So I take my ship out for a quick little cruise (literally less than a mile) and my starboard temp alarm goes off. I shut it down and limp back on one engine. I used the starboard again to dock it and noticed water discharging from the side of the boat but none from the exhaust. This happened with no warning, no gradually running hotter etc.

Can't rule out blockage on the intake but I think unlikely. Do impellers go out like this? Working perfect one day and kaput the next? It is a raw water cooled 8V71 in a 45 FDMY

I will check the strainer and the impeller and I know that if the impeller is broken to try and track down the pieces but beyond one of those two things being the culprit I am not sure what to look at next. Anyone know where I can find a nice primer on how these cooling systems work/troubleshooting?

Also, if I do end up changing the impeller, don't you have to prime it somehow so it will draw water?

Any advice appreciated.





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Also if anyone has a recommended source for the correct impeller? I found one on diesel pro but its $136! Looks like its a Jabsco #17936-001 and there are a ton I see cheaper but I don't want to buy the wrong thing.



Are you sure that your 8v71s are raw water cooled as that would be a very unusual configuration. Most marine applications were fresh water cooled. It is important to know which one your engines are as there are some important maintenance items such as periodicly cleaning the heat exchangers.

You never mentioned how old the original impellers are.

It is not uncommon for an old impeller to fail rapidly as they sort of tend to unravel as one loose blade triggers the failure of other blades, cascade effect.

I suspect that your engines actually are fresh water cooled and one thing that can fail is the engine re-circulation water pump drive belt. This will cause a very rapid engine temperature rise.

Editor Note:   Ron later posted that the 71 series and 92 series are gear driven, only the 53 series is a belt drive.

As far as the impeller, you probably need either the Jabsco 17935-0001-p or else the 17936-0001-p.

The only difference is the 35 is 2.5 in deep and the 36 is 3.5 in deep. The later 8V71s did use the bigger pump but you will find the smaller pump on some of the early 8V71-nonturbo engines so it is best to verify the first impeller change.

The P is nothing special but Jabsco did add it to their part number nomenclature somewhere along the line so many times cross references will not list the P.

1) get the correct impeller. 2) and very important on the Detroit’s buy the puller!!! You can thank me later- and 3) Pack the pump with Vaseline and put some dawn in the sea strainer or incoming hose to the pump. The grease will give the pump suction to prime, and the dawn will give you bubbles out the exhaust to let you know all is well. Oh... did I say GET THe PULLER TOOL?? On a Detroit two screwdrivers won’t do it... dieselpro has the tool.

Great advice Lee, the smallest things can make the job so much easier. I love the idea of the dawn to see bubbles, I'll be using that one, Thanks.

The water coming out the side is by-pass water, a normal condition. If water is coming out the by-pass but not the transom exhaust you have a blockage between the engine and the outlet, maybe something in the muffler. Maybe a critter crawled in and built a nest.We used to watch a muskrat climbing in and out of the exhaust of a boat across from us. Great entertainment. Never saw the boat go out.

The by-pass is adjusted by a globe valve in the raw water on the side if the engine. It is set to adjust the exhaust temperature, usually then safety wired to stay at that setting. I think that even if you or someone had adjusted the by-pass full open there would be some water coming out the exhaust. Run both engines at idle at the dock and see if they are both by-passing water.

Just had the first opportunity to go back out and start troubleshooting the cooling system. I am planning to replace both impellers just so I will know the condition. I started the problem engine and had water flowing from both the side and the exhaust.(see video) I ran the motor at the dock for about 30 min to get to 180 indicated and then it started going up quickly so I shut it down. 

Do you guys still think this is more than likely the impeller? It looked like the same amount of water that used to discharge though the exhaust before this issue developed.




Here’s one little trick to try. If the impeller is just missing a fin or two. At the dock. When the temp starts climbing RAISE the rpms. Say to 15-1600 and watch the gauge. If it levels out or starts dropping it’s an impeller. When impellers are partially done the temps will be higher at low speed because of Lack of flow. Hope this helps diagnose.

OK really dumb question. I'm not a complete dunce when it comes to auto mechanics but given that I thought my engine was raw water cooled I have a lot to learn. The large tank on the front of the engine is the heat exchange tank correct? (mine looks much like the one pictured) I assume all of the raw water fills this tank and the coolant runs through this raw water in tubes of some sort and then back into the engine to cool it. The cap at the top of the heat exchange tank is to top off the coolant mixture which I assume is a half antifreeze half water?

Assuming I am right here (huge assumption Lol) then does the 8V71 have a thermostat that would work just like in a car. Perhaps it is stuck closed not allowing the coolant to to circulate through the heat exchanger? 

What kind of maintenance is needed on the heat exchanger?

Any special care needed in topping the coolant or type/mixture to use?

Am I completely in left field  here or on the right track.

Thanks for all the advice and taking the time to set a novice straight.


I think the one pictured is a later model. Mine are 1974 8V71N's. Here is a video of them running from awhile back.


The large square tank is the heat exchanger. The raw sea water flows through a horizontal tube bundle. The outlet from the bundle is the tube that goes to the exhaust manifold. The inlet is on the opposite side.

The pressure cap on top of the heat exchanger is where you add the coolant and also check it.

Is the tank close to being full? You do need some room for expansion otherwise it will discharge out of the relief port. If the coolant is on the low side, it does reduce the heat exchanger heat transfer capacity. This usually exhibits a slow temperature rise as the heat builds up in the engine.

The heat exchanger tube bundles do need to be removed and boiled out occasionally.

The thermostat is quite large relative to the typical automotive thermostat and is a bypass type similar to the Ford 427 and Crusader 327F. These behave a little differently then the traditional automotive style in that the coolant velocity through the block, heads, and oil cooler remains constant.
Before I would get very involved in troubleshooting your problem, I would do a sanity check on the engine temperature reading and the actual engine coolant temperature. Being that Detroit's usually have electric temp gauges, a short or a bad connection will give you an erroneous temperature at the gauge.

I would check the thermostat operation by first measuring the coolant temperatures at the heat exchanger tube coming out of the thermostat block, the tube going to the engine water pump, and the engine coolant temperature.

It is good to also measure the sea water temperature going into the heat exchanger and the sea water discharge temperature at the heat exchanger sea water discharge.

A clogged heat exchanger will have very little temperature difference across the sea water inlet and outlet. The coolant loop will exhibit similar characteristics.

I would only remove the engine thermostat for checking if the heat exchanger coolant level is full, the engine is at or above operating thermostat, the engine coolant thermostat outlet going to the heat exchanger is at sea water ambient temperature and the engine water pump coolant temperature is at ambient sea water temperature. All of these conditions need to be satisfied before removal of the thermostat is warranted.

The engine water pump is fed from the J shaped tube coming out of the bottom of the heat exchanger and is gear driven off the camshaft.

In my first post I incorrectly mentioned the drive belt, brain cloud moment. The 71 series and 92 series are gear driven, only the 53 series is a belt drive.

The gear driven water pumps on very rare occasions will have the impeller shaft break at the impeller base so the impeller does not turn. That is extremely rare and should only be checked after everything else is verified to be okay.

Thank you all for the replies. Went back to the boat to dig in and had left the engine room light on by accident and ran down one of the batteries. So I look at the rectifier and it is buzzing like it typically does however it is not charging. (AC shore power is on)  Clearly I have many issues all at once but do you guys have an opinion on this? I am going back out today to put a charger on the battery. Seems odd that 1974 rectifier would decide to give up the ghost like this. I checked the fuse and the breaker.

Thanks again, this site is wealth of information.  

I would check the continuity of the positive and negative wires from the rectifier to the battery.

Bad connections tend to fail under heavy current draw especially at the battery.

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