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So, I have a 91 9kt 5spd with 163000 miles. Sometimes when I get on my car, it likes to stall sometimes, idle fluctuates a little.Plus when i first drive my car it runs like a champ. Usually once it warms it runs like crap to say the least, not missing, just lacking power. Any help would be greatly appreciated.
 

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First thing is to clean the throttle plate with FI throttle plate cleaner. Check carefully for vacuum leaks in any vacuum hose. Check the grey and white plastic PCV valve in the small diameter line from the cam cover to the throttle body. This blows only towards the throttle body and should block attempts to blow the other way. It blocks boost pressure from over pressuring the crankcase.Finally, your hooter valve might be leaking. This is the black plastic can shaped thingy on the air intake hose that allows boost to bypass from the compressor outlet directly back to the air intake when you lift off the gas. This prevents the boost pressure from bouncing off the closed throttle plate, which in extreme cases ould damage the compressor vanes. In most cases it just causes the engine to stumble as it reaches idle speed and air leaks into the intake through the hooter.
 

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It might affect turbo boost if the crack is in the cast iron manifold upstream of the turbo but it shouldn't cause the problems you describe.


A cracked manifold needs replacing though as the exhaust gas upstream of the catalytic converter is full of potentially lethal carbon monoxide gas and could make you ill.

In fact, I diagnosed a leaking exhaust manifold (the mounting studs break internally so the manifold comes loose but looks bolted on) with my portable CO detector I have in my house for safety reasons (natural gas furnaces sometimes leak CO when they get close to needing replacement). I stuck it next to the leaking manifld and it went nuts with beeping!
 

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I doubt it would make alot of difference. There's a huge amount of exhaust gas and if the crack were big enough to affect the turbo by much you'd be hearing some pretty loud noises from the crack.
 
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