wrote this earlier ....
then saw that the other replies have it covered
now posting anyways .. :
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ok ..
your previous post mentioned stalling whilst going uphill
was it a case of wrong-gear-by-accident ?
normally .. the td4 has a ton of torque and does not stall easily ..
be as if .. one would have to deliberately try 'n stall it .. type-of-engine ..
i.e. trying to figure if 'stalling' .. 'n struggling to get over 55mph is connected
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now the symptom is not enough power to break 55mph easily ..
so .. is that in 5th gear .. ?
how does it perform if foot-down in 4th gear ? ..
does it start to pull harder when the revs go over 2500 ? .. .. or not ..
will it rev past 3000 ? ..
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and .. what's the exhaust smoke like ??
http://www.uniteddiesel.co.uk/diesel-engine-smoke.php
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suffice to say if it's struggling to pass 55mph ..
engine be out-of-tune .. short on fuel or air .. or both ..
or you have an extremely light foot ..
..

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yeah .. as mentioned .. post more symptoms as it will help narrow down potential causes ..