I have a 1998 Volvo V70 with 68,000. A few weeks ago I started
getting the CHECK ENGINE warning. It comes on for a few days and goes
away for a few days. I checked the code on it and it was P0172 Engine
Too Rich.
I've been looking through the postings on this forum for similiar
problems. What I have read so far is that I either have a leak in a
vacuum hose or I have a faulty O2 sensor. I was wondering if the fact
that it comes and goes might not suggest that it is the sensor rather
than the hose.
Your comments are welcome.
I think I posted here or maybe on one of the brickboards about a
similar problem on a '99 V70 T5.
I was never told what the code was but the problem was fairly obvious
and was caused by the airbox to turbo hose left completely undone by
the mechanic on a top engine mount change. I am surprised that the CEL
goes way and comes back unless you disconnect the battery to
temporarily clear it.
Running too rich is caused by over-fuelling and although the cause can
be various, the most common cause appears to be an air leak around the
inlet path which fools the sensors into adding too much fuel which is
seen by the outlet sensors as way too rich. In my extreme case of
probably about 50% extra over-fuelling the symptoms were that the car
went like a bullet on steroids but wouldn't do more than 17MPG. The
soot build up was rapid and after a couple of hundred miles the black
smoke at tick over was a complete assurance of much too rich.
If yours is not a turbo there is frequent reference to a 90 degree
elbow which splits and that is a common cause. You are dealing with a
series of dependent factors which start with the air box and MAFF
sensor on through to feed air and vacuum pipes then to plugs\leads
exhaust and the O2 sensors.
My cause was obvious but the plugs were soot black (only a week old)
and were thrown away, the first O2 sensor was faulty which was as a
direct result off excessive soot contamination.
I have gone through this long winded explanation to try to highlight
the cascading domino effect that a small problem can have throughout
the system. The reason why is to caution against coming to a too rapid
conclusion and the logic is from an owner who had a catastrophic
sequence of repetitive failures. In that person's situation the fault
was throwing a code and sensor after sensor was changed, because they
were responsible for throwing the code but the eventual fault proved
to be the Electronic Throttle body. Not a cheap job but when you add
three sensors to the bill it was one heck of an ouch! bill.
The lesson I have learned from my two experiences and other people's
more catastrophic experiences is that a garage had be 100% sure that
if a fault caused the sensor to become faulty they had better be 100%
sure that it is cleared before they stick a new £300+ one in. I am
reliably informed that a bad over-fuelling fault can cause a brand new
sensor to be only fit for the scrap bin in 20 to 100 miles<s>.
Good luck with your investigation and hopefully it will be something
very low cost. BTW although mine is a '99 the same may be true of your
model... O2 sensors have been modified and the older ones cannot be
found anywhere... A Vadis software upgrade is needed for the only ones
available to work retrospectively. It means the prospect of a main
dealer fit and fix as the only option if a new sensor is required.
Liam
