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I have a 2001 Executive with an ISM 500 in it. 26K miles on it. I bought it with the red warning light on. It drove ok but was obviously derating. I determined that the ambient air pressure sensor was bad and replaced it which shut off the warning light. I also replaced the temp sensor and the manifold pressure sensor. Today I took it for a shake down ride and the performance is notable improved. I noted that the boost pressure was pinning the 30 lbs gauge when I accelerated hard. I never noticed the boost pressure going past 25 before the sensor changes. Question, is it possible that I'm seeing an overboost condition? I know I have a VGT but I'm not sure how to diagnose turbo problems. I'm concerned that I'll trash the engine if I don't look further. Thanks in advance for your time and advice.

Edited by JohnnyDiesel
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The question is what exactly is the boost?  Most of the ISM and ISX engines will pin a 30# gauge.  But they are running only about 35psi.  

The ECU should program the fuel according to the air density and boost so it shouldn't hurt anything.  If you want to play it safe install an EGT (exhaust gas temperature) gauge.  

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