Re: 760 Turbo, unusual sound on acceleration
"e4fx" <firstname.lastname@example.org> wrote in message
> Hi -
> I have a 1987 760Turbo.
> The last couple of weeks I heard a strange sound when accelerating.
> This happens during normal street driving and towards the middle to
> end of acceleration. sounds like air hissing as if being forced
> through a very small hole - like deflating a pool toy. slows down
> and dissappears as I reach speed. any ideas?
> thanks a lot. happy holidays.
If the turbo ducts (the large hoses) are original, they are at the end of
their life expectancy. One of them may have a hole in it, and if so it will
interfere with engine operation soon.
The turbo generates boost anytime the engine speed gets up in the 2000 rpm
range or higher, with almost any throttle at all. The boost is controlled by
the throttle body so it doesn't show up on the guage, but those ducts are
being pressurized. If a large enough hole is formed, a substantial amount of
the air that has already been metered by the AMM is lost, so the engine runs
very rich when it develops boost. The effect is unmistakable - everything is
fine until the engine starts developing some power, then the power suddenly
sags a lot and doesn't return until you back off on the throttle completely
a couple seconds... then you are back where you started.
I advise you to inspect all the ducts, but I have a caution about the one
from the AMM to the turbo inlet. That one tends to turn to mush at the
bottom, right where it connects to the turbo. Since it is on the inlet side
of the turbo, failure of that duct won't give you much trouble. The duct is
about $200 US from Volvo the last I heard (only paid $150 US for mine so
many years ago!), so if you aren't prepared to replace it you may not want
to disturb it.
For all the other ducts (except maybe that short piece of blue silicone
right on the turbo outlet, which is pretty tough), remove each one and
inspect inside and out for holes and tears. I had one with only a small hole
on the outside but it was torn end to end on the inside. The whole process
is an easy DIY with a screwdriver (and gloves, because they are oily at
least on the inside) and will take an hour if you are as slow as I am. If
any are torn they should be replaced ASAP, and you may consider replacing
all of them (except maybe the turbo inlet, depending on your money
situation). IIRC the ducts besides the inlet one totalled about $200 US, but
I bought most of them one at a time.