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2000 GTP bucking/stumbling/hesitation at WOT between 3800 - 4800 rpms

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3800SC4DR

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Hi, I'm posting looking for help for my dad. He just bought a 2000 Grand Prix GTP Sedan about 2 weeks ago at most, and is already having trouble with it. The car is from Michigan, but we bought it in PA not too far from the NY/PA border. The car is very clean for it's age, and only has 85,000 miles. The car was a one owner (female) that only made 2 mile trips daily, and once in awhile from Michigan to PA where we live.

The first day of having the car it ran great other than a little stumble I heard when he fired first fired it up. Pulled real strong throughout the rev range, no hiccups at all. Brought it home and later went out to drive it again, car immediately became sluggish and had a hesitation. Threw a SES code for the MAF.

Picked up a MAF, installed it, along with a new supercharger tensioner as it was quite noisy. The SES went away and has not come back, but the car still has a major hesitation between 3800 - 4800 rpm area. Almost as if it's hitting a rev limiter. The car bucks, sputters, hesitates, then picks back up again after that 3800 - 4800 rpm range and runs decent, but still feels down on power up to redline. When you aren't WOT, the car doesn't do this at all.

My dad thinks it's a fuel delivery issue, such as a clogged fuel filter and we're about to put that on now. If that does not fix it, what other item should we look into?

The car is stock as far as we know, other than the stock resonator/mid muffler was removed and replaced with a straight pipe as it was rotten.
 


I'd pull the spark plugs and check them over, even just replace them to make sure.

Inspect the wires aswell, if they are super old looking you are better off replacing them too.

Inspect the coil packs for cracks or corrosion.

And make sure to run 91+ octane fuel.

I'm only guessing but it just seems that under a heavy load it's misfiring and my first thoughts are normally to check things spark related.
 
Agree with what Cam said. Personally whenever I buy a used vehicle I always plan on full tune up so I know when its done.

Plugs- auto light 606s or ac delco 41-101 iridiums gapped at .055
Wires- Delphi or delcos
Fuel filter- ac delco only
Pcv valve- doesn't matter brand
Maf cleaned(also keep in mind new mafs can be bad)
seafoam in gas and intake port under supercharger nose
possibly o2 sensors
make sure cat isn't clogged
on these cars around your mileage the lim gaskets go bad aswell as coolant elbows
vac lines can be cracked and cause a vac leak
this wouldn't really cause your issue but changing trans filter and fluid with dex 3 wouldn't be a bad idea

All I can think of at this time
 
I had a problem where it would do the said symptons. Changed the coil packs and it fixed it. I'd do a tune up.
 
Haven't got to the fuel filter today yet, looks like it will have to wait for tomorrow. I thought ignition related as well from other things I have read. He thinks fuel related because he says if it was spark/ignition related it would have thrown a code by now.

I thought maybe the woman who owned it before being that it was only for small commutes, probably never got on it wot. And maybe even was running 87 octane in it to save money and probably never noticed a difference running wise if she was never wot. It had a little under 3/4 tank in it when we got it, and ran the rest out and filled it back up with 93 and a bottle of fuel injector cleaner thinking maybe that had been the case.

Problem hasn't improved though. I drove it to town earlier, it wasn't fully up to temp but I stepped into it and it didn't hesitate. But on the way back after it was at temp it started it's bucking again. It also seems to run quite cool as if it has a lower temp thermostat in it, it settles right at the large mark between 100 and 210. So about 160-180 or so. Not sure what the stock one is at.

Will have to look over the plugs, wires, and coil packs.

Oh also he wants to change the oil in the supercharger, he's kind of stuck on doing that. I doubt it's even been done and the manual says every 50,000 miles I believe. How often do you guys change yours?
 


just do a tune up go from there. with those miles its never had one before for sure. if the po did short trips it likely needs a tune up.

first thing i do when i get a used car is change all the fluids, and a tune up, weather it needs it or not. now i have my own records to go off.

changing the sc fluid is easy and cheap enough, but its not the problem.
 
Random cylinder misfire can often never toss a check engine light. Also you are not always in the bucking/misfire range, only happens when you are on it, and that time frame is not very long. If it was a dead miss on a single cylinder, that would throw a code fairly quick.

That first big line between 160 and 210 is suppose to be 195º, a scanner could confirm that. The factory dash gauge is not something to trust in a way, once it says you are at 210 on the dash it is likely already warmer than that.
 
As already mentioned, full tune up and fluid change along with filters, belts, wires, brakes, struts, rubber! First things first!... Can't chase problems when you don't have a baseline to start from. Could be simply plugs never changed! Lolz. Who would think that? You'd be amazed how different a car rides with new stuff! I'd change the front O2 as well, just for the hell of it. : )
 
Sorry for the super late response. The problem was the new maf was bad! Put a new one in and voila! Also did the air box mod, and a k&n drop in. Working on more mods soon.
 


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