Advice: timing belt sprocket damaged twice

Re:Volvo 740 GL 1989 B230F engine:

I have had engine failure twice for the same problem: there is a
square key on the timing belt sprocket that engages the slot in the
crankshaft. First incident, I found it broke/sheared off.... so the
motor continues to turn, but the camshaft stops turning. I replaced
it with a (brand-new) sprocket from AUtohaus.com, and the car ran for
a few days, then failed when the key broke off the sprocket AGAIN. I
suspect I did not have the timing of camshaft set right. Power seems
down some, and the engine "pings" a lot with good, fresh gas.
I don't see how this is related to timing, but on the first crank
the rotation of the engine would often nearly stall, despite good
battery and starter (they're new).... then the engine would" catch"
and make a loud clattering sound. Once started, no abnormal sounds at
all except for ping under acceleration. If you make a second attempt
to start engine, then the rotation stalling/clattering noise did not
re-occur...it was only on initial start attempt.
I don't know what the clattering noise could be, as I know the
camshaft is non-interference-type.
Could doing something stupid like not timing the engine right put some
abnormal strain/or drag on the rotation of the camshaft (making it
resist rotation abnormally), exceeding what the sprocket key can
withstand? I don't believe it could be that the part was bad, I'm
probably doing something wrong that broke the sprocket key.
The new sprocket didn't break when I was starting, I had driven about
10 miles when it broke/engine died....happened when I was accelerating
from a stop.

Yes, I understand now that you really can't adjust the timing, you
just set the crankshaft position and camshaft position relative to
each other.

Ideas?

 

my guess is that there is something binding in the camshaft, perhaps a
failing bearing or something similar, and that puts excessive strain on the
key, causing it to shear. That key, in addition to indexing the position,
also serves the same purpose as a fuse in an electrical circuit... it's
there to sacrifice itself and prevent further damage in the event something
has gone wrong.....

 

I have exactly the same problem, and it's happened twice, EXACTLY as you
have written, down to every detail. This is happening to my recently
installed 91 K block in my 87 760 Ti wagon. It's a really bummer, that
engine was purring like a kitten before this. My mechanic here in Ottawa has
never seen this before, and he's an old school 25 yr Volvo mechanic. As
well, the guys who installed my engine in Southern Ontario have never seen
or heard of this, that is until a week ago, when one of their customers came
in with exactly the same issue.

My engine is under warranty from these guys, and I'll be heading down there
after Christmas to let them have a look and see what the problem is. I'll
post more on this as I have more info.

RS

 

How do you tighten the crankshaft bolt, at what torque?

Per Hauge

 

Not sure of the torque, I have a mechanic for that kind of thing. But he
says the previous installer of the timing belt may have used an air wrench
on it to tighten it up, because the threads were almost stripped right out.
He installed a new bolt, sprocket, backing plate and pulley. It's been fine
so far, but the engine itself sounds like a handful of nuts and bolts are in
the crank-case.

RS

 

Well, Perhaps the whole problem is that I am not torqueing the damper
pulley to proper spec? My haynes manual does not give the value for
the damper pulley bolt. Can anyone look that up and let me know?

 

According to the Bentley 240 manual, page 215-5, for the B240 motor,
tighten to 60Nm (44 ft.lbs) then an additional "1/6-turn (90 degrees)"
Huh...?

Now, 1/6 of a turn = 60 degrees [360/6]
1/4 of a turn = 90 degrees [360/4]

Maybe someone can explain or clarify.. it would not be the first
mistake I have found in Bentley.

__ __
Randy & \ \/ /alerie's
\__/olvos
'90 245 Estate - '93 965 Estate
"Shelby" & "Kate"

 

(See also the newer thread) - the 60 degree second stage is what Haynes
says.

MIke

 

Drove down to have the car looked at on Tuesday, just got back late last
night. Here's my report;

The ring gear had some pronounced scoring on it, so obviously it had worked
it's was loose. Also, while it wasn't showing any lose rivets or cracking,
my mechanics said it didn't "sound or feel right" when he tapped it. So he
installed another one, and the car works just fine now. Still no exact
reason WHY this would happen, it just did.

cheers

RS