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Re: 99 Boost
Posted by Stottler - http://saabnet.com/tsn/forms/e.php3?em= (more from Stottler - http://saabnet.com/tsn/forms/e.php3?em=) on Wed, 9 Apr 2003 22:45:26
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

In regard to your 99 turbo 'boost' problem, this is the type of deal where I
usually try to think my way through the entire system and check from the
'ground up' that each part is working properly. Otherwise you don't know if
you have multiple problems and you end up shooting in the dark.  You didn't
really say, so I'm going to assume a stock 99 Turbo fuel injection system;
sounds like you have a new fuel tank (cell) and pump, so that should rule
out some issues but maybe introduce some others...

1) Basic fuel supply:  Since you have a new tank, presumably you don't have
the age-old issue where everything is full of little flakes of rust.  Worth
checking the pump pickup/screen, though, to make sure it is clear and that
nothing is clogged or kinked.  You said you had a new pump, don't know if
it's stock or not, but you need to make sure it generates sufficient
pressure AND sufficient flow.  I'm too lazy to go dig the book out just now,
but the base fuel pressure is around 300kpa/3.0bar/45psig.  You also need to
check that the fuel return lines are clear and that the main pressure
regulator is giving you the proper pressure. The regulator is part of the
fuel distributor and rarely needs adjustment, so usually a problem with base
pressure indicates some other problem.  Another critical part is the fuel
filter - make sure you have a new one in there or you can get a flow
restriction.  Two other critical parts are the fuel accumulator (mounted on
the side of the tank in the stock set-up; where is it in your system?) and
the check valve in the pump.   So, assuming you have good basic pressure
(should be constant under any and all operating conditions, we can move on
to the next part of the system.  At this point we've checked out the pickup,
the pump, the check valve, the accumulator, the supply lines, the fuel
filter, the main regulator, and the return lines.

Basically, there's almost no way to work on one of these fuel systems
without the special pressure gauges, so you might start thinking about where
you can lay your hands on those.  I saw some on ebay the other day - if you
can't come up with them let me know; I've been known to lend them out.

2)Fuel distributor: This is a little of an aside, but you need to make sure
that the basic mechanical parts of the airflow sensing system are working
OK.  Often there are hoses, caps, etc. loose or missing that allow
unfiltered air to get to the underside of the airflow plate and arm bringing
in a lot of dirt which can bind things up.  I recommend taking the airflow
meter off the top of the air cleaner, turning it over (probably can do this
without disconnecting the fuel lines) and cleaning everything up real well
with carb cleaner or WD-40.  Make sure that the arm moves freely, the
metering rod moves freely in the bottom of the fuel distributor, that the
airflow plate is in the center of the venturi and not rubbing.  Put all back
together and make sure that all caps, plugs, PCV hoses, evaporative canister
hoses, etc. are in place and in good shape so that no wayward air is getting
in there.  Also, any air leaks between the flow sensor and the intake valves
will cause trouble, so check all your turbo air plumbing.

3) Fuel injectors: Sounds like you've checked these out and cleaned them;
usually problems will show up at idle.  I sometimes pull all 4 injectors
out, put them in a drain pan, and raise the airflow plate with the fuel pump
running (have to jumper the relay) and visually evaluate the spray pattern
and volume.  Officially, I discourage this, as it's a HUGE fire hazard.
Best done outdoors and with an assistant manning the fire extinguisher.  The
cold start injector has no function in a warm engine, but when the engine is
cold, the vacuum contact (black canister hanging on the radiator support)
briefly opens the cold start injector to provide a little extra enrichment
when you open the throttle.  Problems here are usually electrical in
nature - have to trace the circuit back through the thermo-time switch to
the starter.  Do check the vacuum hose, however.  While you're checking
hoses, check the vacuum advance hose to the distributor and the line to the
overpressure switch for the fuel pump.  Might be a good time to replace ALL
the vacuum hoses with new.

4) Control pressure loop:  Here's where it gets interesting.  The fuel
mixture is regulated by the 'Control Pressure'.  This pressure is against
the top of the fuel metering rod in the fuel distributor.  The lower the
control pressure (cold), the higher the metering rod can rise and the more
fuel flows to the injectors and the richer the mixture.  As the engine warms
up, the control pressure rises and the mixture gets leaner.  There's a great
diagram of how all this works on page 254-21 of the Saab service manual.
Often the control pressure regulator (aka warm-up regulator) goes south with
age; again you must have the special pressure guages to check that the
pressure is correct cold and then rises to the correct level as the engine
warms up.

5) Lambda system:  This is the electronic part of the system, and the Turbo
engine will basically run like crap if it is not working properly.  You will
need the 'Pulse Relation Meter' to confirm proper operation of this system.
The Lambda system consists of the Oxygen sensor in the exhaust manifold, the
Electronic Control Unit (mounted under the passenger seat), and the Lambda
valve mounted on the inner fender next to the fuel distributor.  The Lambda
valve varies the fuel pressure in the lower regulator chambers in the fuel
distributor to richer/enlean the mixture according to the reading of the
oxygen sensor.  Whenever the fuel pump is running (check this with ignition
on, engine off, and fuel pump relay jumpered) you should hear the Lambda
valve 'buzzing'.  Another important part of this system is the throttle
valve switch which is a micro switch which is mounted on the throttle body
and closes when the throttle is opened fully.  This forces the lambda system
to it's richest value when the throttle is wide open.  (hint, don't try to
force it there all the time or you will get lots of other problems).  Again,
verify this per the service manual with the Pulse Relation Meter.  I have
rarely seen the ECU or the Lambda valve go bad, but the wiring almost always
goes bad, and you will very likely need a new oxygen sensor.

That's pretty much it.  I haven't addressed the idle air loop (auxilliary
air valve) or the wastegate setting (but don't monkey with this unless the
whole fuel system is working flawlessly, and don't set it above 10psi unless
you really enjoy doing frequent valve jobs).  Likewise, I highly recommend
making sure that the overboost switch which cuts out the fuel pump is
working - if the wastegate diaphragm fails (did I say 'if'? I meant
'when'..) you will burn up pistons and valves in a hurry if the pump doesn't
cut out.  Another potential trouble spot is the air pre-heat valve which
regulates the temperature of the air coming into the air cleaner.  It tends
to fail in the 'closed' position so that you're always sucking warm air.
Likewise, make sure you have a clean air filter element.

If you are lucky and and need only an oxygen sensor and warm up regulator,
you should be able get everything working for under $300.  Round up the
service manual, the pulse relation meter, and the fuel pressure gauge and go
to town!  Probably more than you wanted to hear, but with a 25 year old
system there's not much else but to start at the beginning and work all the
way through.  Note that the fuel flow is controlled only by the airflow
sensor, the oxygen sensor, and the throttle switch - it is not effected by
the boost other than the increase in airflow, so you are really looking at a
fuel system problem, not a boost problem.

Good Luck!

Gary Stottler
'78 99 Turbo 203k miles
'64 96 GT850 98k miles

Posts in this Thread:
StateOfNine.com
SaabClub.com
Jak Stoll Performance
M Car Covers
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