Odd knocking noise in cabin (‘98 940 Estate)

[AllHailKingVolvo]

Hi all,
I have a 940 Estate, 2.3lpt, which is a peach of a car but has an odd and irritating cabin noise at idle.

It’s a dull sounding knocking which seems to emanate from the passenger’s side of the dash/footwell, rises and falls with engine revs but goes away around 2500rpm and disappears completely as soon as the engine is under load.

It only appears when the car is warm, at cold start it’s inaudible/not happening.

I use 15w40 mineral oil and change it every 5000 miles. The car is meticulously maintained and not thrashed-the engine is strong and smooth and the knock doesn’t affect anything performance wise.

I’m stumped as to the cause...noisy fuel pump perhaps? Clutch bearing/s? Although neither seem likely as the noise/tempo rises and falls with engine revs and doesn’t stop or change when I dip the clutch.

Any help/advice would be amazing!

Ta,
Phil

[Laird Scooby]

It sounds like a dodgy big end from your description. However it could also be a broken engine mount although that wouldn't be quite so rhythmic.

Worth checking all your engine mounts and using a stethoscope to see if you can pinpoint the noise - automotive stethoscopes generally have long probes so you don't have to do the "Doctor thing" :

https://www.ebay.co.uk/itm/Automotiv...4AAOSwAFBZuTYt



That one has probe extensions to make life easier. Be careful round the belts with the engine running but it should help you pinpoint the area at least.

[bob12]

Does it come with a white coat and face mask. Sorry Dave couldn't resist that having been used to using the nearest screw driver or a lump of wood.

[aardvarkash10]

I'd be looking for a cable or something similar that is pulsing or rocking slightly and tapping against the underside of the vehicle. If it had one, I'd say speedo cable but for obvious reasons it won't be that...

Sure its not a passenger gently tapping thier foot in disapproval at your driving style? ;-)

[Laird Scooby]

Originally Posted by bob12

Does it come with a white coat and face mask.

No Bob, the Volvo can't identify you so no need to hide your identity and the white coat isn't necessary as it's not leaking oil.

I would have suggested a long screwdriver (as like you i'm more used to using that) but given that i suspect it's a big end as it only does it when hot, if the screwdriver slipped it could get flicked by the belts a bit harder into the lug'ole than you really want!

Ash - i had similar thoughts until i saw the bit about engine temperature, that points to a bearing and/or oil pressure related problem, hence the big end as they tend to go quiet under load.

[AllHailKingVolvo]

Thanks all! I’ll get sleuthing. I live 2 miles from my local Volvo dealer so if I’m stumped they will do a diagnostic check for £47. The big end would be a massive pain in the arse, the car does use a bit of oil so I wonder if the previous owner ran it low on oil and spun the bearings.

I take it I’d need new shells and a crank regrind to sort the big end out, what sort of money would this cost me? I’m not reluctant to sink some wedge into the car if it’s going to sort it, I plan to run it for as long as poss as it’s virtually mint otherwise.

[Laird Scooby]

Originally Posted by AllHailKingVolvo

I take it I’d need new shells and a crank regrind to sort the big end out, what sort of money would this cost me?

That's one of those "piece of string" questions. It all depends how bad it is. As it seems to be a problem only when the engine is warm (and therefore the oil is thinner = lower pressure) that suggests to me you'd probably get away with a new set of shells and hand polishing the crank journals. In fact, you may even find if you clean the breathers out and do an oil and filter change that the problem disappears.

However, if it is the worst case, then yes, you may well need a crank regrind, undersize bearings and so on. As the crank is going to be out, it would make sense to fit new main bearings as well and it would be a wise move to fit a new oil pump as well. After all, something will have caused the big end to go in the first place. Whether the oil pump is past its best and that has caused it, or a previous owner ran low on oil, a previous garage did some "paper servicing" regarding the oil filter (wiped the old one clean instead of renewing it but still charged for a new one) which could have caused a blockage in the filter resulting in low oil pressure or what, it's going to be impossible to find out.

The bug-bear is going to be the labour charge on all of this. Whether it's just the big ends or big ends, mains, new oil pump etc, the sump has to come off either way.
You may well find it's more cost-effective to source and fit a secondhand engine, do an oil and filter change and new cam belt, new coolant and call it good.

You won't know until you find the cause of the knocking. I've suggested the big ends as that fits your description most closely IMHO but i could well be wrong.

It could be as Ash suggests, something making unintentional contact with something else, perhaps a loose exhaust mount (had that on my first 740, the bracket from the exhaust to bellhousing had broken causing a knocking at certain times but not under load) or a dodgy viscous fan clutch causing a knock, loose/broken engine mount - the list goes on!

If you do get stumped and take it to the Volvo dealer for a "Diagnostic Check" make sure what they're offering isn't just hooking it up to the computer to see what fault codes are present on the engine management ECU which won't reveal a big end problem. You may have to book it in for "Investigate knocking at idle when warm" which would almost certainly be on the basis of "it takes as long as it takes" but is often a minimum of an hours labour.
Many main dealers round my neck of the woods charge £80-100 an hour plus the dreaded so if you're on a budget make sure the cost of diagnosing doesn't exceed the cost of fitting a replacement engine.

Let's hope it's something simple, cheap and easy to fix!

[AllHailKingVolvo]

Just a quick update on this-I still haven’t had a chance to get the car in for diagnostics but today I cleaned the throttle body (which was filthy), oil separator tank and all associated breather tubes and it drives like a different car! Much smoother and seems to have a lot more grunt for passing on the dual carriageway etc, even with five people in it as there were this afternoon.

The oil separator tank was gunky, and the little flame trap union thing that attaches the pipe from the top of the sep tank to the vacuum nozzle on the intake manifold was thick with crud. In fact, the smaller spigot that the vacuum hose attaches to was blocked solid and the hose was split, so I sorted all that out and put it back together, and it’s a much nicer car to drive. Pulls away smoothly with no judder now, and really goes well.

The knock/thrumming sound is still there (my eldest daughter summed it up well when she said it sounds like “a faraway helicopter”) but is perhaps a touch less pronounced than before. I’ve just chucked a bottle of injector cleaner in for good measure and will see how it runs after that has gone through.

I’m hoping that today’s work will help stop the oil leak which has been bugging me for ages. If not, it’ll go in for a full diagnostic check and hopefully we’ll have a solution.

Thanks again for your help, all!

[Laird Scooby]

It's surprising how much difference cleaning out the PCV system makes as you've found out!

Give it another thorough clean in about a months time - you'll be amazed how filthy it's got in that time! After that, check it after a month then another month and you should be ok for quite a while. I've found when the system gets really dirty, after the first thorough clean, about a month later it needs cleaning again.

You might find the oil leak is cured now as well, adding some synthetic ATF to the engine oil will probably help on that score as well as it contains an additive that revives and conditions seals. The reason the PCV system being blocked can cause leaks is because it causes the crankcase to pressurise with air that should go elsewhere - firstly this reduces the oil pressure in general and second causes the seals to leak.

Just had another thought about the knocking noise - your daughters description put me onto this idea, could well be the propshaft centre bearing. Mine was very "thrummy" and sounded like i had an Osprey (or Chinook) following me at times.

[TheLeeds]

Could also be the gearbox mount. My wentworth started making a noise which I at first thought was the prop shaft centre bearing support.
I noticed the gearbox mounts were only about £8 on ebay, so I tried that first and the noise stopped. It's a bit difficult to do unless you put the car up on a proper ramp. I had some of those drive up ones, and it was only just far enough off the ground to get to it.

Don't forget to support the back of the gearbox with an axle stand or a jack before starting.