Detroit Diesel (DD671N) Questions
 

Wayne.B wrote in
:

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?

Nope. The way to tell if the compression is good is expressed
by:

"Will it start?"

If it starts on all cylinders...compression is great!

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.

No valves. Intake ports and exhaust ports open as the piston
passes over them near BDC when the blower recharges the air and
blows out the dead gases. Piston rises cutting that off,
approaches TDC, cam rack in the head opens injection just as
compressed air reaches 1200F and KAPOW!, we're on our way down
again after this god-awful loud knocking noise also indicating
compression is great, making enginemen smile...(c;



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?

Worn rings, cracked cylinder head, cracked cylinder sleeve
usually starting at the port opening. Look at the exhaust. If
it starts steaming after it warms up, the head or block is
cracked allowing pressurized water into the cylinder, which turns
to steam on the hot piston. It always starts cold with white
smoke from incomplete combustion. 2-stroke DDs aren't
"environment friendly"....but owner friendly. Shrimp boats
running 4-53s may not have any rings at all and they're still
shrimping...(c;

You can tell when the rings are "too worn". It starts running
away when lube oil from the crankcase splashes past the worn
rings that are supposed to be wiping it off the cylinder walls
before the big bang....and the 2-stroker just runs away wild on
its own lube oil! Most exciting...black smoke for miles behind
boat...shutting down injection pump makes it go faster! Only way
to stop it is shut off its air supply by plugging up the intake,
which is very exciting on an engine that may explode in shrapnel
any second....(c;

But don't let that prospect bother you putting off that ring job
another year....

At least it'll never have a turbo-charger fire....

Aboard USS Everglades (AD-24), a nice old 6-71 powered a big DC
generator putting out 440VDC for the after gun turret from 1952
that had long since been removed for the stupid DASH helo deck.
(We were better off with the gun because at least it worked.)
The calibration laboratory I worked in needed a stable AC power
supply, even when the ship's AC was awful, so a DC to AC motor-
alternator set, 50KVA was installed and wired into the huge,
black bakelite DC power panel with the huge knife switches no
safety bureaucrat would ever approve of. (The lab actually ran
its lighting off a secondary 110VDC, NOISELESS, winding on the
genset.) The 6-71 drove the AC M-G set to 1800 RPM (60 Hz - 4
poles) any time we weren't parked against Pier Papa in
Charleston. (We used to turn back the speed on the M-G set to 50
Hz so the TV didn't wobble in the Med, under threats of "YOU FIX
IT IF YOU BREAK IT" from the Chief Electrician..(c; That 6-71
had over 12,000 hours on its meter on the panel and would almost
crank by hand! They are really great engines...

Oh, diesel fuel? No problemo. It sucked fuel oil from a 90,000
gallon tank.

Detroit Diesel (DD671N) Questions
 

"Wayne.B" wrote in message
...
On Tue, 24 Jan 2006 18:59:06 -0600, dazed and confuzzed
wrote:

look for a leak in
your supply lines.

Any advice on how to do that?



Thinking about it, why not just fire up the hard starting engine then, using
a small container of fuel and an acid brush or similar, lightly wet each
fitting on the fuel lines from your Racors forward to the last fitting you
can get to on the engine. Watch for *suction*. Sort of the opposite of a
pressure bubble test for leaks. I don't know if this would work, but it
might. Or, talk to someone who, unlike me, knows what he is talking about.
:-)

It seems logical though that if that engine is otherwise running fine, you
are getting your RPMs and you are not producing smoke more than the other
engine, that a compression problem due to rings or valves is not likely.

Eisboch

Detroit Diesel (DD671N) Questions
 

On Tue, 24 Jan 2006 20:50:45 -0500, "Eisboch" wrote:

It seems logical though that if that engine is otherwise running fine, you
are getting your RPMs and you are not producing smoke more than the other
engine, that a compression problem due to rings or valves is not likely.

That would be the better out come for sure.

Detroit Diesel (DD671N) Questions
 

On Tue, 24 Jan 2006 20:34:46 -0500, Larry wrote:

No valves. Intake ports and exhaust ports open as the piston
passes over them near BDC when the blower recharges the air and
blows out the dead gases.

That's what I thought but everyone else says there are exhaust valves
in the head.

Detroit Diesel (DD671N) Questions
 

Wayne.B wrote:
On Tue, 24 Jan 2006 20:34:46 -0500, Larry wrote:


No valves. Intake ports and exhaust ports open as the piston
passes over them near BDC when the blower recharges the air and
blows out the dead gases.


That's what I thought but everyone else says there are exhaust valves
in the head.

There are valves.
Check:
http://www.yachtsurvey.com/comparing_diesel_types.htm

Courtesy of Detroit diesel!

Gaz

Detroit Diesel (DD671N) Questions
 

Gary wrote in news:C1EBf.325410$2k.236563@pd7tw1no:

There are valves.
Check:
http://www.yachtsurvey.com/comparing_diesel_types.htm

Courtesy of Detroit diesel!



He's right....Sorry.

Detroit Diesel (DD671N) Questions
 

Wow!! This goes on and on.

As most everyone has said the most likely culprit is fuel system related.
In my experience air leaks are very rare except when the engines have been
recently serviced. If this started after servicing the fuel filters an air
leak is very probable.

Injector tips are another possibility. BTW, injector timing is critical to
a DDA running well. You should probably invest the few dollars for a timing
gage. If you own the engines for a while you'll need one.

These engines are so simple and easy to work on that you would do well to
take a course on them. You could learn everything you'd ever need to know
in about a 40 hour course. That would include a complete teardown and
reassembly phase with a tune up. IMO, these are wonderful engines for the
DIY inclined owner.

Butch
"Larry" wrote in message
...
Gary wrote in
news:C1EBf.325410$2k.236563@pd7tw1no:

There are valves.
Check:
http://www.yachtsurvey.com/comparing_diesel_types.htm

Courtesy of Detroit diesel!



He's right....Sorry.

Detroit Diesel (DD671N) Questions
 

Wayne,

The only way to check the compression is by removing the injector,
their are special tools for this. It's not a difficult job, but you do
have to reset the rack, which can be a little difficult if you've never
done it before and don't have the tools for it. By the way, two cycle
Detroit Diesel's have exhaust valves in the head (usually four valves)
and an intake port cut into the side of the cylinder (no intake
valves). The blower builds pressure in the intake manifold and when the
intake port is open that pressure, along with the open exhaust valves,
is what forces the exhause out.

Detroit Diesel (DD671N) Questions
 

In article ,
Larry wrote:

No valves. Intake ports and exhaust ports open as the piston
passes over them near BDC when the blower recharges the air and
blows out the dead gases. Piston rises cutting that off,
approaches TDC, cam rack in the head opens injection just as
compressed air reaches 1200F and KAPOW!, we're on our way down
again after this god-awful loud knocking noise also indicating
compression is great, making enginemen smile...(c;

Well it is obvious that Larry has never wrenched on a Scream'in
Jimmy of any kind, or he would have known, that they ALL have
Exhaust Valves, in the HEAD, under the VALVE COVER.... where the
Injectors ARE......


Me

Detroit Diesel (DD671N) Questions
 

Wayne,

1st: on a 671... no smoke usually means compression is fine. If
compression was bad, you would get smoke on startup until they warm. if
it warms and they still smoke badly... you need a rebuild fast... If it
runs after fuel is shut off or it runs away then you needed a rebuild
last year.

2nd: Do you have Racors? Is the fuel level in the tank ABOVE or BELOW
the injectors? If the answer is YES and BELOW then go buy a Racor
rebuild kit or at least buy the washer that sits over the metal ball.
That washer will disfigure over time causing the ball to not seat and
the fuel will drain back into the tank. Also make sure all other seals,
especially the top seals are in good shape (A rebuild kit comes with all
new seals)


3rd: Is the engine turning over the normal speed? If not, batteries may
be low. A slow turnover will cause an engine to slow start.

4th: DO your exhaust ports have the heat sheilds over them? If not,
you can use a temp gun to find a bad cylinder. If not, you can play with
the fuel rack with the cover off to find a bad cylinder. (Not
recommended unless you know what you are doing or have 6 fingers on your
right hand) BUT AGAIN... no smoke means you probably have 6 good
cylinders.

5th- Spend 90% of your time playing with the inbound fuel system. A few
hints: While the engine is running.. everything PAST the lift pump
(The first pump after the racor) is under PRESSURE. Check this area
first for leaks while the engine is running-higher speed the better.
Everything BEFOre the lift pump is under SUCTION. This is harder to
find because there will be no visible fuel most of the time. (See #2)
You can put some semi-viscous liquid around the seals and see if it is
sucked in but that can be difficult to see. (Spit, clean oil, etc)

Don's waste your money on a compression check ($500 +++) until you
have tried most everything else

Have fun... I am in a similar situation with one of our boats but we
have smoke and 1 bad cylinder.... kind of a smoking gun...

Ed
ed at Estrobel dot com




Wayne.B wrote:
On Tue, 24 Jan 2006 20:34:46 -0500, Larry wrote:


No valves. Intake ports and exhaust ports open as the piston
passes over them near BDC when the blower recharges the air and
blows out the dead gases.


That's what I thought but everyone else says there are exhaust valves
in the head.