Unfixable Stalling 850 from H**L: Fixed!

I bought a beautiful, low mileage 1995 850 T-5R for a very good price because it had an
"unfixable" intermittent stalling problem

Here is a report of how it took almost six months to resolve

New Fuel Pump:
I started by putting in a new fuel pump, because it was the most common cause of this set of
symptoms, it had never been replaced before and it was 15 years old.

This car was a little behind in servicing, so I also threw a full tune-up at it.

I drove the car around town, to and from work, errands, and it went about 2 weeks before it
stalled again.

Troubleshooting:
Now I've installed a fuel pressure gauge and duct taped it to the windshield to I can see it while
driving

Volvo factory scan tools shows no stored Diagnostic Trouble Codes (DTC's). This is not unusual
or unexpected because in 1995 on-board diagnostic systems were somewhat crude, early stage
systems.

I installed a new air mass meter, because the unit appeared original. No real reason to think its
bad, but it might be the culprit. No luck, the car stalls again about a week later.

Next, Seriously "Throwing parts at it"
Installed new fuel pressure regulator, front and rear oxygen sensors, engine speed sensor

Test drove around the city, after about 2 weeks the car stalled again.

On the bright side, I've new begun to see a pattern. The stalling has only happened when
a) engine fully warmed up
b) the transmission is in drive, foot off the gas, coasting down a hill

I ordered and installed a new ignition coil. I don't think I've ever seen one fail on an 850, but not
too expensive, and I've got to try something.

After about 2 weeks the car stalls again, same conditions, coasting down a hill, warm engine.

One of my technicians now recalls that one time, years ago, he had experience with a defective,
brand new fuel pump. OK, worth a try, so ordered and installed another brand new fuel pump.

Weeks go by, stalled again, still not fixed.

Now, (sound of hand slapping forehead here) tech recalls that we have not yet changed the
camshaft position sensor, and its still the original unit. This revelation is a bit frustrating, as it is a
much more likely suspect for this sort of problem. So in goes a new unit.

Two more weeks of test driving, stalls again.

Now we discover several pieces of vacuum hose may have small leaks. As this car is
Turbocharged, only special heat resistant vacuum hose will do, and we order if from the Volvo
dealer. Special order part, means they don't sell it often. It arrives, we install as needed.

One week later, car stalls again.

Epiphany:

Another, more senior tech is consulted and suggest the main fuel injection control computer
might be the culprit. No one in the shop has ever seen one of these fail before, and common
wisdom has it that when they do fail, the car just won't run at all. Regardless, we order and
install a fuel injection control computer.

Interestingly, and very encouraging is the fact that the engine seems to start a tiny fraction of a
second faster now !

I test drive the car around the city for 2 more weeks, just looking for places where I can coast
downhill.

Oh, did I mention before that when the engine does stall, it won't start up right away? It will
crank over but not start, just like it ran out of gas. This means that I have to choose my test drive
very, very carefully. Don't want to stall on the Bay Bridge, or Market Street in rush hour, eh?

Two more weeks of test driving and I decided to re-install several of the original parts that were
probably OK from the beginning.

After two more weeks of daily driving and I now feel it really fixed.

Can you imagine just how difficult this would have been to sell this package of repairs to a
paying customer? They would have to have the patients of a Saint and the wallet of Bill Gates!

How about to a new customer who does not yet built up a relationship with the shop? I can
imagine that situation because I've been there, more than once. Intermittent problems are not
unique to any particular make or year or model. But they always pose a unique challenge for the
owner and the repair team.

In Hindsight / "Next time"

Many months have now gone by and the problem is resolved with certainty .

The fuel pump is still the place I'd start. We have seen old fuel pumps fail time and again. Never
a code and usually intermittent.

Changing the fuel control computer is simple (if you don't count the cost of parts). And maybe
some customers would be willing to accept the risks involved with test driving ( and getting
stuck).