembled and cleaned out your Heat Exchanger... if you haven't, it might well worth be doing... Maine Sail has an excellent article on doing just that. Broken off zincs, broken impeller blades, mineral deposits can all affect the water flow and cooling ability of the HX.

 

[Stu Jackson]

My guess is debris from the zincs got in there and clogged it. I blew through the heat exchanger and it didn't offer any resistance to flow so I figure it must be pretty clear. I did take off the end plate and didn't notice any significant debris.

That's only half of it. I keep mentioning to folks when they talk about HXs that the initial experience is: "The tubes are clear, must be zinc crap." That's still only half of it.

THE INLET PORT FROM THE RAW WATER PUMP to the HX is where the cooling first begins. That's where the salt starts first dropping out. On our old 2 inch HX, when we first bought our boat which had had a history of overheating just as you described and others reported with their marginal 2 inch HXs, that port was CLOSED UP 40% with salt sediment!!! I cleaned it up and it worked much better, but I still had the growing engine temperature at max throttle. (Our 3 inch HX also exhibits this port blockage tendency - it's physics.)

Simply blowing through your HX WITHOUT TAKING IT OFF THE ENGINE will NOT find this crud.

So, sleep better at night and change your impeller if you feel you must, but take the HX off the engine and check the ports.

BTW, this is the very FIRST time in the 13 years I've been working with M25 series engines that I've EVER heard of a plugged exhaust riser nipple. We hear a lot about it on Yanmars, but that's because the elbow's right on the engine. Ours are remote.

$800. Nonsense. I paid $345 for mine a few years ago, they shouldn't be that much more expensive now.

The more raw water flow the cooler the engine.

Sure, but the faster the engine runs the faster the pump pumps!

 

[jibes138]

But if there is a restriction to flow, the pump can turn faster but not necessarily have a lot more flow. It may just be spinning in the bore and the vanes bending to relieve the pressure. The exhaust nipple might have been a combination of salt accumulating over time and then maybe some zinc debris sealed its' doom. I also noticed some salt deposits in the inlet hose to the pump but no appreciable impediment to flow. The hose from the HX to the exchanger was clear as were the hoses to the fitting to the exhaust elbow. I will take your advice and remove the HX and give it a good once over and maybe replace with the 3". As far as the price goes I agree you can get one for $345, but you can also get one for 800.00. So the question is do you want to spend $800 and have a solid unit with a long life or do you want to spend $345 and smash into the golden gate bridge when it fails unexpectedly on your second time out in the boat with it. You choice or risk level. I really hope it all works out for the best for you.

 

[Stu Jackson]

Yup, I agree, a blockage in a line will restrict flow. Glad to hear you're gong to take it off. When I first took our old 2 inch HX off and shook it, it was like a mariachi band! But all that gunk was in the CLOSED end, not the zinc end, and was from the inlet port.

I've had the 2 inch HX on for two years and have been out under the Gate and back many items since then and haven't run into the bridge. I'm glad I didn't spend $800 for that HX!!! (I bought it from a Universal dealer, known also to sailors as the highest price in town. Who's quoting you $800???)

 

[jibes138]

Our price: $760.43 - each

 

[jibes138]

the closed end IS the zinc end???
So my choice seems to be take the dinghy to the beach and drink beer all day or take the heat exchanger off, shake it around, clean up the antifreeze that leaks out, buy a bigger one for $760 try to get it to fit, find out I need new hoses because the lengths are different and now chafe on the engine, run to the store a dozen times, and reroute the exhaust system for a cleaner run. Option 1 sounds a lot better. I'll let you know what I find when I take it off and play the gourds with it.

 

[Deadline]

You win

Since the impeller looks fine, it obviously hasn't disintegrated and thereby contaminated the engine. But replacing it will ensure that the next one won't be fatigued from the dry run event. Either way, you win.

Don't make my mistake. I replaced the paper gasket and used the O ring as well. The effect was indiscernable because there was no water drip, but air seeped into the pump and caused cavitation in a few minutes. The result was a spectacular explosion of the exhaust hose that frightened everyone onboard, including me.

By all means, check the mixing elbow because that's the first sign of clogging. No use cleaning it, best to replace for around $150 if it's clogged.

 

[DeniseO30]

just a silly question... new pump... are you sure it's the right pump? I think there are 3 models of the Oberdorfer pump