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?

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.

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

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.

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?

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.

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!)

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.

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

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