940 Knock around the clock

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)

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

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. :-)

tap-tap-tap

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:

More info:

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