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Boost strangeness...
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Posted by Eric van Spelde [Email] (more from Eric van Spelde) on Tue, 13 Nov 2001 12:09:13 Share Post by Email
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Hi all,

Decided to take Saabine for a spin to investigate her boost problems
a bit further. I am running a stock T16 APC and stock wastegate
actuator set on the Saab specified basic boost level, as I want this
to be sorted before I tweak the APC box to the former 20+ psi (1.5
bar) level again.

The problem is, that after I warmed the car up a bit and give a bit
more throttle for the first time, max boost as determined by APC
always comes up quickly. But after a quarter of an hour or so of
further driving, when I want to overtake or something there's only a
slight bit more than basic boost until ~4,500 rpm, then the needle
goes to max boost. If this happens in second or third gear and I
shift up, I'll get max boost right away in the higher gear,but by the
time I'm in fifth, chances are it will tail off to near basic boost
again. Stupid thing is - the set up is the same as it was when it
would happily go one or two mm _beyond_ the red zone of the APC gauge
and stay at like 7/8th of the red for as long as I had the nerve to
(talk about wheel-spinning grunt - even the clutch seemed to let go
for a split second once when full boost kicked in, in 5th).
Intercooler fan on or off didn't seem to make any difference, and
even the water injection/5th fuel injector system seems to work -
although it presumably only kicks in at about the orange/red zone
border or even a wee bit beyond (can't tell exactly - as my attempts
to set the trip in point for the system with a small compressor and
manometer failed I calculated the boost value where the system comes
in by caluclating the difference between vacuum at idle and the
maximum setting of the pressure switch, dividing it between the # of
turns at said switch it constituted and counted back accordingly).
Anyways, I had troubleshooted the whole system when stationary and
found the water line to the water injector to be wet after a drive
after pulling it, so I'll take that asa confirmation that it works.

APC box, solenoid, knock sensor, turbo including wastegate and
actuator and coincidentally even the engine have been swapped at
least once in the period that I've experienced intermittent boost,
which coincided with the first time the engine came out. Talked to
Glenn at Trent Saab and he suggested swapping the APC solenoid again
for a third example. So I did. No dice.

Until today I was looking at this as an APC failure, but then after
the sloenoid swap and some extended driving I was gently tootling
along at an indicated ~100 mph with boost gauge needle still in the
athmospheric zone and engine happily spinning, when the needle
suddendly surged slightly into the yellow and back and the car
momentarily started to accelerate without me doing anything!! WTF?

I remember a friend of mine from the USA having had the same 'surging
vacuum' syndrome and nobody could tell what it was. If she happened
tho chase the troll responsible for this out of her car and now I'm
the one having it (him?), I'm not happy at all! ;-)
So I started investigating possible vacuum leaks and found nothing -
only I was hearing a sound like a chattering membrane coming from the
front of the car sometimes when accelerating or driving at 100-110
mph. At idle, I could hear the (athmospheric) turbo dump valve
hissing a bit, so after talking to Graham ('yes, the valves can stick
when they get oily over time') I decided to eliminate this possible
cause by substituting the Big Fat Intercooler Pipe which runs from
the turbo to the IC which carries the dump valve with a not-so-big-
and-fat one withhout the provision for a dump valve. And no, I did
not forget to rempve and plug the vacuum line going from the other
end of the valve to the intake manifold! Same result.

Only thing is, even without the dump valve there is a very
distinct 'pfffft' sound on shifting when under boost, the exact sound
I always thought it was the dump valve and wondered why it sounds
differently than on the 8 valve set up!! Now I'm truly stumped.

The other thing is, that Saabine feels pretty fast even though she
runs as an lpt half the time and as a regular Turbo! the other half.
I don't remember ever having had lurid wheelspin in the dry in second
at a point that the turbo boost gauge needle just moves to about
halfway the yellow zone -and no, I don't think I've got less grip
with the 16" Pirellis than I had before, considering the cornering
speeds I can see on them! It's ballistic - much faster that the
Silver Arrow which consistently points its boost gauge needle to the
yellow-red border on each throttle application and stays at about two
millimetres before the red. And the engine idles & runs silky smooth -
so at least I don't think I have a problem there... :-)

So, anyone got any ideas?

Thanks in advance,

Eric & Saabine



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