1999 Grand Prix GTP DTC P0303

[99GTPMLA]

Hi,

I'm new to this forum and just purchased a 1999 Grand Prix GTP with 113,000 miles. Driving it home from work today it threw the P0303 code which means cylinder 3 misfire. How can I check to see what the problem is? Thanks.

[KeithGTP03]

Check your plug, check your plug wire, check the coil labeled 3 for corrosion. And welcome to the forum!

[99GTPMLA]

Thanks KeithGTP03. I checked the coil for #3 cylinder and it was corroded. I also checked the plug and it was at .070" gap instead of .060". I cleaned the coil stud with sandpaper and it still threw the code. I then switched the #3 spark plug wire with the another one and it didn't throw a code after that. It did start missing again at higher speeds but didn't throw any codes. I will change the wires and plugs tomorrow and clean the coil posts and hope that takes care of it.

[KeithGTP03]

Cool, good luck! If it continues, the next step would be swapping the coils around and see if it follows the coil. If it still misses on #3, time to swap the injector with the one next to it and see if it follows the injector. If it continues after that, you're gonna need to compression test cylinder 3.

[Fivefingerdeathpunch]

Make sure you buy the correct plugs.

[Javi]

Make sure you buy the AC Delco plugs.

fixed

[99GTPMLA]

Just a follow up. I changed the spark plugs, spark plug wires, air filter and changed the oil with Mobil 1 0W-30 synthetic along with the oil filter and it ran great. No missing and it had better power than before. I put in Champion 7940 plugs and gapped them a .058. Thanks for the advice.

[Fivefingerdeathpunch]

Well you didnt buy the correct plugs.

Platinums and champions dont work well with these car.

It may be okay for now, but in a while you'll be back to the original problem because the waste spark ignition system will eat away at the plugs.

Ac Delco Iridiums are the best.

[Fivefingerdeathpunch]

It may be okay for now, but in a while you'll be back to the original problem because the waste spark ignition system will eat away at the plugs.

Champions work best in Chryslers, motorcrafts work best in fords and Ac Delco work best in GM cars.

Iridium plugs are made for the gm cars. Platinums are just cheap plugs that burn out with our ignition system.

[Fivefingerdeathpunch]

I was just hoping he'd research it.

[99GTPMLA]

What is the reason that platinum or Champion plugs won't work well while AC Delco's do?

[99GTPMLA]

Champion also makes plugs for GM original equipment service and SPO. Also some O.E plugs for GM, Ford, Mercedes, Fiat and etc. GM requested the platinum over the iridium because of the cost savings and the waranty rating is about the same. The iridium plugs only advange was the 120,000 mile O.E. waranty and the Platinum plugs were 100,000 mile O.E waranty. I was just curious why AC Delco plugs were favored for the grand prix other than marketing. Thanks.