Water Does not Come out the Exhaust

[priopel]

After working on the cooling system of my Volvo Penta MD 7A engine including removing the manifold, flushing the block, replacing the water supply tube and installing a new impeller in the pump, water does not come out the exhaust. With a tip from a mechanic, I find that if I disconnect the hose from the thermostat at the point where it enters the exhaust (or maybe heat exchanger), and hold it down a couple of feet lower, that water begins to come out of it. I then put the hose back on and the usual amount of water then comes out of the exhaust. It continues to function normally but the next time I start the engine, no water comes out the exhaust unless I repeat the procedure of removing that hose.

 

[Terry Cox]

Just a couple of thoughts. Is the new impeller OEM or off brand? Off brands can cause a priming problem due to a slight difference in clearance between the impeller and pump housing/cover. Also, OEM gasket or off brand?

Another possibility, did you clean the sea water strainer in the process? I've been known to not get a good seal between the cover and the housing, which can also cause a priming problem. I place my finger on the center of the cover with a wiggle, then bring the wing nuts to the cover and give each a quarter turn until snug/tight.

 

[priopel]

Thanks, Terry. Both the impeller and gaskets are OEM purchased from the local Volvo Penta dealer.
I don't know what the strainer is or how to locate it.

 

[Don S/V ILLusion]

Thanks, Terry. Both the impeller and gaskets are OEM purchased from the local Volvo Penta dealer.
I don't know what the strainer is or how to locate it.

It will be at or close to the intake thruhull.

 

[Hayden Watson]

If your sea strainer is above or close to the waterline it could be losing its prime and can cause that problem. Make sure that there are no air leaks at the sea strainer.

 

[dlochner]

The strainer is probably pretty big and will look something like this:

A common failure on these strainers is the gasket above and below the glass housing. It is a cheap fix, buy some gasket material and make a new one. Also the strainer needs to be cleaned out periodically.

 

[jssailem]

I like the Groco ARG 750

They come in 3 designs of basket. Plastic, Stainless Steel, or Monel. I like the monel model.

It is small and compact. If the strainer is located above the water line, I can open the top of the strainer and empty the basket even if the thru hull is open. You can always shut the thru hull to be sure no water enters the boat while you are cleaning the strainer.

 

[priopel]

I was optimistic when I went to my boat this afternoon expecting to clean the water strainer which I was sure had never been serviced in 40 years. What I discovered is that I do not have a strainer. There is a valve at the through-hull and a 1" rubber hose that goes from there directly to the water pump. Hmm

I have installed a flushing T in the 1" hose that goes from the thermostat housing to exhaust as a way to prime the system. Today, when I poured water in at that point, only very little came out the exhaust. I tried pulling off that hose at the end where it goes into the exhaust, pointed it down and nothing at all came out. While that hose was disconnected, I could tell that a bit of pressure from the exhaust comes out the elbow that the hose connects to. That bit of back pressure could inhibit the cooling water from making its way any further.

 

[heritage]

See if water comes out of the hose between the sea cock and sea water pump.

 

[dlochner]

Lets go back to the beginning, do you have a heat exchanger or not?

Not having a raw water strainer is rather unusual as it is necessary to prevent all kinds of crap from entering the cooling system and plugging it up.

The MD7A with a sea water cooling has a water pump that is well before the thermostat. It is located on the aft end of the engine on the starboard side. It draws water from the intake through the transmission and then into the engine block before the thermostat. There is a small copper tube that bypasses the thermostat to allow some water to enter the exhaust system to cool the exhaust.

If the exhaust elbow is plugged, water may not enter the exhaust system. The exhaust elbow is after the exhaust manifold.

Another possibility is the raw water intake. If this has a screen, the screen may be plugged with bottom paint, critters or weeds. This would prevent water from entering the intake.

 

[Hayden Watson]

..What I discovered is that I do not have a strainer. ...

Wow! I have found things ranging from 24" long pieces of eel grass to small fish in my sea strainer. I can't imagine not having one. Given that I would be disconnecting each end of every hose and looking in each to make sure that they are clear.

 

[Apex]

Try a different or the old impeller. I found 0.005 difference created too much clearance on the side of the impeller for it to pump. In the end I had several spares on board: Took everything off the boat and ran the pump with a drill. Tried all the impellers and found 1 that pumped. The remaining were too thin...
Air leaks is also a good suggestion. Both lead to weak pumping of water.

 

[RoyS]

Also, look for wear on the inside of the pump cover. You can flip it over in most cases if there is significant wear.

 

[Terry Cox]

Both the impeller and gaskets are OEM purchased from the local Volvo Penta dealer.

Was the old gasket completely removed from the housing? I've had to scrape ours off. Left on it will create a gap that prevents a pump prime.

 

[priopel]

You guys are great. Your comments are spot on. I am going to pull one end of the water intake at the water pump and hopefully install a strainer after testing the line. Dave, I replaced the exhaust elbow. That the intake line is clogged or partially clogged seems the likely culprit.

 

[NYSail]

I would find it hard to believe there isn’t grass and all sorts of crap in the water passage of the engine. Needs a good flush. As was said…. I pull fish, grass, jelly fish etc from my strainer. Never saw an inboard boat not having one.

Greg

 

[priopel]

Well shucks. I detached the water intake hose at the water pump and water flows freely there unless I lift the end of the hose up. While holding it up, I shoved a stiff wire down the hose. It slid in easily and came out clean. I was hoping I might have narrowed the problem down to this hose but apparently not so.

Water is getting to the pump, the pump has a new impeller, I have temporarily removed the thermostat but that has made no difference. The water supply line from the pump is new. I removed the manifold, cleaned and inspected all the passageways. I flushed the block and could pump water into one of the two apertures for the cooling system and in a couple of seconds the water came squirting out the other opening. The piece that has one chamber for water and the other for exhaust (maybe this is the heat exchanger) was removed, inspected and seems OK. I have had water coming out the exhaust but had leaks along the water supply tube. I have fixed those but no water comes out the stern even after adding water to prime the system. I'm stumped.

 

[Terry Cox]

Had the same problem with our Genset awhile ago and long story short, it was the impeller. The deal sold me a run dry impeller rather than an OEM Onan impeller. The only difference that I or the dealer could tell was the color. The run dry version was blue and the OEM was black; all the dimensions looked the same.

When I installed the OEM version, I also scraped the gasket housing and cover surfaces clean of old gasket material. The tolerance of the gasket is very important; it is supposed to be 1/32' thick. This allows the impeller vanes to slightly touch the cover surface, thus ensuring a good prime.

Another consideration that I checked was the impeller drive shaft seal. In our case the seal was good. If loose/worn, it will allow air to bypass thus preventing a good prime.

 

[Davidasailor26]

Still not clear on whether you have a raw water cooled engine or fresh water cooled. If it’s raw water cooled there really is no heat exchanger. The raw water is pumped through the block, then into the exhaust elbow and out the exhaust. If it’s fresh water cooled the raw water never actually goes through the block. It goes from the pump to a heat exchanger, where the fresh water also runs, then the raw water goes to the exhaust elbow.

I find that if I disconnect the hose from the thermostat at the point where it enters the exhaust (or maybe heat exchanger), and hold it down a couple of feet lower, that water begins to come out of it. I then put the hose back on and the usual amount of water then comes out of the exhaust.

Is the engine running when you do that experiment? The way the raw water pump is designed, as a self priming pump, it really shouldn’t work that way. The output of the pump should be relatively independent of the height of the discharge. (Technically head height might have a small affect, but not the dramatic difference you describe.) It sounds to me like something about the pump or impeller is your problem.

I would find it hard to believe there isn’t grass and all sorts of crap in the water passage of the engine. Needs a good flush. As was said…. I pull fish, grass, jelly fish etc from my strainer. Never saw an inboard boat not having one.

Greg

O’Days of the mid-80’s with small Universal diesels did not come with strainers. Some owners may have added them along the way. We didn’t, and our maintenance was basically removing one end cap off the heat exchanger and cleaning it out a couple times per year.

 

[Tally Ho]

Still not clear on whether you have a raw water cooled engine or fresh water cooled. If it’s raw water cooled there really is no heat exchanger. The raw water is pumped through the block, then into the exhaust elbow and out the exhaust. If it’s fresh water cooled the raw water never actually goes through the block. It goes from the pump to a heat exchanger, where the fresh water also runs, then the raw water goes to the exhaust elbow.


Is the engine running when you do that experiment? The way the raw water pump is designed, as a self priming pump, it really shouldn’t work that way. The output of the pump should be relatively independent of the height of the discharge. (Technically head height might have a small affect, but not the dramatic difference you describe.) It sounds to me like something about the pump or impeller is your problem.



O’Days of the mid-80’s with small Universal diesels did not come with strainers. Some owners may have added them along the way. We didn’t, and our maintenance was basically removing one end cap off the heat exchanger and cleaning it out a couple times per year.

My 1988 O’Day 322 was a Groco strainer. Not sure if was added by a PO or original. But I suspect original.

Greg