APC problem, Saab 9000CS lpt Bucking under boost

There is a lot more to it than that...

Where is the boost gauge when the cut-out happens? Is it way up in the middle of the red? If so, that is where the overpressure switch is supposed to shutdown the engine by cutting spark. If it is low red or orange then the switch may be faulty. It is mounted on the right side firewall.

The turbo is easily capable of making too much boost pressure which has to be limited based on engine running conditions to prevent destroying the engine. The control box (APC) has the job of monitoring engine running conditions and sending the electrical signal to the APC solenoid. The APC solenoid's job is to modulate the vacuum going to the wategate control dashpot. The wastegate controls the boost pressure by dumping excess pressure when opened.

If you were to unplug the electrical connection to the APC solenoid it should route all of the vacuum to the wastegate dashpot and drop to "base boost" only, which should be near the middle of the center orange section of the boost gauge. This mode is actually quite driveable though you will have less maximum power.

If you still get too much boost then either the APC solenoid is stuck closed (actuated) or the wastegate is stuck closed (won't dump pressure).

I would suggest looking at this web site which goes through troubleshooting the APC system quite well, though it may be a bit too technical for the average handy-Joe. http://68.18.142.57/turbo_folder/baseboostdia.htm Good Luck,

-Fred W

Reply to
Fred W.
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I didn't think the 9000 LPT models had a boost gauge :-/

Reply to
tCowjww

Darn, you guys are right. I did not notice the lpt in the subject line. That will make it a bit tougher to troubleshoot at home, but all of the troubleshooting steps should be similar. Unfortunately, I have never owned an lpt so I can't speak from experience.

-Fred W

Reply to
Fred W.

Could be a sticking wastegate valve. You get overboost and the APC cuts the fuel to protect (or limit) the engine.

Reply to
Johannes H Andersen

On the LPT (Light Pressure Turbo), the turbo doesn't kick in. It's on all the time. It works similar to variable power steering. There's less boost at low speeds, more when you need it. But it's always there. My 1995 9000CS LPT has the smoothest power curve I've experienced on a car. Give it more gas and it simply goes faster very fast. I tested a 1997 9000 with regular turbo when I bought the 1995. The feel between the two is very different. That turbo you can feel kicking in. Unfortunately, I can't tell you how to fix yours -- but I'm sure someone else can.

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Reply to
LauraK

Sorry for dropping in on the thread so late. In my 89 non-DI 9000 I (and two previous owners) had exactly the same trouble - it turned out to be a basic ignition problem. Under boost a seal was leaking oil onto a spark lead and grounding it out momentarily so suddenly and quite violently - the engine management system would cut in assuming a knock. The previous owner had rebuilt the tranny and replaced the turbo outright to no avail and my shop only figured it out by accident as the same sort of grounding situation sometimes occurred on Buicks with a distributor head mounted sideways.

You are running DI in that unit I know - but perhaps you want to swap out your plugs and test for grounds. Check around for excess oil under the DI, see if the control wires are clean and tidy and have the DI tested. I'd also pull apart any wire couplings I could find right back to the APC checking for corrosion too.

It may very well be the wastegate and/or a problem with the battery/alternator and/or the APC computer - but I'd confirm the ignition system first given that the knock management system seems to kick in given almost any variation in firing.

Cheers and best wishes.

Reply to
Dexter J

Hi,

I have a 1995 LPT 2.0 CS, and this does exactly the same thing. Except it does not do it all the time. More recently I have found that it starts on 3 cylinders only, then after a period of a few seconds to 1 minute, it starts running normally. I am suspecting the 2 faults are connected, and are due to some grounding or connection problem.

I have been told the DI unit is the likely culprit, but it has made no difference to my Saab when it was changed.

But if you find a solution, please be sure to let us know.

Best Regards and Luck.

Andrew

Reply to
Andy

In your case, if the DI was changed and it still runs on 3 then 4 cyl upon startup, it may be a head gasket starting to go. My 900 did that before I replaced the head gasket. Does the coolant level drop slowly over time?

As for the original post, If the car accelerates very hard, it may be the sticking wastegate. Mine was an lpt, and it just feels like a 3l v6 or so. It does not feel like 'Wow, this thing is fast', it feels like 'This thing is adequate, no more'.

To sets the theory, you'll need some 1/4" hose and a hand pump (bike tire pump, soccer ball pump). disconnect the existing hose from the wastegate actuator and hook the new hose there. If you can suck on the hose and it is free, no resistance, the actuator is bad and must be replaced. Now, pressurize it with the pump. If it moves, it is probably fine. If not, remove the actuator arm from the wastegate arm and see if it will move now, under pressure. If so, the wastegate itself is stuck and needs to be freed? My bets are on the actuator.

Good luck,

KeithG

Andy wrote:

Reply to
KeithG

Hello, I have just bought a 1996 9000 CS eco. It's my first Saab and I think it's great.

It is running fine under normal conditions but when I go to overtake ;) the fuel cuts out as soon as the turbo kicks in. Someone has suggested that an APC Solenoid or control box has gone bad and is protecting the engine when it doesn't need to. The problem starts at around 3500 revs but only if I'm heavy footed.

Could anyone tell me a little more about these gadgets and how to test them and source them cheaply for replacement.

Thank you in advance, Anthony

Reply to
Anthony Cooke

You're right Fred, it doesn't have a boost gauge. It seems to cut out with a jolt as soon as I feel the extra power from the turbo spinning up. It feels like a rev limiter kicking in.

Reply to
Anthony Cooke

sticking wastegate valve.... Does anyone know how I would test it? I could spend a fortune fault finding (well, I could if I had a fortune)

I have to agree with Johannes, it does feel very smooth and powerful until the APC shuts me down. I know I'm going to enjoy my Saab.

Reply to
Anthony Cooke

Thanks Keith,

that is exactly the kind of advise I was after. Pipes and handpumps it is. I'm not adverse to spending money but I didn't want to have to replace the whole lot.I'll see if I can diagnose it using your method.

Anthony

Reply to
Anthony Cooke

I still think it is a sticking waste gate valve, but it is just one possibility. If you potter around within urban speed limits all the time, then the waste gate will rarely function, and it can jam. If it doesn't open when required at high boost, then the ECU or APC or whatever cuts off the full supply. It can be annoying when you're draw into the drivers seat and the power suddenly and unexpectedly disappears, then the spring loading of the seat catapults you forward on the windscreen. This is my theory anyway, it happened to my 1993

9000 lpt some years ago. But it has been so flawless for quite a while that I'm beginning to worry.
Reply to
Johannes H Andersen

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