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  1. #1

    Exclamation Engine help and questions on a marine keel cooled "GM" DD 6-71

    We have just come into a new to us Trawler. We have typically been Sailboat creatures and are comfortable working on our Yanmars, Perkins and Kabotas...we now find ourselves with a beast of an entirely different color. We have spent several days trying to find information and assistance as we feel the engine is not performing as we would have expected.

    The first thing that we found is the impeller was completely toast. The exhaust system had to be replaced. All of this has been done and there is water now gushing through the water-cooled exhaust.

    What we do know:
    This is a keel-cooled system
    There is oil in it
    It cranks up first time every time
    It dies after we have run it for about 20 minutes

    What we need to know
    1. How do you check the coolant on a keel cooled system
    2. How do you tell if your oil filter is clogged and needs replacingj
    3. What should oil pressure be (ideally) at
    Cruising (5 knots - sailboaters remember )
    Full Throttle
    4. What might cause smoke to come from the dip stick (not billowing like an explosion but noticeable to those looking)
    5. Is there a strainer that is in the oil pan or is it somewhere easily accessible and should that be changed
    6. What temp should the motor be expected to run at
    7. What should the ideal RPMS be run at during cruising (vessel is 29 ft with 23 in 3 blade prop)
    8. Can the motor be ran to fast or is there a governor
    9. What type of mileage might we be able to expect traveling at approximately 5 to 6 knots

    Thank you so very much for taking the time to answer any of the questions - we need all the help we can get!!

    Bill and Jess

  2. #2
    Join Date
    Nov 2009
    temecula, ca., usa

    Default Re: Engine help and questions on a marine keel cooled "GM" DD 6-71

    Hi svUb....

    It sounds like it's getting HOT...and shutting down. That is NOT GOOD! If I were you....I wouldn't run it anymore until I found out what is going on. Each and every time it gets hot enough to shut down on it's own, you have engine damage occurring.

    "Starts each and every time" means that you BEGAN with a sound engine. Detroits will be hard to start when they are cold if they have worn cylinders.

    If you are concerned that the oil filter may be plugged....REPLACE IT. There is a bypass valve in the oil filter adapter or housing...whichever you have....that allows oil to flow to the engine in the event the filter does plug up. But you don't want unfiltered oil going to your bearings.

    Oil psi varies on these but should be about 15-20 psig idle and 45-60psig @1500rpm. and above. The low oil pressure alarm is typically set at 10 psig. It is actually possible to have TOO MUCH oil pressure and there is a pressure relief valve in the crankcase. Anything over 70 psig. should be of concern.

    If it is a true keel cooled system....there is no "coolant" to check other than raw sea water flowing. There are, however, systems that utilize a "closed" system with glycol coolant that is cooled by a "heat exchanger" that acts much like the radiator on your car. Only, instead using air as the cooling medium, it uses sea water. In that case, there will be a reservoir with a pressure cap on it that you would use to fill and check the system...again...just like your car.
    YOU have to determine which type you have.

    In either case...You will also have an oil cooler on that marine application as well. And...there may even be a gearbox or transmission cooler.

    The keel cooled systems use coarse filters to strain out debris from the sea water that MUST be frequently checked and cleaned. But, with your description of "gushing" exhaust...it sounds as if you have good flow. You probably already know that there are inlet valves (sea cocks) associated with the incoming sea water. You would always want these to work well as any leak from the sea could sink the boat. I would close them each time I left the vessel. But...you MUST remember to open them again!

    There can also be a "service bypass" valve that a skipper can overlook that allows the raw water pump to move water but would bypass the on engine water pump. Each boat is different and COMPLETE familiarization with the systems should be striven for before even cranking the engine.

    A hard starting engine can be ruined if cranked too long with the sea cocks open. The non firing engine will not be able to force the water in the exhaust away from the exhaust valves and, eventually, the engine will flood. Not your problem now...but something to keep in mind.

    So...my guess is....from this far away...you may have either an overlooked bypass valve or some sort of thermostat issue. The water or coolant may be getting blocked by stuck stats and causing the block and head to overheat. The engine needs to operate in the 160*F to 180*F range for good combustion and, thus, thermostats are used to maintain that temp. Find where they are located on the front of the engine and check or have them checked.

    I do NOT like the part about noticeable smoke coming from the dipstick tube. While these engines DO develop a good deal of crankcase pressure...it shouldn't be "smokey". Are you sure it isn't steam? If it is "white smoke", it is likely steam...either way...it doesn't sound "normal".

    There IS a governor but it can be MIS-adjusted and can allow the engine to over rev. The Detroit 6-71 is happiest...and will return the most bang for the buck...at a MAX rpm of under 2300 NO LOAD. I prefer to see them adjusted to max out at under 2200 for longevity reasons. At max speed, under a load, you should be turning the engine at 1900 to 2100. You will likely "cruise" at between 1400 to 1800rpm but these engine are fully capable of running at 2100 all year long and LOVING IT. The "sweet spot" of the torque curve is at 1800-2100 rpm.
    I have seen governors adjusted as high as 2550 rpm no load but I would not advise it.

    I don't know if I answered all your questions and this probably only made you want to ask more. That's what the forum is for so....

    Good luck.
    Last edited by jgmo; 10-19-2015 at 02:46 PM.

  3. #3
    Join Date
    May 2015

    Default Re: Engine help and questions on a marine keel cooled "GM" DD 6-71

    I'm 67 and have been running 671s since my early teens. I ran them in Vietnam and ran them as a commercial fisherman. I have a pair now. Near the end of WWII, about 60,000 671s a year were being built. When I was a commercial fisherman, it was the most common engine by far. I've seen sets where two or four 671s were on the same shaft.

    I don't think you have a heat/stop problem or an oil problem. You can only stop a hot engine so many times before something big breaks. If it struggles and stops, maybe. But if it just stops or runs erratic and stops, I think you may have a place in the fuel supply where the pump is sucking air. You could have a fuel line rubbing a hole (I've had that) or a filter housing with a bad gasket (I've had that, too). The 671 doesn't need bleeding like injector pump engines. Because fuel is constantly passing thru the injector body, as soon as fuel is restored, it starts. If you have the common secondary filter, there is a plug in the top. You can put a pressure gauge there. Or slowly and carefully, unscrew it to see if fuel or air escapes.

    I've never ran a keel cooled Detroit, but the coolant should be fresh water and antifreeze. I don't know why there would be oil in the keel water. Usually Detroit Marine engines (and Gray Marine) use a heat exchanger on the front of the engine. They have two water pumps. The pump on the rear is a salt water pump and the pump on the front of the blower is the fresh water pump. It sounds like you have a salt water pump just for exhaust cooling water. Never seen that. Depending on how old the engine is, there are two thermostats that are under the expansion tank. They are usually 173, meaning they start to open about 150. The engines I know usually run about 175-180.
    Older engines (Gray Marine to about 1970) often have oil pressure much lower than newer engines. My Gray Marines before overhaul had idle oil pressure of about 10psi and running 25-30psi. These engines were designed in the 1930s and engines of that time had low pressure oil compared to engines of today. One reason is that oil seals were often rope or leather, not high temperature, flexible cup seals like we have now. Years ago I rebuilt a Detroit oil pump and tightened the clearances so running oil pressure was about 70psi. I had oil leaks you wouldn't believe.
    I have 671 natural engines (blower, but no turbo). I run my engines at 1800 rpm and that's good for 10,000 hours or more with proper maintenance. Turbo engines usually last 3-5000 hours between overhauls because of the higher temps. 200 hp vs 485 hp. In the military Detroit's were run as high as 3000rpm. Older engines have a battle setting on the governor. My engines were built before 1950 and looked like they had never been overhauled since new. They could have had 20,000 hours. The governor directly controls the fuel to the injectors. When you move the throttle, you are telling the governor to give more or less rpm. The governor maintains your set speed.
    You should have a incoming water strainer in line before the impeller you found damaged. Strainers should be checked daily. Often these engines have a dual strainer with a lever that allows you to switch between the two strainers while running. That way if you pickup trash you can clean one strainer without stopping.
    Smoke/steam coming from the dip stick tube is probably blow by from worn, but probably not worn out, cylinders. You also should have a vent where the oil goes in. There is an oil strainer in the pan, but not easily accessible.
    Keep your fuel clean. This engine doesn't have an injector pump, just a lift/supply pump. The pump moves 37 gallons/hour. After the secondary filter, the fuel goes into rails on the side of the head and from there to the injectors. Each injector is controlled by the governor and activated by the cam. Inside is a plunger operated by the cam that creates high pressure for the tips. There is also a steel wool type filter in the injector. It normally only gets changed when the injector is overhauled. Fuel is used to cool the tips. If the filter in the injector becomes restricted, less fuel flows and eventually the tip burns. The good part is your fuel is filtered many times while running. You pump 37 gallon/hr and burn about 4-5 gallons/hr. The rest circulates back to the tank, warming the fuel there for a better burn and keeping the tank clean and free of water.
    I cruise at 10 knots with twins. I burn about 8.5 gallons an hour or about 1.15mpg that I think is pretty good for an 83' boat. Before overhaul it was 10 gallons @ 10 knots. AT sailboat speed you'd probably burn about 3 gallons.
    In my opinion, the 671 is the best, most reliable engine of that size. It has no electronics to fail, all the pumps, etc., are heavy duty mechanical pieces. Once started, it runs until you shut it off or the fuel is exhausted.

  4. #4

    Default Re: Engine help and questions on a marine keel cooled "GM" DD 6-71

    Great posts! Thanks learning a lot from jgmo and Lepke! Thanks

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