1999 Grand Prix GTP. Catalytic convertor fine. When you got to wide open throttle it loses power. Super charger pressure looks fine.
Use a scan tool to check for KR.
Check spark plugs / wires.
try a colder range plug if running more boost.
Make sure it always gets premium fuel...even with premium and especially in warmer weather the GTP will frequently perform the factory programmed KR (knock ******.) In WOT it is also more susceptible to running a lean condition with stock programming.
Things you can do to troubleshoot the problem and give more consistant performance with the GTP is run colder range copper core plugs and/or any brand of cold air intake, or even a stand alone 9" open element cone filter. Both are better than stock and provide more airflow even though the stock airbox was redesigned in '99. What else you can do is not limited to that but a start. Good luck.
That is actually quite normal for a stock GTP but here are some things you can do to help eliminate the problem;
Higher octane. For best performance never go lower than 93. At wide open throttle it is more susceptible to have knock ******, a.k.a. spark knock due to a leaner, less than optimal fueling condition that it may create. The stock PCM (engine computer) will reduce engine timing to avoid the knock, hence knock ******, and for every degree of timing reduced by the PCM equates to about a 5 to 6 horsepower reduction in the GTP.
You can also try the next colder heat range of spark plug. What I recommend for your GTP if it is currently with factory parts are copper core Autolite 605 gapped to .055". They are also considerably less expensive than the plugs recommended in the manual.
The next thing you should do in any case is clean your MAF. (mass airflow sensor). It is easy to identify by it being the large, rectangular shaped device mounted into the top of the throttle body with two security tab headed screws. Once removed you will see inside of it two tiny diodes. DO NOT TOUCH THEM OR DROP OR HIT THE MAF. The wire filaments for the diodes are very tiny and can break...without the MAF your car will not run. With rubbing alcohol, carefully pour it over the diodes until shiny...or you can also dip it into a cup of it. Allow to dry and then reinstall back into the throttle body making sure that it sits flat and be careful not to strip the threads.
Air...the GTP needs a lot of air. GM redesigned the box for the air intake but it is still quite restrictive. Changing to a K&N drop in filter and/or drilling holes through the box on the inlet side (in front of the filter only) can make a big difference.
Another thing that helps is to get it running a little cooler to help reduce the knock ******. Leave the engine cover off and take the weather strip on the back of the motor off...it lets heat out.
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SOURCE: Can I take a super charger out of a Grand Prix GTP
Well, the short answer is "yes". But - it isn't nearly as simple as just bolting the supercharger onto the intake and VOILA!, now you're supercharged. The procedure that you're inquiring about is commonly referred to as a "top swap", and there are numerous parts you will need for both engine as well as the transmission. Why the transmission? Because the GT and GTP use different gear ratios and the GTP has a 4T65HD as opposed to the 4T65E that is in the SE/GT. You will also need to have the PCM reprogrammed to accomodate all of these changes.
HERE is a great website that has a ton of very useful information, regarding this and many other Grand Prix topics.
Hope this helps and thanks for using FixYa!
SOURCE: replace thermostat on 2003 grand prix gtp with
The thermostat housing is located in the upper radiator hose. You will see a metal elbow secured by two bolts into what I remember is the upper intake manifold...don't quote me on that but you will see it regardless of whatever I call it. Anyways, GM decided it was a good idea to put stainless steel bolts in there but that makes it easy to strip the threads in the bolt hole so I think it's best when you put your wrench on the bolt to just tap on the handle till you vibrate it loose, then turn it.
When you pull off the housing you'll lose a little coolant and it will create an air pocket which we'll address. It's a 50/50 chance that the thermostat housing gasket will either stick to the housing or the engine...it may be torn, cracked or flaking in either case clean both contact surfaces before you replace the gasket. I recommend an oil impregnated poly material gasket.
Note the position of the thermostat in the hole and place the new one in as such.
I recommend a 180 degree thermostat...stock is 195 but the GTP runs better with the 180.
Put on gasket and reinstall the hose/housing assembly, alternate from one bolt to the other so the housing mates flat and cannot cause a leak.
Next, fill the coolant overflow bottle to the "HOT" mark. If the bottle is dirty and hard to see just put a light on top of the bottle...GM didn't mark it very well so if it's not squeaky clean you can hardly see a damn thing.
*IMPORTANT** if your car still has Dexcool in it, DUMP IT. It will eventually destroy the motor.
If not...continue on. Start the car. Go back to the thermostat housing and place a rag on top of, and twist the peacock valve on top of it to let the air out until it starts to get coolant on the rag. Done.
SOURCE: how to replace throttle position sensor on a 1995
Remove all parts to access the throttle-body, typically the sensor is on the side with 2 screws. Remove the wired plug and then the two screws. Assembly is the reverse being careful to fit the flat piece of the throttle-body into the groove of the sensor. Do not overtighten.
Replace connector and other parts.
Typically the sensor is on the drivers side of the throttle body. May be a torx screw, or phillips (star).
Good luck.
Canada Rocks!
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SOURCE: where is the throttle control
The throttle control sensor is called a TPS which is mounted on the throttle body. You will see three wires that plug into it. The throttle body replaces the old carburetor since the engine is fuel injected as you may know
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Going WOT sometimes causes a lean condition on a stock programmed PCM. How much power loss are we talking about? A slight hesitation or a complete shutdown? If you aren't running premium fuel that will definitely do it and you WILL ruin the motor by not running premium.
I hope your not going by the DIC in the dash for your boost.
Is it gutless, or is it just the normal 3.8 power?
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