760 Turbo, unusual sound on acceleration

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.
eric

 

My bet is on the check valve for your vacuum line having gone bad.
Quick check: Try opening or closing the vents (floor, defroster). If
they don't work, that's it. Probably blew off one of the hoses while
under boost after the valve failed and that's what you're hearing.
Replace the check valve (on the inlet for #4 cylinder IIRC) and
re-attach the hose or hoses to the vent switches.

Bill

 

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.

Mike

 

Good tip on the hoses. I bought my first brick, an 89 740T a few months
ago and have been researching it since. My car needed lots of work and
I've picked up on all the Volvo "issues" for that particular model, the
hoses being one of them. A problem I've been dealing with has been gas
mileage in the 13-15 range. I've done all the usual stuff, tune up, new
ect, checked tires, suspension, brakes, etc all to no avail. Someone
mentioned checking the turbo hoses and sure enough just as you
mentioned the intake one between the amm and turbo had turned to mush
right where the hose connects from the oil trap. I temporarily duct
taped it to seal it off and hope this is the cause of my horrible gas
mileage. I thought it would cause lean running rather than rich but my
Volvo guru's say mileage would decrease because the O2 sensor would
detect a lean mixture and signal the ecu to dump more fuel. Makes
sense and I hope they are right. Strange thing is that after taping it
I now get the same type of noise you mention which wasn't there
before.Anyway what I am getting at is FCP and Eeuroparts has that hose
for something like $65.00 so you don't have to spend $200. You can also
get dealer type items for much cheaper and they ship international

 

What's actually happening is that the air is metered at the air mass
meter right at the airbox, then between the turbo and the engine some of
the air is escaping, but since the air meter is before the leak, the
computer injects enough fuel for the amount of air measured while not
realizing that a portion of that air is leaking out before it gets to
the engine hence the extra fuel.

 

hi - thanks to all of you so much.

exactly what's described below. I've neglected it and the
symptoms are now very severe.

so now i'm in a serious predicament. I have a really sweet woman
who needs to have me drive her around tomorrow starting at 8am.
she's going in for surgery and will be in the hospital for 3-4
months. (I am absolutely NOT kidding.)

I'm not the fastest on cars, but I follow directions well. does
anyone have - or pssibly supply - a link to a nice diagram of
these hoses? main thing is I don't want to mess with the 4th as
mentioned below.

if no pictures, any directions would be great appreciated. I'm
going online to look for something after I write this message.

thank you all again.
my best,
Eric

 

don't think it was clear in previous post. this is what is
happening:

their life expectancy. One of them may have a hole in it, and if
so it wil> l interfere with engine operation s> oon.

The turbo generates boost anytime the engine speed gets up in the
200> 0 rpm range or higher, with almost any throttle at all. The
boost is contro> lled 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 substantia> l amount of the air that has
already been metered by the AMM is lost, so th> e engine runs very
rich when it develops boost. The effect is unmistakable > -
everything is fine until the engine starts developing some power,
then t> he 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.

 

Just remove and inspect each of the rubber hose sections between the air
mass meter and the intake manifold, there's only about half a dozen
pieces, they connect with simple hose clamps so you should be able to
have them all removed from the car in 5-10 minutes if you take your time
and mark where each of them goes. If one has a hole in it you may be
able to squeeze a few days out of it (enough for a new hose to arrive)
with a liberal wrapping of electrical tape, just drive with a light foot.

 

I appreciate your reply.

rented a car for the girl, so now i'm off to work on my 760.
thank you for the instructions!

eric

 

I have had better success with making a "boot" from an aluminum can to go
around the hose right where the hole is and taping that in place - a
roadside repair to be sure. It seemed to provide some support. No doubt
about it, it's hard to hold pressure in a softened, oily hose!

Mike

 

Hey that's a darn good idea, I'll keep it in mind if I ever get a leak
out in the middle of nowhere.