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Re: Rough Cold Idle
Posted by ray name (more from ray name) on Mon, 26 Aug 2002 21:53:59
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

> The engine on my 1985 900S (8V) runs very poorly for the first 2 or 3
> minutes after startup. The engine wants to die with any load, and it is only
> by feathering the pedal that I can keep it from dying. After 2 or 3 minutes,
> it runs fine.

You'll have to rule out more mundane faults, but your symptoms sound
just like a chronic problem I had with mine (also an '85 900S
8-valve).  Does it chug in a cyclic manner with period of about 4
seconds?

The cause for me was a design flaw that let the lambda system go
closed-loop before the engine was warm.  Apparently Saab also thought
there was a design deficiency because in the middle of the model year
they added a thermoswitch to force open-loop operation when the engine
is cold.  Mine must have been made before the change because its FI
wiring more closely resembles '84.  Maybe yours is similar.  My
solution was to add a thermoswitch just as Saab did.

In the CI Fuel system wiring diagram for 1985 on Bentley page 371-57,
the switch in question is item 99 at lower middle edge of page.
Compare to 1984 diagram on prior page - the switch is not present.
Also note the presence of a diode between the decel relay (138) and
the hot start relay (104), which is not present in the '85 diagram.  I
will come back to that later.

The lambda thermoswitch is intended to be in the thermostat housing.
But my thermostat housing only has one hole, fitted with a
thermoswitch to control the cold start injector (CSI).  It is
cold-closed/hot-open and in series with the thermal time switch (TTS)
in the head.  IMO it is unnecessary since the TTS can control the CSI
just fine on its own.  (I can't imagine a scenario when the head is
cold, thus closing the TTS, but the coolant in the thermostat housing
is hot, thus breaking the circuit.  And, FWIW, I have another k-jet
car, albeit non-Saab, that relies solely on the TTS to control the CSI
and it works fine.)  Therefore, what I did was to disconnect the
thermoswitch from the CSI circuit and use it for the lambda circuit.

So there was a lot of research but the fix boiled down to just 3
steps:

1) I disconnected the two green wires to the thermoswitch and bridged
them with a jumper wire.  This gives the TTS full authority over the
CSI.

2) I replaced the original thermoswitch with a red-top thermoswitch
which Saab offers as an upgrade.  The red-top opens at a higher temp
(110F) than the black one (77F), and will keep it open loop until
fully warm.

3) I ran two wires from the new thermoswitch over to the emissions
relay box.  I connected one to ground (I used pin 30 from the empty
'engine speed relay' socket, but any ground point will do).  I
connected the other to terminal X1 of the decel relay, which is wired
to the 'open loop' input of the lambda box.  (A yellow/red ('GL/RD')
wire joins X1 to the inline diode, so I unplugged the diode and
inserted a 'Y' with insulated crimp terminals and used the free leg to
connect the thermoswitch.)


When the car is cold, the thermoswitch will be closed, grounding
terminal X1 and telling the lambda system to stay open loop.  When the
coolant reaches 110F, it will switch to closed loop.

After making the mod, the car has run fine for almost 2 years.  And
there are never problems starting it, so the modified CSI control is
fine.

Before I made these changes, I talked to several 'experts' who
basically said, 'It shouldn't need that if it didn't come from the
factory that way.'  Sorry, that's B.S.  When the engine is cold, the
control-pressure regulator enriches the mixture, and if it
goes closed-loop the lambda system will attempt to lean it out -
exactly what you don't want it to do.  Saab didn't change the design
just for fun, there was a need.  I can't explain for sure how or why
the car *ever* ran well when cold (presumably it did when brand
new... but as long as we have owned it, it never did - until after the
fix).  My only guess is that maybe the original oxygen sensor took
much longer to heat up.  Until it heats up and produces voltage, the
lambda system has no info so it stays open loop.

Ray

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