D3 Horrendous rattle pulling caravan

[meridienmick]

I know there have been various discussions about rattling diesel engines, but nothing seems to cover this particular scenario.

Car is a 2012 XC70 D3 Manual with 40,000 miles and full main dealer service history. We have just come back from a caravan trip to France and mostly the car performed in its usual exemplary fashion, except we had two or three occasions when the engine produced a terrifying rattle - it was so sudden and intense as to cause us alarm when it happened. On each occasion I was working the engine quite hard, pulling the caravan up hill in 6th gear at about 60mph, which is about 1750 rpm. This engine delivers so much torque that changing down isn't normally necessary, so we just bimble along and ask for a bit more oomph when needed. As I say, on a few occasions we would then get a really loud and sudden diesel type rattle which stopped as soon as I eased off the accelerator a little.

Does this sound as if there is something wrong, or was I just being unkind to the engine? Just thought I'd try to get some ideas before commiting to expenditure with my friendly Volvo dealer. I have an extended warranty to the end of the year so if anything is up I want to get it sorted in the next few months.

Thanks all
Mick

[cheshired5]

Gearbox torque mounts have pretty lightweight bushes.
Normal use won't expose a weakness for many miles but I could see heavier loads working them more so have a look.

[Dancake]

Too high a gear for what you were doing if the engine started to vibrate, knock or rattle.

[5cilinder]

Indeed
That an engine can deliver the torque it doesnt mean its sensible to take a caravan uphill with high temperatures wich such a low rpm and high torque
Your not the first writing off an engine (bearings first) this way
You need to get the rpmś in to the 2500 range at least with such a load , at first the harmonic frequencies that amplify vibrations are less arround these revs , second both the oil and coolant pump rev higher so that the increased heat can be easier transported
Third the oilfilm will be more robust with higher flow and less hammering of the pistons

[martin_r_smith]

I have heard of this before and it was the dual mass flywheel vibrating. As said above using that much torque low in the revs is extremely bad for the engine and drive train and just poor mechanical sympathy on your part.

But just to rule it out take the caravan to the same spots it did it before and try and replicate in a more suitable gear, see if it still does it.

[meridienmick]

Thanks to all for the suggestions. I guess I was overcooking the engine, although I had naively assumed that all the time it could produce enough power and torque to increase speed in a given situation it was quite happy. If there were some dials and indicators such as engine temp and oil temp it might help give any early warning that all was not well. Very good point about higher revs equals faster pump speeds so more cooling. Unfortunately I can't go with the idea of repeating the exact exercise as I'm not in France any more, more's the pity!!

Mick

[cheshired5]

Originally Posted by meridienmick

If there were some dials and indicators such as engine temp and oil temp it might help give any early warning that all was not well.

You have a rev counter and your ears.
They should be enough.