1999 Pontiac Grand Prix Car bucking on acceleration

1999 PONTIAC GRAND PRIX
145,000 MILES • 6 CYL • FWD • AUTOMATIC
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BANDITGTP
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I have a 99 GTP with 145k miles. When I was around 90k miles my car started to hesitate when I really punched the throttle. It didn't happen all of the time but enough to be somewhat annoying. My SES light started to come on occasionally and would stay on for several days before it turned off on it's own. After multiple trips to the dealer where they were more interested in selling me tires or replacing my belts instead of telling me what was causing the probelm, I have tried to replace parts that I thought may be causing the problem. The dealer's scan came back with codes P0300 and P0172. I don't remember which code shows the engine is running rich but I had already told the dealer, my MPG dropped from 29mpg to 19mpg. Now 55k miles later, I have gotten back up to 23mpg but the problem has got to the point that I'm nervous merging into highway traffic and I wouldn't dream of passing someone on the highway. It will happen in any gear but it does seem to happen a lot when right at the point of when it upshifts. I always have to get out of the fast lane when going up hill. I drive 100 miles round trip everyday to work and I either need to fix it or junk it. I can't take it any more!!!

Here is a quick break down of what I have replaced already:
(3) ignition coils
(1) ingintion module
(1) catalytic converter
Entire exhaust starting from behind Cat
O2 sensor (rear sensor by U-Bend)
all plugs (twice)
all wires
fuel filter (twice)
air filter (twice)
(2) belts
SC tensioner pulley
Fuel Pressure Regulator

Car has no MODS

I don't remember where in the time frame it happened, but my SC tensioner pulley had fallen off so I was driving for a while without any SC. It either happened right before I started having trouble or right after it started acting up. Could this have caused a problem? Right now it seems that the hesitation happens whenever the SC Boost kicks in.

Any help would be appreciated.
May 27, 2009 at 6:39 PM
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RACEFAN966
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Ok I read you list of parts but none mention the Mass Air Flow Sensor. I would like you to get some mass air flow cleaner (no other cleaner will work) and pull the mass air flow sensor and clean it with the spray really good. Then see how it runs. If this makes an inprovement then we will do more testing on this part. You may need a digital multi meter to test this part so let me know what you can do and we will take it from there. Here is a pic of the part in question. Let me know how it goes.


https://www.2carpros.com/forum/automotive_pictures/249564_396960_1.jpg

May 29, 2009 at 9:59 AM
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BANDITGTP
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I tried removing and cleaning it with the special cleaner and there was no change in performance. I thought I heard somewhere that if you unplug it, the engine may also run better. No luck with this attempt either. It actually ran worse when it wasn;t connected.
I do have a multi meter so if you have any other ideas, it would be greatly welcomed.
May 29, 2009 at 7:18 PM
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RACEFAN966
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Ok lets check out the map sensor first ok. I was really hoping that cleaning the mass air flow sensor would have helped but from the other test you did it appears that it should be ok so to the map sensor now. Here is the test I need you to do.
1. Unplug the electrical connector to the map sensor and check voltage at the gray wire and make sure that there is 4.5 -5 volts with key on engine off. If that is good then go to next step.
2. Now turn ignition off and reconnect the map sensor and then hookup to the light green wire and now if you can check voltage key on engine off and then apply vacuum and record how the voltage changes and let me know what you find. Turn key back off and go to next step
3. Now start car and let it warm up and then disconnect the vacuum hose to it and not how it runs. It should go very rich. Then as you apply vacuum to it is should run leaner don't apply more then 18-20 inches of vacuum to it.
One more thing I would like you to after all this as the car is running use the handle end of a screw drive and lightly tap on the mass air flow sensor and let me know if that makes the car stumble at all or if it dies. Get back to me with what you find and we will take it from there.
May 30, 2009 at 9:04 AM
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BANDITGTP
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What type of tool would I need to apply vacuum to the line? I don't have anything like this. Would a local auto parts store (Pep Boyx/Autozone) have it and do you know about how much it usually costs?
May 31, 2009 at 7:07 AM
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RACEFAN966
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It is a vacuum pump. Yes your local parts store should have it and I would spend more then 20 buck or so on one sence you will only really need it for this test. Is there anyone you know you could borrow one from. Get back to me with what you find.
Jun 1, 2009 at 9:03 AM
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BANDITGTP
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I just tested the MAP sensor on here are my results

1) key on / engine off - 5.07v

2) key on / engine off / no vacuum (green wire) - 2.3v
key on / engine off / with vacuum (green wire) - 0.8v

3) key on / engine on (green wire) - started at 0.8v and then dropped to 0.0v
I didn't notice any change in performance when disconnecting the vacuum or applying vacuum to the map sensor.

I also tried tapping the mass air flow sensor and nothing happened. The engine stayed at a steady idle.

I hope this covers all of the steps. Please let me know if there is anything else I can try.
Thanks
Jun 3, 2009 at 11:59 AM
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RACEFAN966
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OK the the map sensor should be 1.5 v at idle and go to 4.5 under load at wide open throttle (when driving on the highway). So the voltage is low to start off with and should not drop to zero at all. When you pull the vacuum line it should start to run very rich. So at the moment your car is using the TPS only to determine how much fuel your car needs. I would like you to check the orange wire with the black stripe and make sure that it is grounded and check the gray wire to make sure it is getting a constant 4.5 to 5 volt with key on engine off. If this turns out good then with the other test you did according to my books the map is no good.
Jun 5, 2009 at 9:31 AM
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BANDITGTP
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I checked the voltage again this morning and I am getting a steady 5.05 on the gray wire. So, you think it's pointing to the MAP sensor itself?
Jun 6, 2009 at 11:22 AM
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RACEFAN966
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It might be bad. So are you living 1000 ft elevation or lower? That is what the 5.5 volts is telling me. So the light green wire at the map sensor. What does the voltage at the light green wire do as you change vacuum to the sensor with the key on engine off?
Jun 8, 2009 at 1:27 PM
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BANDITGTP
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You are correct that I am below 1000ft. I'm not sure the exact elevation but I live in Central NJ and it is pretty flat around here.
I just checked the MAP again and with the key on engine off, I was getting around 2.5v.
As I applied vaccum, the voltage would drop steadily according to how much vacuum I applied.
After I started the car,the lowest the voltage went was around 7v.
I don't know what happened the other day when I got a reading of 0.0v. I may have been a loose connection or a poor helper(friend of mine).
When I revved the motor in park, the voltage would drop some more but it never dipped below 4.0v.

Any other suggestions would be appreciated.
Jun 10, 2009 at 8:57 PM
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RACEFAN966
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Ok so as the voltage droped when you stepped on the gas it should have come back up as you held the pedal steady again did it do this?
Jun 11, 2009 at 10:09 AM
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BANDITGTP
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This time I hooked up my meter to the map sensor using a long wire so that I could see the readings while I was driving. I must have been in a rush last time I typed my post. The readings I was getting were not 7v and 4v. They were actually .7v and .4v respectively.

Today's readings:
When the car was in park and I gave a quick rev on the accelerator, the voltage would shoot quickly up around 1.7v and then drop back to .4v. It would then settle back around .65v-.7v.
While the car was in gear and stopped at a traffic light, I would get .7v
The harder I pressed on the accelerator, the higher the voltage would go. The highest I ever got was around 3.8v. This was while driving at 65mph and stepping quite hard on the gas.
When going at a steady 65-70mph, I would get around 1.6v.
When I would get off the gas and coast downhill at 50-60mph, the reading would drop down to .35v-.4v.
It seemed that every time the car felt like it was bogging down, the reading was 2.0v or higher.
Jun 15, 2009 at 6:13 PM
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RACEFAN966
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Ok here is what you should be getting. You should see around 1.5 v at idle and 4.5 at wide open throttle. So if your reading are correct the the map sensor is bad and sending false signals to the computer.
Jun 16, 2009 at 10:09 AM
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BANDITGTP
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I just swapped out my MAP Sensor and it is still acting the same and it appears that I'm getting the same readings.

At idle I get around .7v and driving at a steady 75mph I get around 1.5v-1.7v. It still seems that whenever I get a reading over 2.0v, is when the car is hesitating. This is also when I try pressing harder on the gas pedal as when I try to pass someone. I have tried to get it up around 3.5v-4.0v by putting the pedal down to the floor but the engine is bucking so bad that I have to get off the gas.

Any other suggestions would be appreciated.
Jun 19, 2009 at 6:17 PM
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BANDITGTP
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Any ideas or suggestions?
Jun 23, 2009 at 6:51 PM
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BANDITGTP
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It seems that after I changed the MAP Sensor, performance is still the same but i have gained about 1.5 - 2 mpg. Not sure if this helps to explain anything.

Any suggestions would be greatly appreciated.
Jun 26, 2009 at 11:03 AM