2004 320 CDI: problem under hard acceleration

[IAmKloot]

We recently drove from Southern Spain to Gloucestershire in our (acquired for the purpose) W211 estate. Despite its relatively high mileage (215k), it turned the journey into a comparative delight and we are hugely impressed.

A couple of things did concern us and I was hoping the brains trust here might be able to help out...

Firstly, I discovered that when I accelerated hard (away from motorway toll booths, for example), the car would go smoothly until the change from 3rd to 4th (or possibly 2nd to 3rd?), when it would give a big "CLUNK" that felt like the transmission briefly locking up. As soon as it happened, I backed off and everything returned to normal with no apparent ongoing ill effects.
This has only happened about 3 times and only when I've had my foot flat to the floor; reasonably hard acceleration is fine and there are no other anomalies with the transmission.
Obviously I am avoiding a repeat if possible. Does anyone know what it might be?

The other issues are less serious: the parking brake foot pedal does not release when I pull the release handle and a slightly worrying burning plastic smell has started emanating from somewhere in or under the dash.

Any help would be most welcome!

 

[BlackC55]

When was the gearbox oil last changed?

Is the level ok?

I have done a "how to" on here if you want to DIY

The gearbox could be just worn, hence the poor changes.

The parking brake may need adjustment/repair.

Check the injectors for leakage. Take off the engine cover to have a look. I have done a "how to" on here for that also.

 

[IAmKloot]

Thanks, BlackC55, I don't know when the gearbox oil was last changed - if ever. The car came with service stamps (mainly MB) up to 189K miles, but with few details of work done.
Apart from flat-to-the-floor, the gearbox changes impeccably. If it is wear, I'll just have to treat the old girl with a bit of respect! I'll ask the gearbox oil question when the car next goes for a service.

I don't think the parking brake is serious; if I hook my foot under the pedal and raise it, the brake releases, so I think it's just the mechanism for application/release that is in need of adjustment, not the brake itself.

The burning smell is definitely more like plastic than oil, but it's worth a look...

 

[mornin clatter]

What about that smell ... does it appear only when the brake is not releasing? I would worry about that and i would make sure it is not something more serious like something electric. About the gearbox, check the oil level, pilot bush for leaks, and put it on a star. If nothing comes up then it is just old age,( so consider a good nursing home)and as BlackC55 said it is worn. Hope it is a simple fix like low ATF. Keep us informed, and good luck.