Gasoline type issue?

THATGTPGRANDPRIX

MEMBER
2002 PONTIAC GRAND PRIX
3.8L
6 CYL
2WD
AUTOMATIC
99,500 MILES

I bought the car listed above GTP Coupe almost a month ago as my first car for my last year in HS. It has the supercharged 3.8 3800 V6. It needs premium gas (91-93) because of the lower compression and supercharger. The previous owner and the dealer I purchased from filled it with 87. At around 3,500 RPMs the engine shook really violently and threw a P0304 engine code, which is a cylinder 4 misfire code. I filled the tank with 93 and have been running it that was since, and the issue has kind of fixed itself, but not totally. It still does it at highway speeds and still at around half throttle. From what i've read, this is premature fuel detonation, but I cant find a permanent fix. I have a friend with a 98 Regal GS and he has the same issue. I've replaced the fuel filter, used injector cleaner on gas fill ups and new spark plugs and wires. Do I need new injectors? ECU reprogram? Fuel system flush? I need to kill this issue before I start doing any serious performance mods, as I plan on adding an intercooler, a smaller SC pulley and a full exhaust with headers. Any help and ideas would be greatly appreciated. :)

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi,

Premium fuel is needed due to higher compression and performance vehicles, especially ones with turbo or superchargers. The higher-octane fuel helps prevent pinging or knocking from the engine and helps prevent damage.

Also, and you need to confirm this for me, but I thought in 2002 premium fuel was recommended on this vehicle and not required. If that is the case, you can use it regularly. If it is required, one tank shouldn't hurt it.

Now, to the problem. If you had a misfire on cylinder 4 only, are you still get the same code since you replaced the spark plugs and wires? Did you do anything with the ignition coils or the DIC (digital ignition control module) to which the coils mount?

Let me know if the same code is returns.

Take care,

Joe

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Tuesday, January 10th, 2023 AT 7:38 PM

THATGTPGRANDPRIX

MEMBER
3 POSTS

Hello Joe,

I've not only checked the 3800 and grand prix forums, but also the owner's manual, and they both say 91 or higher is required. I've heard people drive these with regular gas and they are fine as long as you don't go over a quarter throttle. As far as I know the previous owner ran regular gas in it throughout her ownership of the vehicle, and that is what the dealer was running in it too. We don't have 91 out here in Chicago so I've been running 93.

I've replaced the plugs and wires, and the issue still continues. Under full throttle in 4th gear, it still knocks and there is a vibration at idle, but the vibration could be blown motor mounts. I've been running injector cleaner and octane booster in my fuel for a while now and it has gotten a lot better. I think the issue was there was still some contamination from the regular gas left over in the system somewhere. It only takes one drop of regular to detonate all of the fuel prematurely.

I have not touched the coils or the DIC but if it comes back those will be one of the first places I look. I know for a fact the original set of coils is still on the car, and I recon they will be out the door soon. I plan on switching to a MSD or ZZP set of coils. I've also read it could be a problem with the engine's knock sensor or the Throttle Position Sensor. I think I'm also going to try a fuel system flushat some point, as my lower intake manifold gasket is leaking, and the fuel lines running over the supercharger need to be disconnected to remove the intake and S/C.

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Tuesday, January 17th, 2023 AT 1:07 PM

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi,

Thanks for the update. It sounds like you made progress. As far as the intake leak, is it leaking coolant?

The knock sensor could certainly cause a knock or ping in the engine. Also, if you have any vacuum leaks (intake) they can cause a rough idle.

I noticed around here as well; 93 octane is basically the only premium fuel offered. There is one place near me that offers 91 octanes, but it's only 4 cents less than 93, so why bother?

Let me know what you find or if anything changes.

Joe

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Tuesday, January 17th, 2023 AT 6:18 PM

THATGTPGRANDPRIX

MEMBER
3 POSTS

Hi Joe,

I had never considered my idle issue to be a vacuum problem. I have had some issues with vacuum on this car, and it wouldn't surprise me that more of the 20+ year old vacuum lines are leaking. I'll look into that more.

There is a bunch of residues around the gasket surface, and it's wet, like coolant. It is not a hard job to do, just time consuming. There is a lot sitting on top of the intake, like the S/C and all of the fuel and EVAP stuff. I was planning on doing an intercooler and will probably add it when doing the gasket.

I'll keep you updated with any progress or changes. :)

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Wednesday, January 18th, 2023 AT 7:35 AM

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi,

When you check for vacuum hoses, make sure to check around the intake or anywhere that could allow unmetered air to get into the engine as well.

Take care and I'll watch for your reply.

Joe