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  1. #1

    Default 6-71 engines bubbling

    Hi all, I'm in the process of possibly purchasing a 1980 F-40 Trojan, and had a good look at the 6-71 engines today. They have both had a great deal of recent work prior to selling, including rebuilding both heat exchangers and replacing all cooling hoses. They light up fast although it's quite cold right now and run well. What has me concerned are small bubbles coming past the (new) cooling system caps and into the coolant overflow reservoirs. I expect coolant to expand past the caps but my prior experience with diesels is that any gasses in the cooling system points to head gasket leakage. I really don't expect this to be the case here as the mechanics recently overhauled the cooling system and pressure tested it to the tune of very big bucks, and both engines are bubbling at exactly the same rate. My question is it normal for this to occur?

  2. #2
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    Default Re: 6-71 engines bubbling

    There is a test to see if combustion gases are in cooling system done with a special solution that gets placed in radiator then tested in another bottle with another solution. forgot where i saw it, maybe 2carpros.com web site

  3. #3
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    Default Re: 6-71 engines bubbling

    This could be entirely normal. But the explanation might seem a bit complicated.

    First, those "over flow" tanks are known by many other names......surge tank......expansion tank.....recovery tank.....and.....de-gas tank. The terms all stem from partial descriptions of what takes place as the system heats up and cools down as well as feeding other systems. Those "other systems" are typically some form of hot water heat exchange for passenger comfort like remote location heater cores and hot water heaters for showers.

    The more this remote heat exchange feature is utilized, the more chances for the engine pump to cavitate and create small amounts of gas that would be observed at the recovery tank sight glass.

    But, for diesels like the Detroit, the most likely cause is the harmonic vibrations that take place at the cylinder liners. The vibrations at the liner can produce high frequency bubbling to occur at the junction of coolant to metal on engines with "wet" cylinder liners. These bubbles are known to collapse with such force, they can actually damage the liner. This phenomenon is known as liner pitting and is a real engine killer.

    These bubbles can be minimized and managed by using cooling system additives designed specifically for the purpose.

    These engines don't use one piece head gaskets either. They use a separate compression seal for each hole and several water and oil passage seals for controlling those liquids. Therefore, it would be highly unlikely that combustion would enter the coolant at the head to block interface.

    You should ask your mechanics about liner pitting and SUPPLEMENTAL COOLANT ADDITIVES and what was done in that regard during initial fill.

    If they look at you funny and say "never heard of it"......well......then.....ooooh boy!

    Good luck.

  4. #4
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    Default Re: 6-71 engines bubbling

    good info jgmo

  5. #5

    Default Re: 6-71 engines bubbling

    Good to know. There aren't any other systems attached to these beasts, but another marine mechanic also told me it's not an issue. I'm glad for this because it's a great boat and the engines run very well and 22 grand worth of overhaul and maintenance was just put into them. But I'd rather know before I drop a cheque, you know? I'm going to have to find myself a service manual and really familiarize myself with them. I have a general idea what I'm looking at, what assorted lumps and fittings are and what they do, but I can see a steep learning curve ahead of me. It's far more complex than the 4 banger auto diesels I've always worked on. Cripes, at 1.9 litres, the genset alone has a bigger engine than my previous 36' sailboat.

  6. #6
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    Default Re: 6-71 engines bubbling

    6-71's were used extensively in the fire service in almost every American LaFrance fire truck from late 60's to 80's+ Very reliable machines

  7. #7
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    Default Re: 6-71 engines bubbling

    Well....I didn't mean to say the bubbles aren't a problem. They may not be. On the other hand, if you aren’t running the right coolant additives then liner pitting can occur. It can happen fairly rapidly and can end up putting holes in the liners. Running low silica coolant containing an additive, like Penn-Cool, that puts a constantly replaced coating on the liners with 50/50 dilution of distilled water will keep you good for a year or so. After that you either test and adjust the additives or do a drain, refill for maintenance. If you run the engines a lot then testing intervals would need to be stepped up.

    I should have asked if these are in line 6-71's or V-6 71's
    If they are older in line engines they may not even have "wet" liners. In that case, liner pitting wouldn't be a concern at all.

    Once you get to know the 2 stroke with unit injectors you will come to understand that it is a much simpler engine than a 4 stroke with a high pressure injection pump.

    If, after you get your CORRECT service manual, concentrate on understanding how the engine takes in air. If you can grasp the 2 stroke air intake (very first pages of the manual) and then learn how to properly time the injectors and set the rack, the rest is very simple. The manual should be the ONLY reading material in the head until you've read through it at least once.

    Good luck.

  8. #8

    Default Re: 6-71 engines bubbling

    Quote Originally Posted by jgmo View Post
    Well....I didn't mean to say the bubbles aren't a problem. They may not be. On the other hand, if you arenít running the right coolant additives then liner pitting can occur. It can happen fairly rapidly and can end up putting holes in the liners. Running low silica coolant containing an additive, like Penn-Cool, that puts a constantly replaced coating on the liners with 50/50 dilution of distilled water will keep you good for a year or so. After that you either test and adjust the additives or do a drain, refill for maintenance. If you run the engines a lot then testing intervals would need to be stepped up.

    I should have asked if these are in line 6-71's or V-6 71's
    If they are older in line engines they may not even have "wet" liners. In that case, liner pitting wouldn't be a concern at all.

    Once you get to know the 2 stroke with unit injectors you will come to understand that it is a much simpler engine than a 4 stroke with a high pressure injection pump.

    If, after you get your CORRECT service manual, concentrate on understanding how the engine takes in air. If you can grasp the 2 stroke air intake (very first pages of the manual) and then learn how to properly time the injectors and set the rack, the rest is very simple. The manual should be the ONLY reading material in the head until you've read through it at least once.

    Good luck.
    They are the inline 6-71's circa 1982. How would I tell if they have wet liners? Interesting about the unit injectors. My 2004 jetta TDI uses unit injectors, a configuration VW dropped after only a couple of years presumably because the design was so troublesome, after many many successful years with bosch rotary injection pumps.

  9. #9
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    Default Re: 6-71 engines bubbling

    If it's an 82 block it should have wet liners. But, there are inspection covers on the side of the block you can pull to inspect and you should be able to see, and even probe with a pick, the lower water seals. These are the same covers you remove to inspect the piston rings through the liner air holes.

    As far as the unit injectors in the V-Dubs....my guess is that they probably encountered problems keeping the tips cool. Or, it may have been they just couldn't lube them good enough. Who knows? VW engineering dept. I bet.

    You have to understand that those injectors would have had to be pretty small in comparison to the ones in Detroits...2 stroke or 4 stroke. They COULDN'T be as robust.

    Cummins runs unit injectors in their big engines but the small truck engines use an injection pump. Heck, even Detroit ran an injection pump on their failed 350 gas to diesel conversion "experiment". Lucky me, I had the dubious honor of working on a few of those TURKEYS. Loved 'em. Made me much money. The owners hated them though. Under powered...noisy ....stunk.....leaked (as all good Detroits do) and went through starters like candy when they got out of tune or ran low quality fuel. Just like the Jetta with units, they were a flash in the pan too.

    But when AUDI and VOLKSWAGEN started winning sports car races with diesel engines a few years back...I was in awe! And I still couldn't tell you how they do THAT!
    Last edited by jgmo; 01-05-2017 at 12:01 AM.

  10. #10

    Default Re: 6-71 engines bubbling

    Unfortunately, those access port are on the other side of the engines and I have no idea how I'll squeeze myself in there to check. Maybe serial numbers will give needed info.

    It seems there was a design issue around lubricating the cam in the PD VW engine. You have to use a special expensive oil and even then wise people keep an eye on the cam bearings for wear after the 300 kilometre mark. The pressures on the cam running both valves and the unit injectors is really high, and there's argument VW didn't provide adequate oil porting to the bearings to deal with the increased load. And those injectors are incredibly expensive - new over $700, and if one goes they want to do the whole set, and with labour you're out 4-5 grand. Funny how they wanted to reinvent the wheel. Tuners love them because with just a software tweak you can get an additional 50 hp and those cars can really fly.

  11. #11

    Default Re: 6-71 engines bubbling

    Thats crazy to have rebuilt the cooling systems on those motors.
    For that kind of money just repower the boat with two 12 or 24 valve inline 6 cummins diesel motors.

    The old 671s in V or inline 6 are twice as heavy as a 12 valve cummins and make about the same power.
    Plus the Cummins will use nearly half the fuel of one of those detroits.

    Just my 2 cents :-)

  12. #12

    Default Re: 6-71 engines bubbling

    My two cents on the Bubbles.

    Bubbles in a closed cooling system are likely a leak in pressure somewhere. Plus you mention the motor just had a lot of cooling system repair.
    Plus now the boat is forsale.
    All of those together kind of have me scratching my head.

  13. #13
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    Default Re: 6-71 engines bubbling

    The nice thing about 71s is no injector pump. No bleeding problems and one less accessory to rebuild. I was a commercial fisherman and in my time the 671 was the most common engine. The reliability is legendary. No power need to continue running, no electronic sensors, circuit boards and so on. The boat I live on has a pair built before I was born, and I'm 69.
    A number of engines have bubbles forming on the water side of the cylinders. A coolant additive, available at most trucker fuel stops solves it.

  14. #14

    Default Re: 6-71 engines bubbling

    Is there any coolant dripping out of air box drains [email protected]

  15. #15

    Default Re: 6-71 engines bubbling

    I have the same symptoms at the original poster BUT I have the 6-71 TIB engine (1990) with dry lined cylinders.
    Also the coolant has become brown. Pretty sure it's a head gasket/o ring issue.
    How bad is this job for someone who's only worked on small car engines and outboards.

    And does anyone have an online manual recommendation for the 6-71TIB engine?
    All on the ones I've seen seem to be from older 6-71s

  16. #16
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    Default Re: 6-71 engines bubbling

    Cavitation is a problem in high hp diesel engines. Wet or dry. It is the forming of bubbles on the surface of the water jacket. When the bubbles pop it causes pits in the water jacket. More noticeable in turbo engines. IT can change the coolant color, especially if you're running water as coolant (lots of rust). Many detailed explanations on the web. There are supplemental coolant additives that help this problem and test strips at most truck stops, Amazon, etc.
    You can test for exhaust gases in the coolant if you think it's a head gasket problem. Kit should be available at any truck stop or auto parts store. I'd do a coolant test first and maybe do a compression test before removing the head.

    Job depends on how good a mechanic and reader you are. 50 years ago I was a trained aircraft radial engine mechanic. I had 2 DD 671s that needed a rebuild and a shop manual. It worked out fine for me. I've done dozens since. If you're only changing the head gasket, you can leave the injectors, rockers, etc., installed and remove as one piece with the head. After the new head gasket is on and the head torqued you'll need to adjust the rockers and the governor. Most manuals of any age will have the info for removing the head. Sometimes a shop manual for your engine is on eBay.

  17. #17
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    Default Re: 6-71 engines bubbling

    The 71 doesn't have what is referred to as a "head gasket". Each individual cylinder is sealed at the head by an individual compression seal. Water and oil transitioning to and from the head is contained by individual seals at passageway interfaces. There is a large perimeter seal at the edge of the block to mainly keep the environment out. If one of the jacket seals leaks there is no path to the combustion chamber.

    Each injector sits in a formed copper tube whose ends are swaged to the head. The tube is water cooled and thus sealed. That would be the area that combustion gasses could enter the cooling system.

    It is fairly common for the tubes to crack or otherwise lose their seal to the head and have combustion enter the coolant and coolant to enter the engine.

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