I have a 2000 bonneville SSEI and having a problem with boost. When the car is cold the boost works corectly but after Im on the road for 60+ miles the boost will only add 3-4 psi. I do have a catalytic converter that is going bad and am wondering if the added back pressure from that is causing the boost to be limited after it gets hot

It shouldn't, if anything it should be making more boost, because it would be backing up from the exhaust. I would get the cat fixed first though, and see where you stand.

How do you know your cat is bad? Have you done a compression check up and downstream of the cat?

General heat in the engine bay can have an effect on the way a boosted engine performs. A PCM that sees a hot engine might pull some timing back now allowing you to get full power. Too many variables to list.

You might want to scan for knock retard.

The cat rattles and has set a code. After it worms up it is for the most part quiet. After doing some more looking around I found 2 vacuume lines that were cracked and going today to hopefully find a replacement for them. I use the car for work and it gets alot of over the road time. It has 217,000 miles on it and still runs great with the exception of the lack of boost in the past 1-2 weeks.

After you replace the vacuum lines, make sure when you blip the throttle from under the hood that the boost bypass valve is moving. It should rest completely down and move significantly when you give it a quick WOT. Also make sure it is moving freely with your hand.

You can do the same check by pulling the vacuum hose off the BCS and plugging the end of the hose with your finger.

Hey guys whn I start my car my boost is like a quarter of the way up ...as it warms up thru out the day it stabilizes a little better but still never lays all the way down at idle like it should....I had the throttle pulled of and cleaned and replaced the cat....no vacuum leaks tht I know of so why won't my need lay flat at idle a quarter is a lot at start...

When the car is not running, the arm on the boost control actuator should be fully extended. When you start the car, the arm "pulls up" into the actuator. Under WOT the arm will again extend (to the position it is in when the car is not running) to open the */C gate valve. In other words, when the arm is retracted into the actuator you have no boost. When the arm is extended out of the actuator you have full boost. High vacuum (idle) no boost, low vacuum (engine off, or WOT) full boost.

One test you might want to try is to remove and plug the vacuum lines gong in to and out of the actuator (this will mean the actuator will not move from the fully extended position it is in when the car is not running) and take the car for a drive. This will give you full boost at all times, regardless of throttle position. If you notice a difference in boost, your problem is a bad bcs or bca. Do not drive the car like this for more than a quick test, or the gods of engine damage will pay you a visit.

unless he is referring to the boost gauge...