A friend of mine bought a Saab 900 Turbo. I’m not sure of the year. Late-'80s/early-'90s? He says everything seems to run fine until he hits the gas hard enough to engage the turbochrger. Then ‘it bucks a bit, like someone doing a bad clutch job’. (Sounds like the stereotypical new manual-driver jearning how to use the clutch, rather than ‘clutch job’ as in a repair.) He says it goes away quickly, but if he were to bring the car to a stop right afterward (stop light/stop sign) it sometimes dies.
I’m guessing the turbocharger is malfunctioning, but I know nothing about fixing cars; so I thought I’d ask here to see if anyone can help my friend out. (I’m also guessing it’s an expensive thing to fix. Any numbers?)
When I first got it, my '91 Dodge Spirit R/T would occasionaly lurch a bit when you got on it. We tracked my problem down to being wheel hop. I solved it by removing the stock rubber-style motor mount and installing a solid-steel mount. There’s a little more vibration in the steering, but it’s barely noticeable. I’m sure there are other possible causes for this problem, but that was mine.
Several things come to mind. It could be wheel hop, but I have never seen that on a 900. Don’t forget that the car is 17 model years old, so a good inspection of all the mounts would be in order.
However the first and foremost thing to check is the waste gate actuator hose. Look at the fresh air side of the turbo. There will be a small hose coming off the side of the outlet (going the engine side). This hose will go one of two places. It will either go straight to the wastegate actuator (round can with a rod sticking out of it) or it will go to a valve and then to the waste gate actuator.
The single most likely problem is that hose is broken. The other likely causes are that the valve is bad and not closing, a previous owner turned the boost up too far, or the wastegate actuator is bad.
Here is the deal, and what is happening. A turbo doesn’t care how much boost it builds. If left uncontrolled it would continue to build boost until the engine broke.
So to control this, they use a wastegate. A wastegate bypasses some of the exhaust gas past the exhaust turbine, and prevents overboost. It is controlled by a wastegate actuator, which is a can with a diaphragm and a big spring. The actuator is connected to the output of the turbo, and when turbo pressure overcomes spring pressure, the wastegate opens and limits the boost.
The valve is for what Volvo called turbo plus, or SAAB called APC. The turbo is set low, and if everything is OK, at a particular RPM if all operating conditions are OK, the valve bleeds some pressure off of the wastegate and allows for extra performance. On the old red engine Volvos (200-700) this was a quick 30 hp boost.
Why the strange driving problems when the hose is broken? As a safety back up, the engine management system will either turn off the fuel injectors or fuel pump if the engine boost is too high to prevent damage to the engine in the event of a broken hose. So the symptoms is when you take off it goes like stink, falls flat on its face, goes like stink, falls flat on it face, lather, rinse, repeat.
