940 Knock around the clock

Hello Thanks to all who offered advice. I checked all the exhaust shields
and other wondrous things under the system,
The knock is driving me crazy!! When warmed up it reduces and with no loss
of power or performance, The compression test is fine, Do I drive her "into
the ground" as Paul the Mech'says or do we have some old magic from the days
of White babbit metal which beside ticker oil will soften the
n-o-i-s-e-********* pardon me. I blame the wife she gets more pleasure with
her right foot then with hubby
Volvo 940 124k







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It could be your exhaust manifold gasket. When it develops a leak the exhaust
gases can produce a sharp knocking sound. At 124K that car has plenty of
life left in it.



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

 

Hello, Trevor:

The knock subsides when the engine warms up? Sounds like piston slap. The
B230F uses lightweight alloy pistons that expand slightly at operating
temperature. When the bore starts to show some wear the result is a less
than perfect fit when the piston is cold, and so the pistons rock in the
bores as they travel through their intake and compression strokes. Pistons
expand as the engine warms up, and it's gone.

Only way to correct this is to pull the motor, rebore the block, and install
oversized pistons. However, it's really more of an annoyance than an actual
problem. The motor will probably run like this for another couple hundred
thousand miles with no ill effects.

 

Our 85 has gone more years than I can count with piston slap. I'd describe
it as a deep rattle or continuous tapping sound rather than a tap-tap-tap
sound. As Scott says, the noise subsides to unnoticable in a couple of
miles.

Mike

 

I'm inspired by your confidence, I'm pretty new to the Volvo, I could never
afford this kind of vehicle when new. Now I have one of these Leviathan's I
would really like to look after her. The old guy at the filling station says
he remembers a product that (if it is a piston slap) would ease the noise
away, he describes it as a paste that is introduced into the oil and takes
up some of the"gap", He rambles on about re-boring the block in situ?
without having to remove the engine from the "straps" (mountings I think he
means) if the worst come to the worst I'll just suffer the annoying "knock".
It really is one of those noises which seems to get louder the more you
listen to it.
Thanks to Boris Scott and Michael for their responses.
Trevor Colluney

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Trevor,

I don't know what paste your service station guy is referring to, but I can
see where it might possibly block oil passages and cause more serious
problems. There was a product on the market in the mid 90's called "Slick
50" that was supposed to prolong engine life by coating the internal
components with a film of teflon. Turns out the Teflon was responsible for
blocking oil passages in some engines which resulted in seizure, thrown
rods, etc. If I remember correctly, DuPont actually sued the makers of Slick
50, stating that Teflon was never intended for use in an internal combustion
engine!

I don't know your guy, and I don't mean to sound "snobby", but sometimes an
independent Volvo garage (or at least one that specializes in European cars)
is a better choice for maintenance and repairs on these old cars. 240s,
740s and 940s have some problem areas that are specific to Volvos. Have
seen (and heard) of some 'horror stories' involving neighborhood mechanics
and Volvos.

That's a nice car, and low mileage. You should get a lot of life (and fun
driving time) out of yet.

 

Just for your information Slick 50 is still made and sold and still does
what it says on the packet. perhaps it wasn't the Teflon that clogged the
oilways but the crud that should have been removed before applying the
treatment. The "paste" goes back a lot further than Slick 50. It was
supposed to fill up the ring grooves to push the rings better into contact
with the cylinder walls, thus restoring compression. It actually did this
quite well and lasted about 10,000 miles per treatment. By then you would
have rebored and fitted new pistons as it was meant as a stop gap till you
could get it fixed. It came out in the late '50s or early '60s.

All the best, Peter.

700/900/90 Register Keeper,
Volvo Owners Club (UK).

 

I think the "paste" being referred to sounds like molybdenum
disulfide. I've seen it sold under the trade name "Moly-slip" (IIRC)
as a motor oil additive (I've even bought it too).

It will stick to pretty much everything inside the engine and tighten
up clearances- bearings and lifters included. I'm not really sure
how long it takes to wear off, about ten thousand miles seems like a
reasonable guess to me. A downside is it will also stick to the
cylinder walls, and smooth out those nice hone marks that are there
to help keep the oil from going past the rings. Among other things
it is a good cheap fix to quiet a noisy engine when trying to sell a
used car.

Slick 50 has been debated to death:

http://groups-beta.google.com/groups?hl=en&lr=&safe=off&q=slick+50&safe=off&qt_s=Search

More info:

http://www.gizoogle.com/index.php?hl=en&ul=&criterion=slick+50

 

Thanks to all who offered their wisdom, anecdotes and advice, I'll continue
to investigate a cure.
Trevor Colluney
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