96 940 Turbo overspin

My brother-in-law has come up with another problem with his car (having
sorted the oil pressure problem with a new pump, following crudy oil on
purchase). I have driven it a verified the problem.

After about 10 minutes hard driving the turbo seems to overspin on
application of any power and makes a loud high pitched wine combined with a
loss of power. All the pipes seem in good condition, turbo is a bit oily
with about 1mm of endfloat movement in the shaft (ie axially).

I don't know much about dump valves but this is where my suspicions lie. It
didn't sounds right (compared to my well sorted 95 940 Turbo, although it is
higher pressure/higher power). There wasn't the normal pssht after gear
change and there seemed to be delay after taking the foot off for the turbo
to spin down. Then if I applied power as the spin down was happening it
seemed to make the dump valve stick and after that the turbo was just
dumping air into the exhaust and would overspin when you put the foot down.

I don't understand is how this works. There is an electrical switch
connected to the pressure line which operates the dump valve (if thats the
thing connected to rigid pipes to the exhaust and inlet manifold), could
this be faulty, what controls it?

Also the lamda sensor light is always on now. Is this because of the
continous mixture problems?

Whats the most likely problem, dump valve, switch? Does it just need a
clean or should I replace it anyway?

 

I've come to the conclusion that the thing I thought was the dump valve
isn't. Where is the dump valve?

Volvo dealer parts guy thinks it is bolted to the side of the turbo.

 

The valve is bolted to the side of the compressor housing on the turbo.
It isn't a dump valve like the kids like to use - on that dumps the
pressure out where you can hear a great whoosh - it just dumps it back
into the compressor inlet. This valve is opened by vacuum in the intake
manifold.

First thing I'd do is read the code, and see what that problem is.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

I think we need to fix the turbo first before looking at the lamda light,
besides thats another thing I don't know how to do, but doesn't look too
difficult.

The dealer said it was just called a 'valve' and cost £150, so either I will
try to clean it or we'll put in a recon turbo (£300 from ECP). The Engine
has a history of sludge, blocked oil pump, oil consumption and the turbo
shaft has about 1mm of axial play, so it would make sense to change the
turbo anyway (if we can still get one). Also I need to check there isn't
sludge in any of the control pipes.

Does the valve come off easily, needs a gasket, or should I just go for the
recon turbo.

 

I had a B230ET I think and had turbo /dump valve problems.The dump
valve on mine was bolted to a metal bracket at the left front of the
head and the hose going to the intercooler was clamped to the other
side of the bracket.My problem was that the valve had come loose on the
bracket and the pressure had blown the joint out.Unless you are getting
a lot of oil in the intercooler or out the exhaust I would not worry
too much about the end float in the turbo.Do you have a pressure gauge
on the dash for the turbo?I also seem to remember that the dump valve
was vacuum controlled and that there was a non return valve in the line
that you had to be careful not to fit the wrong way round.
This might not relate to your engine at all but if it does I have a
manual somewhere.
regards,Mark.