Fitting Catch Can to L300

Just wondering if anyone can help - we've got a L300 Mitz that, when it's cold, can rev itself just about to oblivion, even if you turn off the key and run! Eventually stops - hasn't blown up yet but worried it's gonna. Has been to the garage and they can't find out why - they've had the whole engine apart and when reassembled, it still does it. One of the blokes there suggested fitting a catch can so excess oil can't get to the intake! Anyone had this problem and do you think it will fix it if we get one fitted! We really can't afford to replace the vehicle at the mo, so any help whould be appreciated. Many thanks :-) Anne

find another garage

Sounds like it's trying to diesel runaway. is the oil overfull! Also are the rings still good!

I'm guessing it's got a 2.5 turbo diesel engine!
1. Check turbo bearings and seals.
2. Check rockers and rocker shafts for wear.

only does it when cold start!

has to be oil pooling in the inlet manifold, or the inlet system, question would be where and why, the turbo is a common source of oil, as are guide seals - but guide seals are more prone to show these symptoms when hot than cold
vacuum lines to the inlet need checking too, you may have a leaking diaphram in the trans filling a pipe with oil after switch off, then feeding the engine raw oil on start, disconnect them one at a time and check for wet insides

I'd be guessing it would be smoking a bit when it does the rev to destruct attempts - what colour is the smoke - could even be hydraulic fluid from the booster - colour will suggest a potential area to look at

Thanks for your suggestions, Yes, only does it when cold. Garage has had the motor in bits - checked turbo bearings and seals - all good.
Think rings are ok, had head replaced as it was cracked when we got it. Head gasket has been done twice now due to it not being torqued down correctly (!)first time, Doesn't smoke usually but when attempting to self distruct smoke is black.
Adding to our vehicle dilemma, our Izuzu Irmsher blew it's engine on Sunday taking our caravan for a test drive. AARRGGHH!