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#1
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Detroit Diesel (DD671N) Questions
Not having made any significant displays of ignorance lately that I
know of, thought I'd give it a try: Is there any way to measure compression without removing injectors or other major components on DD671s? Second question: Since the DD671 is a 2 cycle engine, I'm assuming there are no moving valves, just fixed intake and exhaust ports in the cylinder walls. Anyone know if that is correct? If so, I'm also assuming that any loss of compression would have to be from worn rings and/or cylinder bore. Also correct? |
#2
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
Wayne,
First Caveat: I've never worked on any Detroit Diesel engine (that I can remember) Wayne.B wrote: Is there any way to measure compression without removing injectors or other major components on DD671s? Don't know (shrug) Second question: Since the DD671 is a 2 cycle engine, I'm assuming there are no moving valves, just fixed intake and exhaust ports in the cylinder walls. There are exhaust valve/valves in each cylinder in a two-stroke turbo-diesel. See http://auto.howstuffworks.com/diesel-two-stroke1.htm Also see http://www.marineparts.com/partspages/DETROIT/DET12.HTM for a list of exhaust valve part numbers including for the 671. If so, I'm also assuming that any loss of compression would have to be from worn rings and/or cylinder bore. Also correct? See above. Loss of compression could be due to rings/cylinder, exhaust valve, etc. Good luck with it. Don W. |
#3
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
Wayne.B wrote:
Not having made any significant displays of ignorance lately that I know of, thought I'd give it a try: Is there any way to measure compression without removing injectors or other major components on DD671s? No not really & you need to use the correct tester of course, which can be pricey it's probably better to just get a tech to test. Second question: Since the DD671 is a 2 cycle engine, I'm assuming there are no moving valves, just fixed intake and exhaust ports in the cylinder walls. Anyone know if that is correct? No it still has an exhaust valve(s) in the head as usual. The air flow is from; the blower pressurised air box on the side through, tangential inlet ports,(uncovered as the piston approaches BDC like the 2 stroke OBs), incoming fresh air flows to help purge any remaining exhaust, the rising piston rises covering the inlet ports, the exhaust valve closes & compression up to fuel injection just before TDC, The power stroke is till towards the bottom when the exhaust valve opens & the exhaust starts to flow It's certainly a "2" stroke but it has most of the elements of a 4 stroke. This is why when I waaarr on endlessly about how bad 2 stroke OBs are, I'm usually careful to say "crankcase transferred 2 strokes" your diesel is not that. The crankcase is not "deliberately":-)) pressurised (not that you'd guess it given the way they throw oil) & the design is very successful. If so, I'm also assuming that any loss of compression would have to be from worn rings and/or cylinder bore. Also correct? The bottom part of the engine is pretty much the same as a 4 stroke, wet sump, pressure oiling etc so save some accident (air cleaner something broke??) you shouldn't expect the piston/ring/bore area to fail out of the blue. The usual suspects are always there, head, valves (exhaust only but they still get burnt). Your first instinct is probably the best a compression test; it should tell you whats happening or least give you some good clues. K |
#4
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
On Tue, 24 Jan 2006 22:40:54 GMT, Don W
wrote: See above. Loss of compression could be due to rings/cylinder, exhaust valve, etc. Good luck with it. Things are not at a critical state since they are both running well, but the port side engine is taking a few seconds longer to cold start than it used to, so I'm assuming that something is mildly amiss. |
#5
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
Wayne,
K answered your questions very well, but I'm curious why you think you have low compression, as that rarely happens on a detroit. I suspect an underlying story here. Secondly, that engine has been made for 70 years, there are many variations. Which one do you have? Please note there are many different cylinder heads as well. The older ones are 2 valve and the modern ones are 4 enhaust valves per cylinder. I think you should identify symtoms, if you want meaningful help. Steve "Wayne.B" wrote in message ... Not having made any significant displays of ignorance lately that I know of, thought I'd give it a try: Is there any way to measure compression without removing injectors or other major components on DD671s? Second question: Since the DD671 is a 2 cycle engine, I'm assuming there are no moving valves, just fixed intake and exhaust ports in the cylinder walls. Anyone know if that is correct? If so, I'm also assuming that any loss of compression would have to be from worn rings and/or cylinder bore. Also correct? |
#6
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
On Wed, 25 Jan 2006 00:31:55 +0100, "Steve Lusardi"
wrote: I think you should identify symtoms, if you want meaningful help. It's a 1981 Johnson & Towers marine conversion, supposedly rebuilt within the last 1,000 hours or so by the previous owner. My port side engine is taking longer to cold start than it used to, sometimes needing 8 to 12 seconds of cranking. It used to start on the first or second turn even when stone cold, and the starboard engine still does. If it has been run within a day or two it still starts very quickly. There is very little exhaust smoke at startup even when cold, and what little there is goes away quickly. The engine is running well and making full power as far as I can determine. |
#7
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
"Wayne.B" wrote in message ... On Wed, 25 Jan 2006 00:31:55 +0100, "Steve Lusardi" wrote: I think you should identify symtoms, if you want meaningful help. It's a 1981 Johnson & Towers marine conversion, supposedly rebuilt within the last 1,000 hours or so by the previous owner. My port side engine is taking longer to cold start than it used to, sometimes needing 8 to 12 seconds of cranking. It used to start on the first or second turn even when stone cold, and the starboard engine still does. If it has been run within a day or two it still starts very quickly. There is very little exhaust smoke at startup even when cold, and what little there is goes away quickly. The engine is running well and making full power as far as I can determine. There's a guy at our marina who had a somewhat similar problem for a couple of years. Turns out he had a very small air leak in the fuel supply line for one engine. It would run fine after starting, (I assume because the leak was overcome by fuel, but that is pure speculation), but was hard starting. Ain't boats fun? Eisboch (grandkid #5 is a girl!) |
#8
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
On Tue, 24 Jan 2006 19:17:09 -0500, "Eisboch" wrote:
Ain't boats fun? Eisboch (grandkid #5 is a girl!) Congratulations! We are still at #0 but both sons are engaged so that's a good start. Yes, small air leak is a possibility under consideration. This issue is fortunately not at a critical state yet but I like to stay on top of things so I don't get a failure when I'm in the boondocks somewhere. |
#9
posted to rec.boats,rec.boats.cruising
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Detroit Diesel (DD671N) Questions
"Wayne.B" wrote in message ... On Tue, 24 Jan 2006 19:17:09 -0500, "Eisboch" wrote: Ain't boats fun? Eisboch (grandkid #5 is a girl!) Congratulations! We are still at #0 but both sons are engaged so that's a good start. Yes, small air leak is a possibility under consideration. This issue is fortunately not at a critical state yet but I like to stay on top of things so I don't get a failure when I'm in the boondocks somewhere. Are your 671s turboed or normally aspirated? It's funny when you get to know how the various engines sound. I can identify the sound of those 671s anytime I hear them start up. A friend has a pair of normally aspirated 671s in a 1972 Hat 38 - actually a conversion from original gas engines. They have over close to 10,000 hours on them and run like a top. Great engines. Eisboch |
#10
posted to rec.boats,rec.boats.cruising,rec.boats.building
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Detroit Diesel (DD671N) Questions
Wayne.B wrote:
On Wed, 25 Jan 2006 00:31:55 +0100, "Steve Lusardi" wrote: I think you should identify symtoms, if you want meaningful help. It's a 1981 Johnson & Towers marine conversion, supposedly rebuilt within the last 1,000 hours or so by the previous owner. My port side engine is taking longer to cold start than it used to, sometimes needing 8 to 12 seconds of cranking. It used to start on the first or second turn even when stone cold, and the starboard engine still does. If it has been run within a day or two it still starts very quickly. There is very little exhaust smoke at startup even when cold, and what little there is goes away quickly. The engine is running well and making full power as far as I can determine. Change the fuel filter, and if that doesn't help, look for a leak in your supply lines. -- "A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences." - Proverbs 22:3 |
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