"Chris Hogg" wrote in message
...
On Thu, 23 Nov 2017 17:41:25 -0000, Terry Casey
wrote:

I saw a report somewhere -
might even have been here - from someone who did a lot of
short trips which had destroyed the filter because the
exhaust frequently never gets hot enough for long enough to
ensure that all the water vapour is exhausted. Otherwise any
vapour remaining condenses back to water at the end of the
run which destroys the filter - and replacements are not
cheap!)

I went instead for a petrol Focus which I kept for five
years with no problems.

A few months later I was talking to another retired man who
had fallen into the diesel trap and paying heavily for the
mistake.

Hmm... I've not heard filters being destroyed for that reason. I
always understood that on short journeys the filter never got hot
enough to burn off the carbon it collected, so eventually became
choked. The actual filter medium is ceramic, so I wouldn't expect it
to be affected by moisture. Perhaps other components in there don't
like being wet. OTOH the early filters were based on the ceramic
cordierite, while I believe that some these days are made of silicon
carbide, which may behave differently.

Yes I had heard that the main problem with short runs is the build-up of
soot (carbon) which never gets burned off. When I first got a DPF error
warning on my car, the garage advised me to do a long motorway run or else a
shorter journey in third gear (to keep the engine revs high and to heat up
the DPF more quickly) to see if that burned off the soot. Sadly it didn't,
hence the £1000 bill for new DPF and cat, though I consoled myself that this
was after some 150,000 miles so DPF and cat had had a good life.

Although I did quite a few short runs, I also used the car at weekends for
long (50-100 mile) journeys on country lanes at 40-60, so the DPF would have
been warmed up on those journeys. In between it got a bit of a blast if I
happened to go on the dual carriageway near me: it certainly didn't spend
its time pootling at 30 or doing stop-start in traffic.

My car also has a "plastic pouch" filled with a "liquid for cleaning the
exhaust" (as explained to me by the garage) - probably similar to AdBlue as
used in buses and coaches. About a year before the DPF error, this pouch
split, spilling its contents, which caused the car to enter limp-home mode.
Sod's Law was in operation that day because it was as I was accelerating
hard to pull out of a junction with poor visibility: to find the car lose
most of its power when an HGV is bearing down on you is not an experience I
want to repeat :-(