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1999 Grand Prix GTP exhaust

tomtom123

tomtom123

New member
I have a few questions. I know with many cars, if you removed the catalytic converter the car would freak out because the pcm wasn't reading what it is supposed to read. But I heard that the 1999 - 2000 Grand Prix GT could have the catalytic converter removed without tripping any codes. My question, does the 1999 Grand Prix GTP pcm also not trip codes when the exhaust is tampered with? For example, removing bends and backpressure often causes a Service Engine Light and messes with the cars performance and economy. Does anyone know more information about the 1999 GTP and if it is affected by removing/messing with the backpressure? I would like to take some of the bends out of the exhaust system and, if possible, use a bit bigger diameter piping. I am just wondering if doing that would trip a code or not. Thanks in advance for any answers =)
 


The factory cat back is of no concern to you. You will not outflow the pipe size untill roughly 350WHP or more. Your car now has maybe 200 WHP if stock.

The cat converter is not an issue either, that flows fine. Its the pipe size of the downpipe into the converter and the u-bend in the exhaust for the rear o2 sensor. Most people buy a 3in downpipe to remove both problems and give the car a small bump in power and help reduce KR.


So in short, back pressure is pretty much a myth with these cars and on 97-00 cars the cat converter will not matter as the vehicle does not use the rear o2 sensor to control anything. The sensor is only there to see if the cat is there or not. So if you remove the converter the light may come on but it means nothing.
 
Okay, thank you for the information. I have one other question. How high should my boost get stock? My boost gage, the stock one, only gets up to about half way. Is that about where it should be stock?
 
It should be able to fill the lego blocks at stock levels. You may have a vac leak. Check your vac lines.
 
A real vacuum/boost gauge would help alot more.

Its too hard to say if its the info center thats wrong or if indeed you have a vacuum leak or a plugged up exhaust system that happens as the car warms up.

I'd check all vacuum lines for cracks and such as its free and replace them as needed.
 


I'd be willing to bet its a vacuum leak, like most of the time the boost will build at max half way and sometimes bounce between half and half+1 block. My exhaust I can safely say should not be plugged since for the past week or so it has been disconnected from the catalytic converter which I am fixing today. I had a "wastegate solenoid" code come up once but it came on when the boost gage ended up reading max boost.
 
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Okay, I have checked and found no vacuum leak anywhere. The car still only builds between just under half and just over half boost.
 
I don't see how a "real" boost gauge is going to solve the issue. It is quite noticeable when it's measuring half boost compared to when it's measuring full boost. That to me indicates there is something more than just the boost gauge not properly measuring the amount of boost. Am I wrong?
 


Blue, how do I test that? Just by replacing it? I was thinking it might be that solenoid but I haven't been able to find it anywhere other than from GM in a kit they sell that I heard was around $250 =/
 
I had the same issue before. Let me see if I can find the post. It may just be the solenoid. I'm sure you can get one from the wanted parts section.

Sent from my HTC One using Tapatalk 2
 
Alright Blue. Like the car randomly will build full boost, but most of the time builds between 4 - 6 blocks. I was messing with the solenoid but couldn't find anything that seemed to help fix the issue.
 


I don't believe any of the vacuum lines are leaking, can't find a leak anywhere. When the car is cold, when I first start the car in the morning it will build full boost, but after a few minutes it starts only building about half
 
I'm pretty sure its the boost bypass solenoid, or the sensor for it because I unplugged the pressure line coming into it and now it builds full boost.
 
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