Logo
Click for Detroit Diesel manuals!
Results 1 to 12 of 12
  1. #1
    Join Date
    Jul 2016
    Location
    Northern WI
    Posts
    2

    Default 6-71NA Overheat mystery

    Hey all, thanks for taking a look at my post. I work on a tug with 4 6-71's for propulsion, 2,600hrs since last overhaul. The engines are keel cooled, with each engine to it's own cooler, coolers aren't shared or connected to each other in anyway. Normal running speed for us is 1,700rpm, never any faster since we never get over 4 knots no matter what. I'm now having to run 3 of the 4 engines without thermostats to keep a mid 180's temp on them at 1,600rpm. On really warm summer days we have to back down to 1,500 to keep from running hot. I've chemically flushed cooling systems and replaced thermostats before finally removing them completely. These engines only have the fresh water pump on the blowers for coolant circulation and have been running fine the last 4 years. I replaced water pumps with no avail. I've re-plumbed the trans coolers out of the engine cooling circuits with no luck. I have a 25* temp differential on my keel cooler in/out, and as soon as we throttle down my temps plummet immediately, so I know my coolers are shedding heat. I've discussed this with a handful of other tugs in my area and we're out of ideas. This issue has developed in the last couple months and I don't have the luxury of being able to shutdown more than one engine at a time for more than a few hours.

    I have very limited chances for cell coverage/internet connecting here in Western Alaska, but will try my best to check in often. I'm stumped and appreciate any help.

  2. #2

    Default Re: 6-71NA Overheat mystery

    Hi, have the keel coolers been cleaned on the sea water side?? To me it looks like the keel coolers are not shedding enough heat to the sea water. Idling down slows water speed thru the keel cooler allowing rapid cool down. Barnacles on the props?? More load more heat!
    Charlie w.

  3. #3
    Join Date
    Jul 2016
    Location
    Northern WI
    Posts
    2

    Default Re: 6-71NA Overheat mystery

    Coolers and props are clean. I failed to mention the tug is out of the water every fall before ice up and 95% of the time we're in fresh water. I think that's all the pertinent info I forgot about.

  4. #4
    Join Date
    May 2015
    Location
    Oregon
    Posts
    72

    Default Re: 6-71NA Overheat mystery

    One issue, more than 50:50 anti freeze doesn't cool very well...
    I use a descaling product called Rydlyme. I suppose it's mainly for the sea water side, but since I started using it on the FW side I've solved many overheat problems without taking things apart. Often on boats some other shop flushed or even manually cleaned the heat exchangers. Since Rydlyme is also reported to remove rust, I've assumed it removed insulating barriers of rust off the cylinders and water passages. Maybe allowing faster flow. I don't know, it just seems to work where others failed. I'm long retired, but use it on my 671s in both fresh and salt sides. My mains are 1947 & 8. I run at 1800 and never go over 175°.
    Rydlyme is biodegradable and can be dumped over the side or storm drains in any place I've been. Ebay, Amazon or better marine stores.
    When I was young I ran a quad tug. I've been running and overhauling Detroit Diesels for nearly 60 years.

  5. #5
    Join Date
    Nov 2009
    Location
    temecula, ca., usa
    Posts
    2,680

    Default Re: 6-71NA Overheat mystery

    Lepke may have the solution. Scale buildup on coolant passages serves to act as a very effective insulation against heat transfer. The minerals in water tend to "plate out" more prodigiously on the hottest surfaces so the fire deck of the head seems to get impacted the worst. I was trained that 1/32" inch of mineral scale will provide the same resistance to heat conduction as 5" of cast iron. Ironically, fresh water can deposit as much or more scale in an engine than salt water sometimes.

    You didn't mention water strainers or filters but I trust those have been checked and cleaned. I have seen the brass "brillo pad" type strainers trap enough debris and retain it to the point that the strainer pads couldn’t be effectively cleaned and had to be replaced.

    Removing the T-stats to resolve overheating issues could bacfire. While it sounds like it's worked, somewhat, for you , consider that most of these engines have what is called a reheat bypass system that utilizes full blocking thermostats.

    The therostats, when open, allow heated coolant to exit the head and recirculate immediately back into the block. This action allows for more rapid engine warmup from cold start. The thermostats modulate this action until maximum cooling is required at which time they fully open and fully block the reheat passages.

    Completely removing the stats leaves the reheat passages open at all times. Long story short: The engine can't obtain max cooling with the stats removed.

    Try Lepke's descaler because it sounds like it couldn’t hurt and might help.

    Good luck
    Last edited by jgmo; 07-18-2016 at 02:45 AM.

  6. #6
    Join Date
    Nov 2009
    Location
    temecula, ca., usa
    Posts
    2,680

    Default Re: 6-71NA Overheat mystery

    Just reread my post.....I should have said:
    When the stats are CLOSED coolant is diverted away from the heat exchanger and partially directed back into the engine block for rapid warm up. Then, when FULLY open the reheat circuit is fully blocked and all flow is directed to the heat exchanger.

    This is why engines will overheat if the weong TYPE of thermostats are used. Sorry for any confusion I may have caused.

  7. #7
    Join Date
    May 2015
    Location
    Oregon
    Posts
    72

    Default Re: 6-71NA Overheat mystery

    When I do Detroit fresh water side, after getting rid of anti-freeze, I run the engine at idle for about an hour. Removing the thermostats is a PIA, so hot is the easy way. Rydlyme is advertised to work between 0° and 180° so as long as the engine doesn't overheat... Over 180° good metal may be effected. On the salt water side I circulate it cold. I do my generators coolant the same but lock the injector pump down to idle.
    Another issue: 25-30 years ago, the caustic cleaners were much more effective. I remember when the change came and I bought several cases before the change. I assume it was the EPA at work that screwed that up, too.

  8. #8
    Join Date
    Nov 2009
    Location
    temecula, ca., usa
    Posts
    2,680

    Default Re: 6-71NA Overheat mystery

    Something else on "heat rejection" that often goes overlooked but it's a basic concept for all liquid cooled, internal combustion engines.

    What I'm referring to is the 1/3rd rule. Meaning that the heat of combustion is removed from an engine in 3 main ways. The ratio of one third, one third, one third isn't exact but is used to illustrate that heat isn't rejected SOLELY by the coolant. While the "cooling system" is the obvious intended method for controlling temperature, a large amount of heat leaves the engine via simple radiation to the engine's surroundings. An engine caked in grime or mud or located in a poorly vented compartment will tend to run hot.

    But, the most overlooked form of transferring heat away from engines is the exhaust. A restricted exhaust or otherwise "inefficient" exhaust.....as in one that is undersized for example, could have an engine running excessively hot. Many mechanics, myself included, don't always think of the exhaust when faced with an overheating condition. Mostly because it's not all that common. But it has to be taken into consideration if the solution isn't forthcoming.

    Or....in other words.....
    Just sayin'.
    Last edited by jgmo; 07-24-2016 at 12:43 AM.

  9. #9
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,549

    Default Re: 6-71NA Overheat mystery

    You say you removed the thermostats that could be part of the issue. If the coolant is moving too fast it will not transfer heat properly either. Instead of a thermostat put in a large washer to restrict the coolant flowing through the heat exchanger so the heat has more time to transfer. Just an idea you can try different size holes in the washer to get the best flow rate through the keel cooler to transfer heat more effecient?

  10. #10
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,549

    Default Re: 6-71NA Overheat mystery

    That could also explain why the temp plummets when you throttle down the coolant is moving slower through the heat exchanger as well as the demand is less.

  11. #11
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,549

    Default Re: 6-71NA Overheat mystery

    Just thinking about it you could possibly put a large gate valve in the coolant line you could control the coolant flow depending on the demand maybe worth a shot?

  12. #12

    Default Re: 6-71NA Overheat mystery

    Hi Joey
    Just wanted to know what part of wi. you are at I you are near you should get a hold of Mike Ojar, he is good with Detroit's. Superior wi.

Similar Threads

  1. Mystery
    By joereeves in forum Johnson Evinrude Outboard Forum
    Replies: 5
    Last Post: 03-04-2012, 11:33 AM
  2. Marvel Mystery Oil
    By tbrophy in forum Ford Lehman
    Replies: 0
    Last Post: 10-24-2008, 02:04 PM
  3. Mystery engine
    By peterbc2 in forum Other Gas Inboard Mfgs IO and Sterndrive Questions
    Replies: 2
    Last Post: 04-15-2008, 01:10 AM
  4. A real Mystery
    By tracy hayes in forum Archive 2003-2009
    Replies: 1
    Last Post: 10-06-2003, 08:20 PM
  5. OPTIMAX MYSTERY 135
    By CAPT. PO in forum Archive 2003-2009
    Replies: 2
    Last Post: 09-10-2003, 03:00 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •