Low boost issue 95 850
 

This is a regular issue that I've been trying to hunt down since I did the PCV last year. The car will only boost to around 5-6psi unless I get a heavy rain then it boosts normal. I ran a computer check and i said bad front O2 so i replaced that, but i still have the issue. I installed a A/F gauge and it ALWAYS reads rich, my MPG confirms that as i only get about 18mpg in town and 23hwy. after the O2 it said there was another issue that from what I've read is means a vacuum leak, so i smoked it and got nothing. Is it possible for the little pump thats attached to the vacuum tree to go bad and cause this? Could my low boost be caused by a bad relay? Any help would be great.

I'd think if the vacuum pump was bad, you'd notice it when starting out in the morning - one of its job is to build vacuum in the brake booster right after start up (otherwise you'd get a hard pedal until you've driving a 100 yards or so). If you've smoked the small vacuum lines, then next thing is to check for leaks in the large air pipes and the intercooler. Also there's an 90 degree tube under the drivers side of the radiator that can slip off and not be obvious when looking for leaks in the smaller tubes coming off the vacuum tree. Note that O2 sensor issues can tell the CPU to limit boost as well so if you have a code you need to clear it or fix what's going on. Some other people have complained that non-OEM O2 sensors can be problematic as well - not sure about that myself since most sensors use the same design. One trick people use when running tuned ECUs and bigger turbos is to install a "CEL BOSS" spacer to move the sensor out of the main airflow as a way to avoid the sensor limited their boost.